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FIBRE OPTIC TECHNOLOGIES AND TECHNIQUES RESEARCH COLLECTION

by Sulaiman Wadi Harun et al.

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Modern life would be unimaginable without the technologies of telecommunications, engineering and computing, in which fibre optics play a critical role. As such, this book will be of interest to a wide array of researchers and technicians in those fields. Chapters include ones on optical amplifier automated measurements and fibre-optic remote tests, and the applications of both fibre-optic

displacement sensors and microfibers. Subsequent contributions address doped fibre amplifiers’ characteristics under both internal and external perturbation, and the use of a rare-earth doped femtosecond fibre oscillator as an ultra-wideband multiwavelength light source. The book concludes with a discussion of passive Q-switched and mode-locked fibre lasers using carbon-based saturable absorbers. This book will be required reading for researchers and engineers.

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ISBN 978-953-51-2345-3

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Fibre Optic Technologies and Techniques - Research Collectionby Sulaiman Wadi Harun et al.

Published by InTechJaneza Trdine 9, 51000 Rijeka, Croatia

Edition 2016

© InTech and the Author(s) 2016The moral rights of the author have been asserted.

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Additional hard copies can be obtained from [email protected]

Fibre Optic Technologies and Techniques - Research Collection by Sulaiman Wadi Harun et al.

p. cm.

ISBN 978-953-51-2345-3

Contents

Preface VII

LabVIEW Applications for Optical Amplifier Automated Measurements, Fiber-Optic Remote Test and Fiber Sensor Systems 9 S. W. Harun, S. D. Emami, H. Arof, P. Hajireza and H. Ahmad

Fiber Optic Displacement Sensors and Their Applications 45 S. W. Harun, M. Yasin, H. Z. Yang and H. Ahmad

Fabrication and Applications of Microfiber 79 K. S. Lim, S. W. Harun, H. Arof and H. Ahmad

Doped Fiber Amplifier Characteristic Under Internal and External Perturbation 115 Siamak Emami, Hairul Azhar Abdul Rashid, Seyed Edris Mirnia, Arman Zarei, Sulaiman Wadi Harun and Harith Ahmad

Ultra-Wideband Multiwavelength Light Source Utilizing Rare Earth Doped Femtosecond Fiber Oscillator 143 Nurul Shahrizan Shahabuddin, Marinah Othman and Sulaiman Wadi Harun

Passive Q-Switched and Mode-Locked Fiber Lasers Using Carbon-Based Saturable Absorbers 157 Mohd Afiq Ismail, Sulaiman Wadi Harun, Harith Ahmad and Mukul Chandra Paul

Modern life would be unimaginable without the technologies of telecommunications, engi-neering and computing, in which fibre optics play a critical role. As such, this book will be of interest to a wide array of researchers and technicians in those fields. Chapters include ones on optical amplifier automated measurements and fibre-optic remote tests, and the appli-cations of both fibre-optic displacement sensors and microfibers. Subsequent contributions address doped fibre amplifiers’ characteristics under both internal and external perturbation, and the use of a rare-earth doped femtosecond fibre oscillator as an ultra-wideband multi-wavelength light source. The book concludes with a discussion of passive Q-switched and mode-locked fibre lasers using carbon-based saturable absorbers. This book will be required reading for researchers and engineers.

Preface

Chapters in this book were also published in:

S. W. Harun, S. D. Emami, H. Arof, P. Hajireza and H. Ahmad (2011). LabVIEW Applications for Optical Amplifier Au-tomated Measurements, Fiber-Optic Remote Test and Fiber Sensor Systems, Modeling, Programming and Simulations Using LabVIEW™ Software, Dr Riccardo De Asmundis (Ed.), InTech, DOI: 10.5772/13247.

S. W. Harun, M. Yasin, H. Z. Yang and H. Ahmad (2012). Fiber Optic Displacement Sensors and Their Applications, Fiber Optic Sensors, Dr Moh. Yasin (Ed.), InTech, DOI: 10.5772/18564.

K. S. Lim, S. W. Harun, H. Arof and H. Ahmad (2012). Fabrication and Applications of Microfiber, Selected Topics on Optical Fiber Technology, Dr Moh. Yasin (Ed.), InTech, DOI: 10.5772/31123.

Siamak Emami, Hairul Azhar Abdul Rashid, Seyed Edris Mirnia, Arman Zarei, Sulaiman Wadi Harun and Harith Ah-mad (2012). Doped Fiber Amplifier Characteristic Under Internal and External Perturbation, Selected Topics on Optical Amplifiers in Present Scenario, Dr. Sisir Garai (Ed.), InTech, DOI: 10.5772/34064.

Nurul Shahrizan Shahabuddin, Marinah Othman and Sulaiman Wadi Harun (2012). Ultra-Wideband Multiwavelength Light Source Utilizing Rare Earth Doped Femtosecond Fiber Oscillator, Semiconductor Laser Diode Technology and Ap-plications, Dr. Dnyaneshwar Shaligram Patil (Ed.), InTech, DOI: 10.5772/35030.

Mohd Afiq Ismail, Sulaiman Wadi Harun, Harith Ahmad and Mukul Chandra Paul (2016). Passive Q-switched and Mode-locked Fiber Lasers Using Carbon-based Saturable Absorbers, Fiber Laser, Dr. Mukul Paul (Ed.), InTech, DOI: 10.5772/61703.

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LabVIEW Applications for Optical Amplifier Automated Measurements, Fiber-Optic Remote

Test and Fiber Sensor Systems S. W. Harun, S. D. Emami, H. Arof, P. Hajireza and H. Ahmad

University of Malaya Malaysia

1. Introduction For the past 20 years, intensive researches in the fields of optoelectronics and fiber optic communications have resulted in the invention of new devices that revolutionized our life. The optoelectronics devices such as compact disc players, laser printers, bar code scanners, and laser pointers are some of the examples [1]. In fiber optic communications, revolution in the telecommunications system has been initiated by the introduction of higher bandwidth, more reliable telecommunication links with lower bandwidth cost [2]. Recently, fiber optic sensor technology has gained interest from the research community. Optical fiber sensors offer a number of advantages over conventional electrical sensing technologies, which make them attractive for a wide range of application areas. These advantages include intrinsic safety in chemically hostile or explosive environments, low susceptibility to electromagnetic interference, electrically passive operation and high sensitivity, compatible with composites, light weight and geometrical versatility [3]. Improvement in fiber optic technology creates a necessity for a virtual instrumentation system. A virtual instrumentation system is software used for developing a computerized test system, a measurement system, a calibration system, a control system for an external measurement hardware device and a display system for test or measurement data [4]. The best virtual instrumentation system that has been developed so far is LabVIEW. LabVIEW is an application development program that was developed by National Instruments in 1986 to integrate science and engineering tasks by interfacing computers with instruments for collecting, storing, analyzing, and transmitting data while, at the same time, providing an effective user interface. Different from other development software such as C/C++, FORTRAN, Basic, etc., LabVIEW utilizes its own integrated programming language known as the Graphical Programming Language, which uses graphics as code sequences in the application being developed, making the software development process significantly easier [5]. LabVIEW is powerful programming software that can interface with over 7,000 instruments to provide data acquisition, industrial measurement, automated testing, and instrument control. Integrated through LabVIEW, instruments such as sensors, optical time domain reflectometer (OTDR), oscilloscopes, optical spectrum analyzer (OSA) and RF generators can work alongside the GPIB hardware application software that makes fiber optic communication system research activities significantly easier [6].

LabVIEW Applications for Optical Amplifier Automated Measurements, Fiber-Optic Remote Test and Fiber Sensor Systems

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In this chapter, applications of LabVIEW in automatic test measurement of fiber optic system are demonstrated. In the first section, the LabVIEW applications in fiber optic system and the basics of instrument connectivity are presented. Then, the aspects of hardware communication to external instruments through GPIB and serial interfaces are analyzed. Next, self-calibrating automated characterization system for depressed cladding applications is demonstrated utilizing the LabVIEW’s GPIB interface. The automation system consists of a tunable laser source (TLS), optical spectrum analyzer (OSA), attenuator, laser diode controller, and a personal computer all networked using GPIB cables. Results of the manual and automatic measurements and the analysis of the measurement trace obtained from the optical time domain reflectometer (OTDR) are shown. Subsequently, the communication methods between the OTDR device and personal computer along with the details of the automation program developed using LabVIEW are presented. In the end, two applications of LabVIEW in fiber optic sensor system are discussed.

2. LabVIEW for fiber optic applications Fiber optic systems have become in high demand for use in telecommunication and sensor systems. The optical systems, whether transmitting data across continents or providing real time measurement consist of dozens of components. These components are made from many different types of exotic materials and the manufacturing technologies are so new. Traditional labor intensive techniques cannot keep up with market demands, which require a cheaper solution. This section explains the automatic test measurement, fiber sensor and remote testing, which can be used to solve many problems in fiber optic system.

2.1 Self-calibration automated measurement Automatic test measurement is a vital part of the telecommunication and fiber optic communication test scene today. Automatic test measurement enables self-calibrating test to be done very swiftly and accurately. The amount of time consumed in implementing a fiber optic sensor system forms the bulk of the development cost and thus it is necessary to reduce the troubleshooting time to the shortest possible. This can be achieved with the use of automatic test measurement techniques [6]. There are a variety of different approaches that can be used for automatic test measurement systems. Each type has its own advantages and disadvantages, and can be used to great effect in the right circumstances. Automatic test equipment and automatic test software are the two main types of test measurement systems. Equipments such as automatic optical inspection (AOI), automated X-Ray inspection (AXI) and In-Circuit Test (ICT) are common forms of automatic test equipment which are used today in optical and electrical science [4]. One the best solution in automatic test equipment is a board or unit that can be tested using a stack of remotely controlled test equipment. The most popular method of controlling the test equipment is the General Purpose Interface Bus (GPIB). There may also be an interface adapter necessary to control and interface with the item under test. While the GPIB is relatively slow and has been in existence for over 30 years it is still widely used as it provides a very flexible tool of test. This type of systems use test instruments on a board that can be slotted into a standard slot thus saving both space and cost when compared to the stand-alone. Laboratory test equipment can often be used as most items of lab test equipment have a GPIB port. The main drawback of GPIB is its speed and the cost of writing the programmes although packages like LabVIEW can be used to aid programme generation and execution in the test environment [5].

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2.2 Fiber optic sensor Fiber optic sensor is a device in which variations in the transmitted power or the rate of transmission of light in optical fiber are the means of measurement or control. The information could also be derived in terms of intensity, phase, frequency, polarization, spectral content, or other quantities. Physical parameters such as strain, temperature, pressure, velocity, and acceleration can be measured by fiber optic sensor. Immunity to electromagnetic interference (EMI) and radio frequency interference (RFI), elimination of conductive paths in high-voltage environments, inherent safety and suitability for extreme vibration and explosive environments, tolerant of high temperatures (>1450 C) and corrosive environments, light weight, and high sensitivity are the main advantages of fiber optic sensor over normal sensor [7]. Depending on the usage, fiber optic sensors can be divided into Extrinsic (or Hybrid) and Intrinsic (or All-Fiber) types. Figure 1 shows an extrinsic sensor which consists of an optical fiber that transmits modulated light from a conventional sensor. A major feature of extrinsic sensors, which makes them so useful in such a large number of applications, is their ability to reach places which are otherwise inaccessible. In an extrinsic sensor, sensing takes place in a region outside of the fiber [8]. An intrinsic sensor relies on the properties of the optical fiber itself to convert signals from the environmental into a modulation of the light beam passing through it. Intrinsic sensors can modulate the intensity, phase, polarization, wavelength or transit time of light. Sensors which modulate light intensity tend to use mainly multimode fibers, but only single mode cables are used to modulate other light parameters. A particularly useful feature of intrinsic fiber optic sensors is that they can, if required, provide distributed sensing over distances of up to 1 meter [7].

Fig. 1. Extrinsic sensor

Fig. 2. Intrinsic sensor

Input Fiber Output Fiber

Light Modulator

Environment Signal

Environment Signal

Fiber Optic

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Different types of fiber optic sensors are designed based on the characteristics of light modulated by the environmental effects. One of the most popular fiber optic sensors is the intensity based fiber sensor. Intensity-based fiber optic sensors depend on the principle that light can be modulated in intensity by an environmental effect. Multimode fiber reflective sensor and bending the fiber are two examples of fiber optic sensors. Figure 3 shows a displacement sensor using a multimode fiber. In this sensor, a light leaves the fiber end in a cone pattern, and strikes a movable reflector as shown in Figure 3. The intensity of reflected light is related to the distance of the fiber-reflector from the fiber’s end [9].

Fig. 3. An example of a fiber optic displacement sensor

Bending effect is another option which can be used to affect light intensity in an optical fiber. Figure 4 shows the fiber optic displacement sensor using the light intensity modulation based on the bending effect. As shown in the figure, the fiber loss increases as the deformer closer to the fiber. The displacement of the deformer is linearly dependent on the intensity of the transmitted light [10]. In interferometric fiber optic sensors, the optical phase of the light passing through the fiber is modulated by the field to be detected. This phase modulation is then detected interferometrically, by comparing the phase of the light in the signal fiber to that in a reference fiber [8].

Fig. 4. Fiber optic displacement sensor based on bending effect

Light Mirror

Displacement

Fiber Optic

Deformer


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2.3 Remote fiber test system A Remote Fiber Test System (RFTS) is used mainly for error detection in optical networks that may arise due to mechanical faults or quality losses in the optical fibers and connectors that form the optical communication network [11]. Optical Time Domain Reflectometer (OTDR) is the main device for measurement and software control of the measurement process in RFTS. Figure 5 shows a simple RFTS measurement system. The control unit is a personal computer which is used for pre and post-measurement management. The OTDR functions as a measurement unit and the optical power splitter divides an input power equally into output ports. The optical power splitter can be replaced by an optical switch [12].

Fig. 5. RFTS measurement system.

The measurement process begins as the OTDR injects a short light pulse into the optical power splitter. In the splitter, the input power is divided into several output ports with power ratio specified by the splitter type. As the light pulse travels along the test fiber, a fraction of light is reflected back to the optical power splitter due to Rayleigh scattering and Fresnel reflection. All the scattered signals from the test fiber form a complex measurement trace as they reach the OTDR. Finally, the OTDR trace is saved in the computer memory and the analysis of the measurement result begins. In the standard RFTS, the error detection system is based on the measurement of the total loss in the optical link. Another approach that can be used is by comparing the reference and test measurement results. The reference measurement is collected just after the installation of the optical communication system [4]. As the test measurement is performed, the resulting test OTDR trace is compared with the reference trace. If the difference between the optical power level of the reference and the test measurement at certain measurement point exceeds the predefined threshold, the RFTS reports an error and stops the measurement. The time for the measurement to repeat is controlled by the user. The whole measurement process is software controlled and managed by LabVIEW [13].

3. Instrument communication methods There are different types of communication methods available for controlling laboratory instruments as well as optical components. These methods are serial, parallel, GPIB, VXI,

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PXI and others, which the choice is mainly based on the applications. Two most common instrument communication methods are GPIB and serial port communication. In 1965, Hewlett-Packard designed the Hewlett-Packard Interface Bus (HP-IB) to connect their line of programmable instruments to their computers. Because of its high transfer rates (nominally 1 MB/s), this interface bus quickly gained popularity. It was later accepted as IEEE Standard 488-1975. Today, the General Purpose Interface Bus (GPIB) is more widely used than the HP-IB. This is due to its ability to connect to different devices to the same GPIB bus. Any devices must have a unique GPIB address between 0 and 30, so that the data source and destinations can be specified by this number. Address 0 is assigned to the GPIB interface board. The GPIB has one Controller, usually a computer that controls the bus management functions. The LabVIEW GPIB VI automatically handles the addressing and most other bus management functions, saving user from the hassle of low-level programming. To use GPIB as part of in any virtual instrumentation systems, the GPIB driver software is required and can be installed in our computer according to the directions that accompany LabVIEW or the board. Installing a GPIB board is usually easy [14]. To configure the GPIB, serial interface or any communication method, NI Measurement & Automation Explorer (NI-MAX) simulation software can be used. The GPIB boards can be integrated with the NI-MAX so that it can be configured under the Devices and Interfaces tree easily. For the other GPIB devices, their installation procedures are described in the manual that comes with the GPIB board. Figure 6 illustrates the MAX windows which shows the GPIB interface number as 0. It is followed by an instrument with the primary address 2.

Fig. 6. Measurement & automation explorer windows

Serial communication is the other popular means of transmitting data between a computer and a peripheral device such as a programmable instrument or even another computer. Serial communication uses a transmitter to send data, one bit at a time, over a single communication line to a receiver. This method is normally used when data transfer rates are low or the data needs to be transferred over long distances. Serial communication is


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popular because most industrial computers have at least one serial port available. However, for laptops and smaller desktops that do not have a built-in serial port, an inexpensive USB-to-RS-232 adaptor can also be used. Serial communication requires that you specify four parameters: the baud rate of the transmission, the number of data bits encoding a character, the sense of the optional parity bit, and the number of stop bits. The requirements for port setting features are shown in the Ports (serial and parallel) menu in Figure 7. For the port binding, it is used for specifying the port to be used for the serial/parallel device. An alternative port can be selected to link to the ASRL resource, this setting does not change the logical port number (COMx) of the physical serial port, it only changes which COM port this VISA resource is binding to. After that, click the “Validate” button to test whether NI-VISA can access the resource with the specified configuration. The result shown on Figure 8 should pop up.

Fig. 7. Serial port setting

Fig. 8. Validation of NI-MAX

When a VISA session to “ASRL1::INSTR” is successfully opened, communication with the device can be started by going to the Property Node (Set) and modifying the value of the specified attribute as shown in Figure 9. The values for serial baud rate, serial data bits, serial stop bits, serial flow control and others are set to the same values as those chosen during Port Setting. Then click the execute button. Next, as shown in Figure 10, click the “Write” tab and fill up the blank box under the “Buffer” with a proper command ending with “\r\n” to initiate the communication with the serial device. The function of “\r\n” is just like the “ENTER” button on a personal computer keypad. Without “\r\n”, the command is incomplete, and hence, NI-MAX cannot recognize it. This is the same as when

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the command at Hyper Terminal has been inserted, the “ENTER” key must be pressed to start the communication with the equipment such as OTDR.

Fig. 9. Property node setting

Fig. 10. Write buffer of NI-MAX

4. LabVIEW application examples 4.1 Self-calibrating automated characterization system for EDFA Ultrahigh- capacity optical transmission systems are a vital component for 21st-century telecommunications networks that support various expanding information systems such as the Internet, mobile communications, and digital cable television [15].Wavelength-division-multiplexing (WDM)systems employing optical amplifiers such as the erbium-dopedfiber amplifier (EDFA) are considered the most effective solution to increase data transmission capacity [16–17]. The main characteristics of EDFAs are their gain and noise figure, and their values depend on the input signal wavelength, input signal power, and pump power [4]. The gain is determined by measuring the difference between the output signal power and


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the input signal power, whereas the noise figure is calculated from the gain, amplified spontaneous emission (ASE), and resolution of the optical spectrum analyzer (OSA). The accurate ASE level is measured using the interpolation technique, and thus, even a slight error in the initial measurement of the input power level will cause a cascading effect that will render the final ASE measurement invalid. In addition, the accuracy of the gain value is also dependent on the accuracy of the input measurement [5]. This makes the current manual measurement techniques not suitable for gain value measurement as even the slightest deviation in the initial input measurement will result in inaccurate gain values. As such, using manual measurement techniques for EDFA experiments could lead to inaccurate results and long experiment times. EDFAs are typically characterized using OSAs, tunable laser sources (TLSs), optical attenuators, laser diode controllers, and optical power meters. From experience, EDFA research with manual control of these instruments will lead to delays and inaccuracies in research due to the large number of values that have to be taken and recalibrated for each parameter. To cope with this problem, a self-calibrating automated measurement system for EDFAs has been developed based on the LabVIEW program and GPIB hardware. The basic architecture of a standard EDFA is depicted in Figure 11. The setup consists of a piece of EDF, a wavelength division multiplexing (WDM) coupler, a pump laser and two isolators. In this work, a depressed cladding EDF (DC-EDF) with Erbium ion concentration of approximately 500 ppm is used as gain medium for amplification in S-band region. The EDF is pumped by a 980 nm laser diode to generate population inversion for amplification. A WDM coupler is used to combine the pump light with the input signal. Optical isolators are used to ensure unidirectional operation of the optical amplifier [15].

Fig. 11. Configuration of the EDFA

The automation system consists of a TLS, OSA, attenuator, laser diode controller, and a personal computer. All these instruments are networked using GPIB cables and are individually designated as Talkers, Listeners, and/or Controllers. By having these functions, all the involved instruments can automatically be controlled through a single computer for the input signal measurement; the experiment is setup as shown in Figure. 11, using only the TLS, attenuator, and OSA. Figure 12 shows the user interface of the automation system. The program has two main menus, i.e., the “input” menu and the “output” menu. Under the “input” menu, there are three submenus, i.e., “Multiwavelength/Power −30 dBm,” “Multiwavelength/Power 0 dBm,” and “Multipower.” The “Multiwavelength/Power −30 dBm” submenu measures data by fixing the input power at −30 dBm and varying the input signal wavelengths. This submenu automatically changes the TLS wavelength values and determines the most accurate corresponding attenuation value to obtain a constant input

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power of −30 dBm. The OSA measures the output data and gives the necessary feedback to both the TLS and the attenuator, and the process repeats until the desired wavelength values and corresponding attenuation values are obtained.

Fig. 11. The automation system for input signal measurement

Fig. 12. The automation system user interface for input signal measurement

The automated measurement system is able to obtain a highly accurate input value by using a “while loop” subroutine that is contained within the application. The “while loop” will require the system to first obtain a rough attenuation value by adjusting the attenuation value so that the OSA is able to obtain a reading that is within ±0.9 dBm range of the


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required −30dBm value, as per the requirement of the multiwavelength menus or as per the required power as determined by the user in the multipower menu. Once the rough attenuation of the value has been determined, the system will then adjust the attenuation value based on the OSA readings to obtain the exact attenuation value required for the subsequent experiment. The “while” loop is illustrated in Figure 13. The submenu “Multiwavelength/Power 0 dBm” gathers data at a constant 0 dBm input power at different wavelengths. With regard to wavelength, the software is programmed for the S-band wavelength region from 1480 to 1530 nm. The “Multipower” submenu obtains the input power for a constant center wavelength, but at different input powers, which has been programmed from −40 to 5 dBm. All submenu functions are automatic, and the software records all required data on either diskette or hard disk.

Fig. 13. “while loop” subroutine

TLS ON

ATTN 0dB

OSA Obtain Input Power Value

Attenuation =input signal Value- 40 dBm

OSA Attenuation Rough calibrations

39.1>M>40.9 No

OSA Attenuation Fine

Store Attn Value

Yes

M=40 dBm

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For EDFA gain and noise figure measurements, the apparatus is set up as shown in Figure 14. The EDFA is placed between TLS and OSA, with the laser diode controller pumping the EDF (the pump power value must first be determined). The EDFA software “Output” menu is designed to automate this setup, as shown in Figure 15. Under the “Output” menu, there are two submenus, i.e., “Multiwavelength” and “Multipower.” The “Multiwavelength” submenu obtains such output data as the gain, noise figure, ASE, peak power, and peak wavelength, and the obtained data are compared with the input data taken from the “Multiwavelength −30 dBm” or “Multiwavelength 0 dBm” submenu first. This comparison of input and output data is done to obtain the gain and noise figure values using interpolation techniques.

Fig. 14. The automation system for output gain and noise figure measurements

Fig. 15. The automation system user interface for gain and noise figure measurement


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The “Multipower” submenu obtains output data at a center wavelength of 1500 nm but at different input powers. As with the “Multiwavelength” submenu, all data obtained from the “output” menu “Multipower” submenu are compared with data obtained from the “input” menu “Multipower” submenu to obtain values such as gain, noise figures, etc. All the “output” submenus plot graphs after one experiment cycle is complete, as shown in Figure 16, saving time for data analysis. In addition, the obtained data can also be stored in Word, Powerpoint, Excel, and other formats for further analysis. Figure 17 shows the flowchart for the automation system. The basic software design methodology possesses the required characteristics of being time efficient and the ability to accurately and consistently gather data while simultaneously reducing the uncertainty value, as these characteristics are very important in ensuring that the software can be utilized to gather data. The EDFA automation system is tested based on the aforementioned characteristics. The software is proven to reduce the EDFA testing time by more than 80%, as compared to manual EDFA testing methods.

Fig. 16. S-band EDFA software “Output” menu

Accuracy is also increased, as at times, the experimentalist can make mistakes in acquiring and analyzing the data. By using the automation software, the human error factor can be eliminated, thus providing a higher degree of accuracy. The uncertainty value is also determined to be approximately ±0.012 dB, as compared to manual testing methods, as shown in Figures 18 and 19. However, each new experiment cycle will require the recalibration of equipment. This is due to changing environmental conditions such as temperature and experimental setup alignment, which have a significant effect on the measurements taken by the equipment. In addition, the optical connectors cannot be removed during the experiment cycle, and a recalibration must be performed by the automation software if the optical connectors are removed. For OSA calibration, standard lamps are frequently used to calibrate the OSA, whereas power measurements are made using a standard lamp and a power detector. In addition to increased accuracy and a reduction in the experiment time, the software also reduces the number of personnel needed for each experiment, thus allowing personnel to focus more on the interpretation of the acquired data. In addition, the software is designed to be user friendly and does not require specialist training to operate.

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Fig. 17. Flowchart for the automation system.

Fig. 18. Automate and manual EDFA’s gain versus input signal power

Start

Select Menu

Select Sub Menu

Input Output

Multiwavelengthwith -30dBm Input Power

Multi Power Multiwaveleng

Multi Power

Start Automatic Measurement

Data Display and Save

Stop

Select Sub

Menu

Multiwavelengt

hwith 0dBm

Input Power


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Fig. 19. Automate and manual EDFA’s gain versus input signal wavelength

4.2 Remote fiber test system using Optical Time Domain Reflectometer (OTDR) DWDM has increased fiber traffic capacity and thus it is important to proactively monitor and maintain the fiber networks. Due to huge penalties specified in Service Level Agreements (SLAs) and Quality of Service (QoS) commitments, carriers are keen to take measures in maintaining their fiber network. Optical Time Domain Reflectometer is a well-known optoelectronic instrument used for testing a fiber optic cable assembly in optical network [18]. An OTDR emits a pulsed ray into the fiber under test and acquired reflected ray. The strength of the return pulses is measured and integrated as a function of time, and is plotted as a function of fiber length. The plot can be used to analyse the optical fiber and detect the approximate location of the fault event that occurred. Different pulse widths in OTDR will affect the distance resolution and dynamic range. A longer laser pulse width will improve dynamic range and attenuation measurement resolution at the expense of distance resolution. This is useful for getting the overall characterization of a link, but is weak when trying to locate faults. A short pulse width will improve distance resolution of optical events, but will also reduce measuring range and attenuation measurement resolution [19]. Theoretically, OTDR has good distance measuring accuracy since it is software based and with its crystal clock possesses an inherent accuracy of better than 0.01%. Further calibration process is not needed since the practical cable length measuring accuracy is typically limited to about 1% [20]. Figure 20 shows the block diagram of an OTDR where a laser diode is driven by an electrical pulse generator that produces a train of short optical pulses. The backscattered optical power from the fiber through a directional coupler is detected by photodiode (PD). The detected waveform is amplified by an amplifier and converted to digital signal through the analog to digital converter (ADC) and then processed by the digital signal processing (DSP) unit. The timing of the DSP is synchronized with the source of the optical pulses so that delay in the propagation of each scattered pulse can be precisely calculated. Figure 21 is a typical commercial OTDR trace, showing among other things, the fiber attenuation. This figure shows that different losses will have different shapes in the trace. Hence, the analysis of the trace can be done to estimate the cause of a fault event [21].

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Fig. 20. Block diagram of an OTDR One of the automation programs suggested for Remote fiber Test System is LabVIEW whose configurations will be explained in this section. The front panel or the graphic user interface of the automation program is shown in Figure 22. There are five sections in the program as indicated by arrow A, B, C, D and E. Section A is the communication settings required by LabVIEW to enable the communication link with the OTDR device. The settings in this section are VISA resource name, baud rate, data bits, parity, stop bits, flow control and timeout. The VISA resource name is the COM port which the RS-232C cable is connected to. Timeout sets the timeout value for the Write and Read operations. The baud rate, data bits, parity, stop bits and flow control must be set to the same value as the settings at the OTDR device and RS-232C cable in order for the communication link to be successful. Failure to do these will cause errors in the LabVIEW when running the automation program. Section B is the setting for measurement conditions. Examples of measurement conditions are wavelength of the signal used, distance range, pulse width, attenuation, average time and index group. For the AQ7260 OTDR default setting, there are only two possible wavelengths which are 1310nm and 1550nm. The possible distance range is from 2km to

Fig. 21. Simulation of an OTDR trace.

Laser Diode

Display

ADC

PD

Electrical Pulse

Generator

Digital Signal Processing

OTDR Timing

Amp.

Fiber under Test


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Fig. 22. Front panel of the automation program

640km. The pulse width is from 0ns to 20us as provided by the manufacturer. The possible attenuation settings are from 0dB to 26.25dB according to the AQ7260 OTDR user manual. Finally, the group index with is varying from 1.00000 to 1.99999. Next is the measurement conditions output given in Section C. The blank space will display the output of the “fiber type with wavelength, distance range, pulse width, attenuation and resolution of the measurement”. The byte count1 value is set to more than the possible return count1 to prevent any loss of data. Users can enable or disable this function by clicking the “Save to USB” button. Finally, section E provides the measurement results. The measurement results in term of “event number, distance, splice loss, return loss, cumulative loss, dB/km, event type, and total span ORL” are shown in the blank area after the completed run of the program. For the event number, it will show “END” if the event is at end of the fiber, else it will show “1”, “2” and so on according to the number of the detected event. Some of the measurement results will be neglected if they are not detected by the OTDR measurement, and the only display values are for those detected by the OTDR. The whole program consists of 6 main stacked sequences and 20 sub stacked sequences. Figure 23 depicts the configuration for the distance range setting for the second main stacked sequence or the sub stacked sequence. VISA Write is used to write the data from write buffer to the device. The command for distance range is Rm\r\n, where “m” depends on the distance range chosen by the user. Figure 24 shows the configuration for pulse width programming. In the second sub stacked sequence, the command needed for the pulse width is PWm\r\n. The configuration for the programming of group index is shown in Figure 25. The command needed is IORm\r\n, in this case, the “m” is the range from 1.00000 to 1.99999 depending on the value needed for the user. The “concatenate strings” function will combine the two constants (IOR and \r\n) and the control (value of group index) in the arrangement from top to bottom to form IORm\r\n. Thereafter, the concatenate output string will pass to

C

A B D E

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218

Fig. 23. Programming for distance range

Fig. 24. Programming for pulse width

VISA Write to initialize the related operation. The next step is to initialize the OTDR device to start the measurement from the personal computer. The command needed to initialize the real time measurement is ST2\r\n. This command is sent to the VISA and the output is connected to the next sub stacked sequence. The duration needed for the OTDR measurement to be completed is approximately 15sec, so, a 20 second delay is applied to the next sub stacked sequence as shown in Figure 26 to ensure the measurement process is 100 percent complete. Then, to stop the measurement after 20 seconds, a stop measurement command is needed which is ST0\r\n, as shown in Figure 26. Without this stop command, the measurement will keep going without end.

VISA write

Concatenate strings

2nd Sub stacked sequence

2nd main stacked sequence Sub stacked sequence

Concatenate string

Number to decimal strings

Sequence local


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Fig. 25. Programming for group index

Fig. 26. Programming for stop measurement

For the automation program, the user has the option to choose whether they want to save a copy of the measurement trace and data or not. If the user does not wish to save the measurement trace, the program configuration at LabVIEW is as shown in Figure 27. The case structure is at the false condition (save function disable at the front panel). There is nothing to process or pass through during this sequence, hence, the program will continue to the next main sequence. If the user wishes to save a copy of the measurement results, the program configuration is shown in Figure 28. The case structure is at the true condition (save function enabled at the front panel). The configuration shown in Figure 28 is for the drive setting and file name to be saved for the measurement results. The command used is FDAmp\r\n (refer to Table 1), where “m” in this case is for drive setting and is equal to 5,

6th Sub stacked sequence

VISA write

Concatenate strings

VISA write

30


220

which sets the drive used to save data to USB drive. While, “p” indicates any file name that the user wishes to save. The strings will be combined into a single output by the “concatenate strings” function and act as the input to the VISA Write. A delay is implemented to reduce the traffic to the OTDR to prevent the occurrence of a hang situation.

Fig. 27. Programming for false (save function disable)

Fig. 28. Programming for true (save enable) case, drive setting and file name to save

Figure 29 shows the configuration to set the file type of the measurement trace. Examples of the possible file types are BMP, SOR (Telcordia) and so on. The command used here is FF7\r\n, which will set the file type to be saved in the SOR format. This file format will assist the user when analyzing or modifying the measurement trace by using Yokogawa OTDR Viewer software. Again, a delay is applied to reduce the traffic to the OTDR to prevent hang up. Finally the “data save” command is used to save the data. Figure 30 shows that the command FST\r\n is sent to the input of the VISA Write. Then, the measurement trace or data will be completely saved into the device (pen drive or thumb drive) plugged into the USB port of the OTDR device. A delay of few seconds is needed for the measurement trace or data to be successfully saved into the pendrive or thumbdrive.

Case structure (for false case)

6th main stacked sequence


31


221

Fig. 29. Programming for “file type save”

Fig. 30. Programming for data save.

The end of the program is as shown in Figure 31. This automation program ends with a VISA Close. The VISA Close closes a device session or event object specified by the VISA resource name. Finally, the simple error handling function indicates whether an error has occurred. If an error occurs during the program execution, it will return a description of the error and optionally displays it in a dialog box. In this section, the measurement data obtained from the automation program are presented. These data will be used to compare with the data obtained without using the automation program in the next section. The results obtained from the automation program for 1310nm wavelength single mode fiber at 100ns, 200ns and 500ns pulse width are printed on the screen and shown in Figures 32, 33 and 34. The data shown are the event number, distance, splice loss, return loss, cumulative loss, dB/km, event type and the total return loss. The data are separated by comma and is ignored if there is no such data for the certain event. The event number is in accordance with the sequence of the events.

VISA write Wait (ms)

VISA write

32


222

Fig. 31. Closes of the program

Fig. 32. 1310nm SMF at 100ns pulse width (automation program)



To verify the reliability of the automation program, a comparison between the results obtained from the automation program and the manually measured OTDR device is made. The data of Figures 35, 36 and 37 are obtained manually for 1310nm wavelength SMF at 100ns, 200ns and 500ns pulse width respectively. These results are compare with those obtained from the automation program. By comparing Figures 32, 33 and 34 with Figures 35, 36 and 37, it can be observed that the results are identical without any deviation. Hence,

VISA close

Simple error handler


33


223

it is concluded that the automation program is very reliable and the results obtained from automation program are exactly the same as those obtained from the manually measured data from the OTDR device.

Fig. 35. 1310nm SMF at 100ns pulse width (manually measurement)



34


224

4.3 Automated fiber optic multiphase dynamic fluid differentiation measurement Nowadays the main concern for many industries, especially those related to power production and more importantly the oil and gas industry is the discrimination of the immiscible phases of fluids in a flow system. The phase differentiation method is one of the effective, low-cost and highly accurate methods to discriminate phases in a flow system [22]. This ability to discern between the various phases of a flow system is important as this will ultimately affect the production outcome of the system. Oblique tips is one of the effective ways to achieve phase discrimination in fiber optic sensor. However, a proper automation and analysis software is necessary for the interpretation of the data from the optical probes [23], [24]. The LabVIEW software is a smart analysis program which is able to discriminate the individual phases of a multiphase system using Boolean logic and it presents the data in a real-time manner. Discrimination of the immiscible phases of fluids in a flow system using the phase differentiation measurement method is demonstrated by Kavintheran in 2007 [25]. The software was designed to monitor up to 8 individual sensor probes and it can also be configured to monitor two-phase or even three-phase flows. The automated measurement of the phase differentiation measurement system is demonstrated in Figure 38. The system consists of four optic sensor heads, four laser diodes, four optical circulators, an optical coupler and an OSA. The laser diodes work as signal sources which send signal through the circulator to the optic sensors. The sensor head assembly is designed such that the sensor probes are arranged 90 degrees apart at 0, 90, 270 and 360 respectively. The reflected signal from the optic sensor head goes back to the circulator and is redirected to combine with reflections from the other laser sources through the OSA. The OSA is connected via GPIB hardware to a computer equipped with the LabVIEW software and a phase discrimination program that uses Boolean logic to accurately determine the various phase fractions of the multiphase system in real-time.

Fig. 38. Sensor probe location in sensor head assembly

The system is designed so that the output for each probe that computes a reflectivity between 0.015 and 0.040 generates a digital “1” which indicate water, and a digital “0” for each probe that computes a reflectivity of less than 0.015 to indicate air. The system


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incorporates a Boolean logic function that allows the phase level measurement to be determined accurately and also without any overlay in data. Figure 39 shows the flowchart for the operation of the program, while Figure 40 below shows the Boolean logic circuit. To detect three possible indications of the phase level, the Boolean logic function is designed to accommodate up to eight sensor probes. The three possible results are one-quarter water three-quarters air, three-quarters water one-quarter air and full water. Visual representation of the phase level is the result of the outputs of the logic circuit which is connected to the indicators. Each sensor probe is connected to an OR gate. In order to allow the logic functions to operate with only one input, OR gates are used. The OR gate allows the deployment of either four sensor probes or eight sensor probes. The program will refresh the phase fraction levels every 1 second, which is the scan time of the OSA [25].

Fig. 39. Phase discrimination program flowchart

Fig. 40. Boolean logic function for phase discrimination

Read Probe 1

0.015 < R< 0.040

Indicate=1 Indicate=0

Yes

No

Read Probe 1

0.015 < R< 0.040


Yes No

Read Probe 1

0.015 < R< 0.040


Yes No

Read Probe 1

0.015 < R< 0.040


Yes No

Start

Indicate Phase Level

Boolean Logic Function

36


226

The reflectivity of the probes as a function of time when the water level in the flow system is at one-quarter is shown in Figure 41. As depicted, the reflectivity of probe 3 in water is shown to be 0.032 by the software, while the reflectivity’s of probes 1, 2 and 4 is shown to be 0.002. To determine the phase level of the system, the Boolean logic function is done automatically and immediately. When digital “1” signal is generated by probe 3, it immediately passes through the respective logic gates to determine the phase level. Once this has been done, the system will represent the phase level using a diagram as shown in Figure 42. When the flow system is half-full, the reflectivity of probes 2, 3 and 4 is 0.0316, while the reflectivity of probe 1 remains at 0.002. Figure 43 shows the reflectivity of the four probes against time.

Fig. 41. Reflected power (dBm) of sensor probe vs. time for one-quarter full flow system

Fig. 42. LabVIEW phase level representation for one-quarter full flow system

Similarly, the Boolean logic gate function will determine the phase level of the system as shown in Figure 44. When all four probes detect water, the reflectivity of all four probes is 0.002, as shown in Figure 45. The Boolean logic system will then determine the phase level of the system to be fully water, and will indicate the condition of the system as shown in Figure 46.


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227

Fig. 43. Sensor probe reflectivity of vs. time for half-full flow system

Fig. 44. LabVIEW phase level representation for half-quarter full flow system.

Fig. 45. Sensor probe reflectivity of vs. time for full flow system

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Fig. 46. LabVIEW phase level representation for full flow system

4.4 Polarimetric fiber-optic pressure sensor Due to the possibility of it being used to distribute high sensitivity sensors and of it being configurable in many different shapes, the polarization–based FOS is widely used. Recently different types of polarimetric sensors have been investigated for the measurement of various physical parameters such as: pressure, strain, stress and temperature. Polarimetric fiber optical sensors are sensitive to the variations in the polarization state of light transmitted by a single mode optical fiber. The state of polarization may vary according to physical factors such as stress, pressure, temperature, etc [26]. So far, the polarimetric fiber sensors demonstrated in the literature are based on interferometric schemes. In interferometric schemes high-birefringent (HB) polarization maintaining (PM) fibers are used. The proposed fibers have strong asymmetries so the quasi-degeneracy of the two orthogonally polarized modes can be avoided and thus forcing a single polarization mode under normal operations. A low-cost distributed deformation sensor based on the change of polarization in a standard single-mode fiber of which data is obtained by the help of LabVIEW has been successfully developed by G. C. Contantin [27]. The schematics of the sensor is shown in Figure 47 which consists of a laser source, a mechanical polarization controller, a fiber polarizer and an optical receiver that is connected to a PC via a digital acquisition card (DAQ). The Power source consists of a laser diode and the biasing circuit. The laser source which is used is a telecommunications laser diode mounted inside a butterfly package. To acquire the diode temperature and monitor its temporal evolution, a laser diode is connected to a PC via a digital acquisition card. The light emitted by the laser source feeds the pressure transducer via the mechanical polarization controller that is used to align the incident polarization orthogonal to the transmission axis of the polarizer in the absence of the applied pressure stimulus. In this situation, the receiver reads a “0” voltage in the absence of any perturbations. It is also possible to align the light polarization in order to maximize the receiver reading in the idle state. As shown in Figure 47, the pressure sensor is made of a fiber sandwiched between two plexiglass plates [27]. Under pressure, the polarization state of the single-mode fiber changes and the value of the power transmitted from the fiber polarizer is affected. Therefore, if an optical power of zero is registered in the absence of


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perturbation, once pressure is applied to the transducer an optical power other than zero is obtained at the receiver [27].

Fig. 47. Distributed deformation sensor schematic

Figure 48 shows the data acquisition program which has been developed with LabVIEW software. The main results obtained using the polarimetric fiber-optic pressure sensor is shown in Figure 49. In these figures, the dependency of the sensor output on time is shown. Figure 49(a) shows the output voltage (V) versus time in the absence of deformation. Figure 49 (c) presents the output normalized tension for different values of the pressure. The

Fig. 48. Data acquisition program

Receiver

Mechanical Polarization Controller

In-line Polarizer

Laser Diode

Optical Coiled Fiber

Pressure

Plexiglass

40


230

(a)

(b)

(c)

(d)

(e)

Fig. 49. Figure 49 (a) shows the dependence of the sensor output on time in the absence of deformation, Figures 49 (b), (d) and (e): show the output normalized voltage (dB) at constant pressure. Figure 49 (c): presents the output normalized tension for different values of the pressure. Figure 49(f): show the dependence of the sensor output on weight.


41


231

output normalized voltage (dB) at constant pressure is shown in Figures. 5 (b), (d) and (e). The normalized value is calculated by [27]

(0)

out infnorm

out inf

V VV

V V−

=−

Where Vnorm represents the output normalized voltage, Vout is the value of the acquired voltage, Vinf =− 0.5V is the lower limit of the output voltage and out (0) V is the output voltage in the absence of deformation.

4.5 Fiber Bragg Grating (FBG) sensor interrogation system Recently in the field of architecture and civil, infrastructures such as long-span bridges, high-rise buildings, large dams, nuclear power stations, off shore platforms and others, there is a great demand for high performance sensors for structural health monitoring (SHM) systems to guarantee the efficiency/safety of the structures. Optical Fiber sensors such as the Fiber Bragg Grating (FBG) shows distinguishing advantages: immunity to electromagnetic interference and power fluctuation along the optical path and high precision. An FBG is a type of Bragg reflector constructed in a short part of optical fiber that reflects particular wavelengths of light and transmits all others. A key research area in FBG sensors is FBG fabrication, FBG demodulation and FBG encapsulation. In order to push forward the applications of the FBG sensors, reliability and accuracy of the FBG sensors must be ascertained. To achieve accuracy in the interrogation system, there is a great need in a monitoring program that could process the data and display the data collected for each sensor in graphical forms [28]. The optics, hardware and software architecture of such system was developed in year 2005 by Toh Yue Khing [29]. In this FBG fabrication system, digital filtering, peak detection and the ability to lock onto the reference wavelength are implemented by the LabVIEW program. In this research work, FBG is fabricated by using photosensitized optical fibers where the fiber reflective index is imprinted within the fiber core. The FBG sensor is fiber with different reflective indices which are connected in series to increase the number of sensors in a single fiber optic channel. Under tension or compression, changes in the reflective indices in the fiber will cause a shift in the reflected wavelength. By measuring the change in wavelength of the reflected light, the FBG interrogation system will be able to measure temperature, pressure or strain changes, depending on the types of material embedded in the FBG [29]. Figure 50 shows the interrogation System designed for the FBG fabrication system. As shown, a PCI-7831R FPGA Module, Fiber Fabry Perot Tunable Filter (FFP-TF), 8 photodiodes, 8 Pre-amps and an 8-Channel Fiber Optic Configuration are used in the system. Fiber Fabry Perot is a tunable filter which allows a narrow band of wavelength to pass through the fiber and filters the broad band laser. The Fabry Perot Tunable Filter is very sensitive to voltage change and serious noise disturbance produced by the electronic circuitry can also affect the accuracy of the result. Therefore, a high precision and low noise DAC voltage set is required. The PCI-7831R is a national instrument multifunction board featuring a user-programmable FPGA chip for onboard processing and flexible I/O operation. All analog and digital functionality is configured using the NI LabVIEW graphical block diagrams and the LabVIEW FPGA Module. When the PCI-7831R receives

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232

instructions from LabVIEW to perform a scan, it will instruct the DAC to send out a whole range of voltage from 0 to 10V. At each change of DAC output, the FPGA will read from the 8 ADCs simultaneously and store the raw data onto the onboard memory. Once completed, data in the memory will be digitally filtered using the LabVIEW program. The main software filters the noise from the data that is caused by both optical and electrical disturbance. Since the tunable filter is sensitive to temperature, the characteristic of the filter tends to drift over time. This drift will cause an error in the measurement. Thus, the software will automatically lock onto a reference FBG to obtain an accurate reading. The software front panel consists of two tabs. Figure 51 shows the sensor view tab. As shown in Figure 50, every peak in the FBG sensors is represented by the strain, pressure or temperature gauge. Any shift in wavelength of the FBG sensors will cause the measurement gauge to respond.

Fig. 50. Interrogation System design using FBGs

The scope view tab of the waveform acquired from the interrogation system developed for one of the channels is shown in Figure 52. These tabs consist of two scope displays. The top display shows the filtered data, while the bottom display shows the raw data from the Interrogation System. The first peak is the reference FBG and the rest of the 10 peaks are from 10 different types of FBG sensors. The shifting of peak 9 is caused by a change in pressure applied to the FBG sensor.

Onboard Memory

0-10V

User Configurable FPGA

ADC

DAC

ADC

PD

PD

x8

8 Channels Fiber Optic

Configuration

Tunable filter

x8

FBG Sensor

FBG Sensor

FBG Sensor

PCI-7831R FPGA Module


43


233

Fig. 51. Software front panel, sensor view tab

Fig. 52. Software front panel, scope view tab

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234

5. Conclusion The techniques and capabilities of LabVIEW development applied in fiber optic science are demonstrated. The application of LabVIEW in self-calibration and automatic measurement system, fiber remote test system and optical sensor are explained. The self-calibrating automated EDFA characterization system using the LabVIEW software and GPIB hardware is a more effective solution to testing. The overall experiment time is reduced by more than 80%, whereas data acquisition is more accurate and consistent with a low uncertainty value of ±0.012 dB. The automation program for remote fiber test systems using the optical time domain reflectometer (OTDR) is also presented. Polarimetric fiber-optic pressure sensor and multiphase dynamic fluid differentiation measurement are two examples of fiber optic sensors which are discussed. A low cost multi-channel interrogating system is developed with the NI PCI-7831R Module. This minimizes the electronic hardware configuration, simplifies and reduces the time required in the development process. The use of virtual instrumentation has not only provided a modern and interesting way for users to perform experiments but also reduced the amount of time necessary to make connections. The real-time display of quantities, such as gain, spectrum, temperature, strain power, and so on has made a great improvement in the users’ ability to visualize these quantities and understand their relation to one another.

6. References [1] B.E.A. Saleh and M.C. Teich, "Fundamentals of Photonics", Wiley, 1991 [2] Gred, Keiser "Optical Fiber Communications" Singapore: McGraw-HILL, 2000 [3] J. Chen, W. J. Bock, "A Novel Fiber-Optic Pressure Sensor Operated at 1300-nm

Wavelength" IEEE Transactions on Instrumentation and Measurement 53(1), 10 [4] D. Derickson, "Fiber Optic Test and Measurement. Upper Saddle River," NJ: Prentice-

Hall, 1998. [5] R. A. Sherry and S. M. Lord, "LabVIEW as an Effective Enhancement to an

Optoelectronic Laboratory Experiment", In Process Frontier Conference, pp. 897–900. 1997

[6] M.Z. Zulkifli, S.W. Harun, K. Thambiratnam and H. Ahmad, "Self-Calibrating Automated Characterization System for Depressed Cladding EDFA Applications using LabVIEW Software with GPIB", IEEE Transaction on Instrumentation and Measurement, vol. 57, no. 11, November 2008.

[7] A. W. Domanski, A. Gorecki and M. Swillo, "Dynamic Strain Measurements by Use of the Polarimetric Fiber Optic Sensors", IEEE Journal of Quantum Electronics, 31(8), 816 (1995).

[8] D. A. Krohn, "Fiber Optic Sensors: Fundamental and Applications", Instrument Society of America, Research Triangle Park, North Carolina, 1988.

[9] N. Lagokos, L. Litovitz, P. Macedo, and R. Mohr, "Multimode Optical Fiber Displacement Sensor", Applied Optic, Vol. 20, p. 167, 1981

[10] S. K. Yao and C. K. Asawa, "Fiber Optical Intensity Sensors", IEEE Journal of Selective Areas in Communication, SAC-1(3), 1983.


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[11] Kreso Zmak, Zvonimir Sipus and Petar Basic, "A New Approach to Remote Fiber Testing in Optical Networks", 17th International Conference on Applied Electromagnettics and Communications, Croatia, 2003.

[12] U. Hilbk, M.Burmeister, B.Hoen, T.Hermes, and J.Saniter, " Selective OTDR Measurements at The Central Office of Individual Fiber Links in A PON", in Proc. Optical Fiber Communication Conference, Dallas, TX, Feb 1997, pp. 54-55, Paper TuK3.

[13] F.A Maier, H.Seeger, "Automation of Optical Time Domain Reflectometry Measurements", Hewlett Packard Journal, 46(1), pp 57-62, 1995.

[14] "GPIB Instrument Control Tutorial". National Instruments. August 2009. [15] S. W. Harun, K. Dimyati, K. K. Jayapalan, and H. Ahmad, "An Overview on S-band

Erbium-Doped Fiber Amplifiers," Laser Physics Letter, vol. 4, no. 1, pp. 10–15, 2007. [16] S. W. Harun, F. Abd Rahman, K. Dimyati, and H. Ahmad, "An efficient gain-flattened

C-band erbium-doped fiber amplifier," Laser Physics Letter, vol. 3, no. 11, pp. 536–538, 2006.

[17] N.M. Samsuri, S.W. Harun, and H. Ahmad, "Comparison of Performances between Partial double-Pass and Full Double-Pass Systems in Two-Stage L-Band EDFA," Laser Physics Letter, vol. 1, no. 12, pp. 610–612, 2004.

[18] Ozawa, K; Hishikawa,Y; Watanabe,K and Maki,H, "Remote Fiber Test System with Network Element Database and Geographic Information System", IEEE communication, vol. 2, page 1204-1207, 1999.

[19] R. Ramaswami, K.N.Sivarajan, "Optical Networks: A Practical Perspective" Morgan Kauffman Publishers, Los Altos, CA 1998.

[20] I. Yamashita, "The Latest FTTH Technologies for Full Service Access Networks", Proceedings of the IEEE, November 1996 Korea.

[21] D. N. Harres, B. Company, St.Louis, "Built-In Test for Fiber Optic Networks Enabled by OTDR”, IEEE communication, 2006.

[22] C.P. Lenn, F.J. Kuchuk, J.Rounce and P. Hook, "Horizontal Well Performance Evaluation and Fluid Entry Mechanisms", Process of Social Petrol Engineering. Conferance and Exhibition, New Orleans, LA, SPE preprint 49089,. 1998.

[23] R.T. Tamos and E.J. Fordham, "Oblique-Tip Fiber-Optic Sensors for Multiphase Fluid Discrimination", Journal of Lightwave Technology, Vol. 17, No. 8, August 1999.

[24] R.T. Ramos, A. Holmes, X. Wu and E. Dussan, "A Local Optical Probe using Fluorescence and Reflectance for Measurement of Volume Fractions in Multi-Phase Flows", Meas. Sci. Technol., 12 (2001) 871-876.

[25] Kavintheran, Thambiratnam "Automated Fiber Optic Multiphase Dynamic Fluid Differentiation Measurement using LabVIEW Software with a Boolean Logic Function", ASEAN Virtual Instrumentation Applications Contest Submission, 2007.

[26] A. W. Domanski, A. Gorecki and M. Swillo. "Dynamic strain measurements by use of the polarimetric fiber optic sensors", IEEE Journal of Quantum Electronics QE-31(8), 816 (1995).

[27] G. C. Contantin, G. Perrone, S. Abrate, N. N. Puscaz "Fabrication and Characterization of Low-cost Polarimetric Fiber-Optic Pressure Sensor" Journal of Optoelectronics and Materials Vol. 8, No. 4, August 2006.

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[28] Z. Zhou, T.W. Graver, L. Hsu, J. Ou, "Techniques of Advanced FBG Sensors: Fabrication, Demodulation, Encapsulation and Their Application in the Structural Health Monitoring of Bridges" Pacific Science Review, vol. 5, 2003

[29] Khing, T. Y.; Teck, P. K.; Voon L. K., Ee; L. T. & Wan, L. Y. (2005). Development of a Low Cost Fiber Bragg Grating (FBG) Sensor Interrogation System, ASEAN Virtual Instrumentation Applications Contest Submission.


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15

Fiber Optic Displacement Sensors and Their Applications

S. W. Harun1,2, M. Yasin1,3, H. Z. Yang1 and H. Ahmad1 1Photonic Research Center, University of Malaya, Kuala Lumpur

2Department of Electrical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur

3Department of Physics, Faculty of Science and Technology, Airlangga University, Surabaya

1,2Malaysia 3Indonesia

1. Introduction Optical fiber-based sensor technology offers the possibility of developing a variety of physical sensors for a wide range of physical parameters (Nalwa, 2004). Compared to conventional transducers, optical fiber sensors show very high performances in their response to many physical parameters such as displacement, pressure, temperature and electric field. Recently, high precision fiber displacement sensors have received significant attention for applications ranging from industrial to medical fields that include reverse engineering and micro-assembly (Laurence et al., 1998; Shimamoto & Tanaka, 2001); Spooncer et al., 1992; Murphy et al., 1991). This is attributed to their inherent advantages such as simplicity, small size, mobility, wide frequency capability, extremely low detection limit and non-contact properties. One of the interesting and important methods of displacement measurement is based on interferometer technique (Bergamin et al., 1993). However, this technique is quite complicated although it can provide very good sensitivity. Alternatively, an intensity modulation technique can be used in conjunction with a multimode fiber as the probe. The multimode fiber probes are preferred because of their coupling ability, large core radius and high numerical aperture, which allow the probe to receive a significant amount of the reflected or transmitted light from a target (Yasin et al., 2009; Yasin et al., 2010; Murphy et al., 1994). For future applications, there is a need for better resolution, longer range, better linearity, simple construction and low cost unit.

In this chapter, fiber-optic displacement sensors (FODS) are demonstrated using an intensity modulation technique. This technique is one of the simplest techniques for the displacement measurement, which is based on comparing the transmitted light intensity against that of the launch light to provide information on the displacement between the probe and the target. A silicon photo-diode is used to measure the transmitted and reflected light intensity. The sensor performances are investigated for various laser sources, different probes types and arrangements and different target. The theoretical analysis and the corresponding


48

Fiber Optic Sensors

360

results on various bundled fiber based sensors are also presented in this chapter. The application of the FODSs in liquid refractive index measurement is investigated theoretically and experimentally. In the last part of this chapter, a continue monitoring the liquid level is also demonstrated by using the FODS.

2. FODS with beam-through technique The intensity based sensors can be achieved by either beam-though or reflective techniques. A change in displacement of the through-beam and reflective sensors are manifested as a variation in the transmitted light and reflected light intensity, respectively. This section demonstrates a simple design for an intensity-based displacement sensor using a multimode plastic fiber in conjunction of beam-through technique. The performances of this sensor are investigated for both lateral and axial displacements. In the sensor, light is transmitted through a transmitting fiber to a receiving fiber and the received light is then measured by a silicon detector.

Fig. 1 shows a schematic diagram of the proposed sensor, which consists of two set of fiber, one set is connected to a light source and is termed as the transmitting fiber, and the other set is connected to a silicon detector and is known as the receiving fiber. In the experiment, the transmitting fiber located opposite to the receiving fiber is moved laterally and axially. The light is scattered after travelling out from the transmitting fiber and the receiving fiber collect a portion of the scattered light to transmit into the silicon detector where its intensity is measured. The intensity of the collected light is a function of axial and lateral displacement of the fiber. The light source is a He-Ne laser with a peak wavelength of 633 nm. The light is modulated externally by chopper with a frequency of 200 Hz, which is connected to lock-in amplifier to reduce the dc drift and interference of ambient stray light. For an axial and lateral displacement, a flexible adjusting mechanism

Fig. 1. Schematic diagram for lateral and axial displacement sensing using beam-through technique.

Laser He-Ne 633nm

Chopper

x-y-z stage Fixed end

Silicon detector

Lock-in amplifier

RS-232

reference

Multimode fiber (transmitting) Multimode fiber

(receiving)

Computer (display)


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361

using piezoelectric is required, so the receiving fiber tip is mounted on a translational stage, which provides fine movement of the transmitting fiber surface in the axial and lateral direction. In this experiment, the axial and lateral micro distance is varied and the lock-in amplifier output voltage of the transmitted light is directly recorded by a computer automatically using Delphi software through serial port RS232. The piezoelectric micrometer can provide precise changes of about 25 and 30 nm for every positive and negative pulse, respectively, and in this experiment the displacement measurement is taken in successive steps of 45nm.

To analyze the performance of the proposed FODS, the output voltage from a receiving fiber is related to the axial and lateral displacements of the transmitting fiber. Both fibers should be mounted perpendicular to each other and positioned flush against the surface. The output voltage of the sensor should be highest in this position. The transmitting fiber is then moved away laterally and axially from the receiving fiber tip by still maintaining perpendicularity between them. Fig. 2 shows the output voltage of the lock-in amplifier against the lateral displacement between the two ends of the fibers. In the experiment, the core diameter for both ends is varied. As expected, the voltage is highest at zero displacement from the center and the lateral movements of the transmitting fiber away from the receiving fiber resulted in a reduced output voltage as shown in the figure. The power drop pattern follows the theoretical analysis by Van Etten & Van der Plaats (1991) in which the output transmission function is given by:

(1)

where η, d, and a are coupling efficiency, lateral displacement, and fiber core radius, respectively. η is defined as the ratio of output voltage over the maximum voltage. The sensitivity of the sensor is determined by a slope of a straight-line portion in the curves.

As shown in Fig. 2, the beam-through type of sensor has two symmetrical slopes and the sensitivity is higher at the smaller core diameter. At core diameter of 0.5 mm for both transmitting and receiving fibers, the sensitivity is obtained at around 0.0008mV/μm and the slope shows a good linearity of more than 99% within a range of 420μm. The linear range increases to around 800μm for the both slopes as the core diameter increased to 1.0 mm. The linear range can be further increased to more than 1000μm by using a larger core for the receiving fiber as shown in Fig. 2. However, the voltage is unchanged at a small lateral displacement for this sensor due to the larger receiving core, which covers the whole diverged beam from the transmitting fiber. The highest resolution of 13μm is obtained with core diameter of 0.5 mm for both fibers. In this work, the resolution is defined as the minimum displacement which can be detected by this sensor.

The performance of the sensor with lateral displacement is summarized as shown in Table 1. The output voltage from a receiving fiber is related to axial displacement of end surface of the transmitting fiber. Fig. 3 shows the output voltage of the lock-in amplifier against the axial displacement for the different core diameters. In this experiment, the end surface of transmitting fiber is moved away from the receiving fiber tip by still maintaining perpendicularity between them. As expected, the voltage is highest at zero displacement from the center and the output voltage reduces as the axial displacement increases for all

22 cos 1d d dacra a a

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Fig. 2. The output voltage of the lock-in amplifier against the lateral displacement of the transmitting fiber.

Fiber's core diameter

The left slope The right slope Resolution (μm) Sensitivity

(mV/μm) Linear range

(μm) Sensitivity (mV/μm)

Linear range (μm)

1.0mm /1.0mm 0.0003 855(585-1440) 0.0004 765 (1845-

2610) 33

0.5mm /0.5mm 0.0008 420(120-540) 0.0007 420 (750-

1170) 13

0.5mm /1.0mm 0.0005 1035(450-1485) 0.0005 1125 (3060-

4185) 20

Table 1. Performance of the Lateral Displacement Sensor

core diameters used. The power drop pattern follows the theoretical analysis from (Van Etten & Van der Plaats, 1991) in which the output transmission function is given by:

2

2 )(1)arcsin()(

21 NANANANAa

z

(2)

where η, z, a, and NA are coupling efficiency, axial displacement, core radius, and numerical aperture, respectively.

As shown in Fig. 3, the sensors only have one slope and the sensitivity is higher at the smaller core diameter. At core diameter of 0.5 mm (for both transmitting and receiving fibers), the sensitivity is obtained at around 0.0002 mV/μm, which is the highest and the slope shows a good linearity of more than 99% within a range of 900μm. The linear range increases to 3195 m with the larger core diameter of 1.0 mm. In case of the receiving core is bigger than the transmitting core, the voltage is almost constant at small axial displacements due to the coherent light source, which has a small divergence angle. The highest resolution

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-1000 -500 0 500 1000

Lateral displacement (micrometer)

Out

put v

olta

ge (m

V)

1.0mm0.5mm0.5mm/1.0mm


51


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Fig. 3. The output voltage of the lock-in amplifier against the axial displacement of the transmitting fiber.

is obtained at 50 m with 0.5 mm core diameter for both fibers. The performance of the sensors with axial displacement is summarized as shown in Table 2. The stability of these sensors is observed to be less than 0.01 mV (3%). The experimental results are capable of offering quantitative guidance for the design and implementation of the displacement sensor. This sensor requires two probes, which is precisely aligned and therefore the applications are limited. In the next section, various reflective types of FODSs are proposed to improve the sensitivity, linearity and dynamic range of the sensor.

Fiber's core diameter Sensitivity (mV/μm) Linear range (μm) Resolution (μm) 1.0mm/1.0mm 0.0001 3195 (2340-5535) 100 0.5mm/0.5mm 0.0002 900(0-900) 50 0.5mm/1.0mm 0.0001 1530(0-1530) 100

Table 2. Performance of the Axial Displacement Sensor

3. FODS with reflective configuration The FODS based on intensity modulation and reflective arrangement provides a promising solution for displacement measurement in terms of wide dynamic range, with high potential for ultra-precise non-contact sensing. It also provides flexibility in incorporating the optical sensors permanently into composite structures for monitoring purpose [Wang et al., 1997). In the simplest design of reflective FODS, a probe with a pair of fibers is normally used as the media to transfer/collect the light to/from the target and its theoretical analysis is fully contributed (Faria, 1998). In the design of FODS system the sensor probe is playing a majority role comparison with the selection of laser source and reflector. Hence, the researchers are paid more attentions in the development of sensor probe to improve the

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performance of FODS. The FODSs described in this section are focus on the various configurations of sensor probe since they are mainly influence the performance of FODS.

3.1 FODS using a probe with two receiving fibers

In this section, a new configuration of the FODS is reported by using two receiving fibers which are bundled together. The mathematical analysis of FODS is developed to simulate the theoretical results, which is then compared to the experimental result. The performance of this FODS is also compared experimentally with the conventional FODS. The probe structure of the proposed FODS is shown in Fig. 4. It consists of one transmitting and two receiving plastic multi-mode fibers bundled together in parallel. To analyze the theory of this sensor setup, a more realistic approach – Gaussian beam is used to describe the light leaving the transmitting fiber. The irradiance of emitted light is obeying an exponential law according to

2

2 2

2 2( , ) exp( ) ( )EP rI r zz z

(3)

where PE is the emitted power from the light source, r is the radial coordinate and z is the longitudinal coordinate. is the beam radius which is also a function of z,

. The waist radius ω0 and Rayleigh range zR are the important parameters in

the Gaussian Beam function.

Fig. 4. Side, Front and Overlap Views of FODS probe structure with two receiving fibers

The optical power received by the receiving fiber can be evaluated by integrating the irradiance, I over the surface area of the receiving fiber end, Sr.

( ) ( , )

rrS

P z I r z dS (4)

The overlapping area of the reflected light area and the core of the receiving fibers is also illustrated in Fig 4. The power value of reflected light collected by two receiving fibers increased with the increased of displacement of probe and target mirror. Two receiving fibers collected the reflecting light results the receiving light power increased. Based on this geometrical analysis two receiving fibers collected the reflected light significantly affects the transfer function of the FODS.


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The Eq. (3) can be described in other expressions in order to simulate conveniently. The power collected by the first and second receiving fibers are denoted as P1 and P2, respectively where P1 is closer to the transmitting fiber. From Eq. (3), P1 and P2 can also be written as;

2 2

2 2

2 2

1 2 2

2 2( )( ) exp( ) ( )

r t r r

r t r r

R R R R yE

y R x R R R y

P x yP z dxdyz z

(5)

2 2

2 2

2 2

1 2 2

2 2 ( )( ) e x p( ) ( )

r d t r r

r d t r r

R R R R R yE

y R x R R R R y

P x yP z d x d yz z

(6)

where the Rt, Rr is the radius of transmitting fiber and receiving fiber, respectively. The Rd is the diameter of the receiving fiber. The radial coordinate r is expressed by in Cartesian coordinate system. Then the total amount of power collected by both receiving fibers is;

P = P1+P2 (7)

The conventional FODS only collects power P1 using a pair of fibers bundled together. Compared to this sensor, the proposed FODS collects more reflected light due the additional P2 power, which increases the dynamic range of the sensor.

The software simulation is programmed and implemented in MATLAB. Some important parameters are specified in the programming, wavelength of the laser source λ = 594nm, transmitting fiber core radius Rt = 0.5mm and numerical aperture value NA = 0.25. Fiber diameter Rd = 2mm. The theoretical analysis transfer function of proposed displacement sensor in Eq. (6) can be normalized by its maximum collected power Pmax, Pn = P/Pmax. The normalized distance , while power increase to maximum for small values h from 0 to z, power decrease to zero according to the h values higher than z. The simulation results are then compared with the experimental results.

The experiment setup for the FODS with two receiving fibers is shown in Fig. 5. It consists of a light source, a chopper, a sensor probe, a flat mirror, a silicon detector, a lock-in amplifier and a PC. The sensor probe consists of one transmitting and two receiving plastic multi-mode fibers which are bundled together in parallel. The transmitting and receiving fiber length is 2m with a core diameter of 1mm. A 594nm yellow He-Ne laser is used as a light source. It has the maximum output power of 4mW and beam divergence of 0.92mRads, which is modulated by external chopper with a frequency 200Hz. The transmitting end of fiber probe radiates the modulated light from the laser to the mirror. The flat mirror is controlled by a piezoelectric motor and driver. The distance between the fiber probe and the mirror is varied in successive steps of 4µm and the light voltage which is represented the optical power is measured against the change in the mirror displacement stage. Then the mirror reflects the transmitting light into the receiving end of fiber probe. The reflected light through receiving end can be detected by the silicon detector. The photon energy collected by detector is converted into a voltage. The output of the detector transfers into the lock-in amplifier to deduce the dc drift and filter out the undesired noise. The lock-in amplifier is connected to a PC using RS232 data series line. From the PC, the output light voltage is monitored.

/nh h z

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Fig. 5. Experiment setup of proposed FODS with two receiving fibers.

In both theoretical and experimental analysis, the results are processed and displayed in the normalized forms which the output power is normalized by the maximum collected optical power and the displacement is normalized by the distance parameter z. This is to make the output function a dimensionless function and eliminate the dependency of the FODS output function to the fiber core radius and divergence angle. Fig. 6 shows the comparison of the simulated and the observed response of the proposed FODS. As shown in Fig. 6, both curves exhibit a maximum with a steep linear front slope and back slope which follows an almost inverse square law relationship for the reflected light intensity versus distance of the mirror from transmitting fiber end. The signal is minimal at zero distance because the light cone does not reach the core of both receiving fibers. As the displacement increases, the size of cone of the reflected light at the plane of fiber also increases, which then starts to overlap with the receiving fiber cores leading to a small output voltage. Further increases in the displacement lead to larger overlapping which in turn results in an increase in the output voltage. However, after reaching the maximum value, the output voltage starts decreasing even though the displacement increases. This is due to the large increase in the size of the light cone and the power density decreases with the increase in the size of the cone of light.

Fig. 6. Normalized collected optical power versus the normalized displacement curves for both theoretical and experimental results.

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7

Normalized Distance

Normalized output

Comparison the Transfer Function of Theory and Experiment

Theory

Experiment

He-Ne Laser

Modulator

Lock-In Amplifier

Silicon Detector

PC

Object Mirror

Chopper


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367

The maximum normalized output powers of 1 are obtained at the normalized displacement distances of 1.2 for the theoretical curve and 1.3 for the experimental curve as shown in Fig. 6. The close agreement of theoretical and experimental results in the Fig. 6 is quite evident. The small difference can be accounted for experimental limitations mainly due to the geometry error of the fiber used and positioning error. Table 3 summarizes the performance comparison of the simulated and the experimental result of the sensor. The slope of the response curve is the sensitivity, which is expressed in the unit of mW/μm. As shown in the table, the linearity range at the back slope is nearly 3 times that of the linearity range at the front slope. However, the sensitivity at the front slope is nearly 3~4 times higher than that of the sensitivity at the back slope.

Methods The front slopes The back slopes

Linearity range Sensitivity Linearity range Sensitivity

Theoretical 0.8 µm (0.3 ~ 1.1) 0.06 mV/µm 2.5 µm

(1.4 ~ 3.9) 0.015mV/µ

m

Experimental 0.9 µm (0.2 ~ 1.1) 0.06 mV/µm 2.5 µm

(1.5 ~ 4.0) 0.017mV/µ

m

Table 3. The performance comparison between theoretical and experiment results.

The experiment is also repeated for the conventional sensor with one transmitting and one receiving fibers. The response of the conventional FODS is then compared to that of the proposed FODS as shown in Fig. 7. As shown in the figure, the maximum power collected by the receiver is obtained at a shorter distance for the conventional sensor. The sensor performance comparison between the proposed and the conventional sensors is summarized in Table 4. The sensitivity of both sensors is almost similar. However, the proposed sensor obviously has a better linearity range as shown in Table 4. This is attributed to the amount of the collected light intensity, which is higher in the proposed sensor compared to that of the conventional one.

Fig. 7. The displacement curves for both proposed (1TF, 2RF) and conventional (1TF, 1RF) sensors

0

0.01

0.02

0.03

0.04

0.05

0.06

0 1 2 3 4 5 6

Normalized Distance

Output power, mV

1TF,2RF

1TF,1RF

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Methods The front slopes The back slopes Linearity range Sensitivity Linearity range Sensitivity

Conventional 0.5 µm (0.2 ~ 0.7) 0.06 mV/µm 1.4 µm

(0.9 ~ 2.3) 0.016

mV/µm

Proposed 0.9 µm (0.2 ~ 1.1) 0.06 mV/µm 2.5 µm

(1.5 ~ 4.0) 0.017

mV/µm

Table 4. The performance comparison between the proposed and conventional sensors

The maximum linearity ranges of 2.5 µm and 0.9 µm are obtained at the back and front slopes respectively for the proposed sensor as shown in Table 4. The linearity range of the proposed sensor is improved by about 44% for both slopes compared to the conventional sensor. As indicated by the above results, we can conclude that the employment of two receiving fibers increases the linearity range of the sensor, which is very useful for the large displacement measurements. Both theoretical and experimental results are capable of offering quantitative guidance for the design and implementation of the displacement sensor.

3.2 The FODS with two asymmetrical fibers bundled

As discussed in most of the FODSs [Ko et al., 1995; Elasar et al., 2002; Oiwa & Nishitani, 2004; Cao et al., 2007), the radius of the transmitting and receiving fibers are often made the same for the convenience of analytical study and experiments. However, there is a lack of research work on the displacement sensor using bundled fiber with different core radius. In this work, a mathematical model of displacement sensor using asymmetrical bundled fiber is developed. Some simulations were carried out based on the mathematical model and experimental results were also obtained to validate the MATLAB simulated results. The effect of different core radial ratios (CRRs) to the dynamic range, sensitivity and illumination area of bundled fiber are analyzed and discussed in this section.

Theoretical analysis

The proposed FODS consists of a transmitting and receiving fibers as well as a reflecting mirror. Both fibers are of different core radius and are bundled together in parallels as shown in Fig. 8. Let and denote the core radius of the transmitting fiber and the core radius of the receiving fiber. The core radial ratio, CRR is the ratio of transmitting fiber and receiving fiber core radius, as given below:

CRR,

R

T

rkr

(8)

Fig. 8. Side and front views of the transmitting and receiving fiber ends.

Tr Rr


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369

Fig. 9 shows the geometrically illustration of the overlapping area between the reflected light circle and the core of the receiving fiber at different CRR. Based on this figure, in the same core radius of transmitting fiber; the reflected light power collected by the receiving fiber increases with the increasing core radius of the receiving fiber. The larger receiving fiber core radius and core area, the bigger fraction of reflected light can be collected by the receiving fiber. In the previous report (Faria, 1998), two major approaches have been introduced for theoretical analysis, namely geometrical and Gaussian Beam approaches. For the former approach, the simple assumption is made that the light intensity is constant within the reflected light circle. On the other hand, the light intensity outside the reflected light circle is null. This approach is apparently less accurate compared to the second approach. Gaussian Beam approach is a more realistic and more accurate method. The intensity of the light emitted from the transmitting fiber is described with Gaussian distribution as shown in Eq. (3).

Fig. 9. Geometrical illustration for the overlapping area between the reflected light circle and the core of the receiving fiber at different CRRs.

The light power collected by the receiving fiber can be evaluated by using integral as shown in Eq. 4. However, the exact integration is tedious and impossible. Therefore, assumptions and approximation were used to dissolve the integration. For points situated in the far-field, z >>zR the following relations with the divergence angle can be obtained

0

0

( )tana aR

zz z

(9)

Core radius of the transmitting fiber and receiving fibers are given by the approximation

tanT a a a az z (10) and

R T a ak kz (11) where za is the distance between the beam source to the fiber end (Wang et al., 1997). The core area of the receiving fiber is computed from

2 2 2 2

a R a aS k z (12)

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The radial distance between the two core centers of the transmitting fiber and receiving fiber is determined from

(1 )T R T T a ar k k z (13) The path of the beam from the beam source to the bundle end after the reflection is given by

2az h

(14)

The displacement parameter in the normalized form is presented as

2a

a

z hz

(15)

or

1 2 Nh

(16)

where hN = h/za . To relate the displacement between the reflective mirror to the fiber end, h

to the transfer function, with the help of the results determined above, the collected power of the receiving fiber can be expressed as

2

2 2

2 2( ) exp( 2 ) ( 2 )

Ea

a a

P rP h Sz h z h

2 2 2

2 2

2 2((1 ) )exp( 2 ) ( 2 )

E a a

a a

P k z k zz h z h

(17)

By substituting Eq. (15) into this equation, we obtain

2 2

2 2

2 2(1 )( ) expEP k kP

(18)

The maximum received power is achieved when '( ) 0P , and this leads to

2 (1 )m ax k (19) Based on the relation in Eq. (3-16), the maximum h is given by

max

2 2 12

kh

(20)

The maximum power is given by

2

max 2( 2 (1 )) exp( 1)( 1)

Ek PP P kk

(21)

In the normalized form, Eq. (3-15) is rewritten as


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371

2 2

2 2max

( ) 2( 1) 2(1 )( ) exp 1NP k kPP

(22)

In the analysis, the theoretical model of the FODS is modeled based on the similar parameters used in the experiment: Wavelength of the laser source = 594nm, transmitting fiber core radius Rr =0.5mm and numerical aperture value NA = 0.4. Based on the same parameters, four analytical model were simulated for k = 0.5, 1, 2 and 3 which were based on the available fiber core radius combinations in the experiments.

Experiment

The experiment for the FODS with two asymmetrical bundled fibers is carried using the similar set-up as shown in Fig. 5, but using bundled fiber with different core radius. The asymmetrical bundled fiber is constructed by pairing two different plastic fibers with the core radiuses of either 0.25mm or 0.50mm or 0.75mm. Due to the limited selections of core diameters, six combinations were selected for the experiments: [ , , ]T Rk r r = [0.5, 0.5mm, 0.25mm], [1.0, 0.5mm, 0.5mm], [2.0, 0.25mm, 0.5mm] and [3.0, 0.25mm, 0.75mm]. k is the core radial ratio. A precise displacement reference between the bundle end and the reflecting mirror is imperative for the experiment. Therefore, a New Focus 9061 motorized stages which is driven by a picomotor is used to change the displacement of the reflecting mirror from the fiber probe. Each increment step in the displacement is made identical and accurate. The collected light power in the receiving fiber is converted by a silicon detector into electrical power. Lastly, the electrical signal is filtered by a lock-in amplifier and recorded in the computer.

Characteristic of the sensor

In both theoretical and experimental analysis, the results are processed and displayed in the normalized forms which the output power is normalized by the maximum output power and the displacement is normalized by the parameter za. This is to make the output function a dimensionless function and eliminate the dependency of the FODS output function to the fiber core radius and divergence angle. Figs. 10 and 11 show the analytical and experimental results respectively for the proposed FODS. As shown in both figures, the location of the maximum output is shifted toward the right along the axis of displacement as the value of k increases. Besides, the linear range on the front slope and back slope gets larger for every larger value of k. Both graphs exhibit the almost the same trend of characteristics in the curves as the value of k becomes larger. This phenomenon can be explained by the use of distinctive core radius of the two fibers. As shown in Fig. 9, at the same displacement the fraction of overlap area in receiving fiber core by the reflected light circle (percentage of shaded area in the receiving fiber core) is differ for different CRR. For a larger value of k, the fiber displacement sensor requires further displacement to achieve maximum overlap area. Adversely, the sensitivity of the fiber displacement sensor decreases as the CRR increases. On the other hand, some error in the initial displacement (0< h < 0.3) is observed if the two overlaid graphs are compared. This error is accounted to the approximation used in the theoretical analysis.

The performance of the proposed FODS from the experimental results is summarized in Table 5. The results show that the magnitude of the sensitivity decreases as the CRR or k value becomes larger while the linear range is larger for a larger value of k. The sensitivity

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Fig. 10. The experimental result of proposed FODS model at different CRRs or k values.

Fig. 11. The theoretical result of the fiber displacement sensor at different CRRs or k values.

CRR , k Front Slope Sensitivity

Linear Range (mm)

Back Slope Sensitivity

Linear Range (mm)

0.5 1.7615 0.320 - 0.800 -0.5270 1.312 - 1.176 1 1.0984 0.230 - 0.998 -0.3378 1.536 - 3.302 2 0.9688 0.288 - 1.296 -0.2581 1.728 - 4.320 3 0.6955 0.320 - 1.440 -0.1366 2.240 - 5.400

Table 5. The sensitivity and linear range for different k values

characteristic trend is consistent with the theoretical plot as shown in Fig. 12. Fig. 12 shows the normalized sensitivity against normalized displacement at various k values. The curve width of the graph represents the linear range of the sensor. As shown in the figure, the linear range of the sensor increases with the value of k which is in agreement with the


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373

Fig. 12. Theoretical normalized sensitivity curves of the FODS at various k values.

experimental result in Table 5. This property provides a greater enhancement in FODS applications in terms of flexibility, wider dynamic range and high precision displacement measurement. The maximum sensitivity of 1.76 is obtained at k=0.5. The largest dynamic range of Fig. 10 is obtained at k=3. The conventional FODS which uses two fibers with identical core radius often encounters several restrictions due to limited linear range for the measurement. Besides, the limited option of sensitivity often becomes a challenge in the high-precision measurement. This restriction can be avoided using a suitable CRR or k value. The k value can be chosen in a way to provide the optimum performance.

3.3 The FODSs with two asymmetrical fibers inclined

To date, many works have been reported on the intensity modulation based FODSs (Bock et al., 2001; Cui et al., 2008; Saunders & Scully, 2007; Miclos & Zisu, 2001; Kulchin et al., 2007), which the probe consists of a pair of fibers used for transmitting and receiving the light. For instance, Buchade and Shaligram, et al. (2006) presented a FODS using two fibers inclined with a same angular angle and reported the sensitivity was enhanced compared with the conventional sensor with parallel bundled fibers. It also reported that the performances of the FODS with two fibers are depended mainly on four parameters: the offset, the lateral separation and the angle between the transmitting and receiving fiber tips, and the angle of the reflector (Buchade & Shaligram, 2007). However, there is still a lack of research work on the FODSs with different geometry of the receiving fiber. Hence, a study of this type of FODS is depicted in this section theoretically and experimentally.

Theoretical

Fig. 13 shows the geometry of the inclined displacement sensor, which consists of a transmitting fiber, receiving fiber and a reflector. The sensor performance is studied at various core radiuses of transmitting and receiving fiber. Fig. 13 (a) (Fig. 13(b)) shows the

0 0.5 1 1.5 2 2.5 3 3.5 4-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Normalized Displacement

Nor

mal

ized

Sen

sitiv

ity

k=2.0

k=1.0

k=0.5

k=3.0

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geometry of the sensor in case of the receiving core is bigger (smaller) than the transmitting core. Two asymmetrical transmitting and receiving fibers are mounted at an angle ‘θ’ with reference to the normal to the reflector. This ensures the receiving fiber core to collect the maximum power from cone of emitting light of the transmitting fiber. The shortest distances between the sensor probe tips and reflector are x1 and x2 for transmitting fiber and receiving fiber, respectively. The dash lines in receiving fiber is represented the size of diameter value same as the transmitting fiber. The image of transmitting (receiving) fiber is formed at a further distance x1 (x2) opposite to the transmitting (receiving) fiber beyond the reflector. The image fiber is thus seen located at 2x1 or 2x2 from the original position of the probe. Effectively the reflected light appears to form a cone and reaches the receiving fiber, which is parallel aligned in the cone as shown in Fig. 13.

(a) (b)

(a) r2>r1 (b) r2 <r1

Fig. 13. The structure of sensor probes with two asymmetrical inclined fibers

As shown in Fig. 13, the core radius of the transmitting and receiving fibers are denoted as r1 and r2, respectively. Meanwhile, the diameters of transmitting fiber and receiving fiber are rd1 and rd2, respectively. We assume that the ratio between the radius of two fibers is k1, k1 = r1/r2 and the ratio of diameter of two fibers is k2 = rd1/rd2. From the geometry analysis of Fig. 13, the distance between the two sides of image of transmitting and receiving fibers is given by:

)sin(2)2cos( 11 xrf d (23)


63


375

Then, the distance between two fiber core, D is obtained as;

2)1(

22212 krfrrfD ddd

(24)

The acceptance angle of transmitting fiber α is given by , where NA is a

numerical aperture for the transmitting fiber. The distance between emitting point of transmitted light to the receiving flat core is denoted as z, which is given by;

z = z1+z2+z3 (25)

where z1 =r1×cotα=k1r2×cotα, z2= 2x1×cos(θ) and z3 = rd1×sin(2θ) = k2rd2×sin(2θ) as illustrated in Fig. 13.

To analyze the power collected by the receiving fiber, we simply analysis the light inside the fiber by using a Gaussion beam approach. The irradiance of emitted light is obeying an exponential law according to Eq. (3). The optical power received by the receiving fiber can be evaluated by integrating the irradiance, I over the surface area of the receiving fiber end which is given by Eq. (4). To simulate conveniently, the Eq. (4) can be described in other expressions;

2 22 2

2 22 2

2 2

1 1 2 2 2

2 2( )( , , ) exp( ) ( )

r D r yE

y r x D r y

P x yP k k z dxdyz z

(26)

The P (k1, k2, z) is the power collected by the receiving fiber corresponding the parameters k1 and k2. The radial coordinate r is expressed by 2 2x y in Cartesian coordinate system.

Fig. 14 illustrates the overlap area of the reflected light area and the core of the receiving fiber. The overlap area is zero at x2 = 0 (Fig. 13(a)) or x1 = 0 (Fig. 13(b)) and at a very small displacement (blind area) where the jacket of the two fibers blocks the reflected. As the displacement is increased further, the overlap area increases and thus increases the total power collected by the receiving core. The total power is maxima when the receiving cone covers the entire receiving core area. After that, the received optical power starts to decay exponentially as the displacement continues to increase. The received optical power is strongly dependent on the core size of the receiving fiber. At inclination angle of 2θ between the transmitting and receiving fibers, the distance x1 between the sensor probe tip and reflector is given by (Buchade & Shaligram, 2006)

x1=rd1(cosec-2sin)/2 (27)

From the geometrical analysis of Fig. 13, the distance x2 is obtained as; x2 = x1- rd3sinθ for rd1< rd2 (Fig. 13(a)) or x2 = x1+ rd3sinθ for rd1≥ rd2 (Fig. 13(b)) where rd3 = rd2 - rd1. Therefore, the distance between sensor probe tip and reflector mirror can be summarized as;

22 2 2cos 2sin 1 2

2drx k ec k

(rd1> rd2)

1sin ( )NAn

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2 2 cos 2sin2

dk r ec

(rd1= rd2)

2

2 cos 2sin2dr k ec

(rd1< rd2)

(28)

(a) (b) (c)

(a) rd1< rd2; (b) rd1= rd2; (c) rd1> rd2

Fig. 14. Overlap area view

The proposed sensor is simulated by using a MATLAB programming. To simplify the analysis, the k1 values of 0.5, 0.667, 1, 1.5 and 2 are used. The k2 value is set based on the availability of the fiber in our laboratory. In this simulation, the k2 values of 0.5, 1 and 2 are used. The wavelength of the laser source λ is set at 594nm. The numerical aperture values NA1 = 0.27, NA2 = 0.32 and NA3 = 0.4 are used for the core radius of 0.25mm, 0.5mm and 0.75mm, respectively.

Experiment

To verify the simulated results the FODS is constructed by mounting the transmitting and receiving fibers on the plastic board at angle with reference to the normal of the reflector as shown in Fig. 15. Separate samples with various fiber diameters and core radius are prepared for angle = 10o, 20o and 30o. Light from 594 nm He-Ne laser is modulated by an external chopper at frequency of 200 Hz and launched into the transmitting fiber. The light has an average output power of 3.0 mW, beam diameter of 0.75mm and beam divergence of 0.92 mRads. The length of transmitting and receiving fiber length is approximately 2m. The transmitting fiber radiates the modulated light from the light source to the target mirror, the displacement of sensor probe tip between mirror is controlled by a piezoelectric & driver. The reflective light from target mirror, which is mounted in the bottom of tank, is collected by the receiving fiber whose carriers the light into the silicon detector. A lock-in amplifier is connected with the detector to deduce the dc drift. The initial experiment is carried out by varying the inclination angle between the fibers.

Results and discussion

Fig. 16 compares the experimental and theoretical plots of the normalized output collected against normalized displacement between probe and reflector with air medium in between. In this study, the ratios k1, k2, and angle θ are set at 0.667, 0.5, 10° respectively. As shown in the figure, the theoretical curve is in good agreement with the experimental curve, verifying


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Fig. 15. Experiment setup of proposed FODS with two inclined asymmetrical fibers (Yang, et al., 2009).

Fig. 16. Comparison between theoretical and experimental curves of the FODS with air medium in between the gap

the feasibility of our theoretical model. It is also observed that up to 0.3 (0.5) of separation for experimental (theoretical), light in transmitting fiber would be reflected back into itself and little or no light would be transferred to receiving fiber. This is then referred to as the blind region. As the distance increases, the reflected cone overlaps the receiving fiber core and hence the output intensity increases. This relation is continued until the entire face of receiving fiber is illuminated with the reflected light. This point is called optical peak and corresponds to maximum voltage. As the gap increases beyond this transition region, the intensity drops off following roughly an inverse-square law. The small discrepancy between the theoretical and experimental results is due to the noise sources such as shot noise and thermal noise, which are added to the value of the experimental results and are not calculated in theoretical analysis.

Chopper

He-Ne Laser

Modulator

Lock-In Amplifier

Silicon Detector

PCC RS-232

Piezoelectric

Mirror/object

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The experiments are also carried out to study the effect of k1 and k2 values as well as angle θ on the performance of FODS. Fig. 17 shows the normalized output power against displacement for the FODS at various k1, k2 and angle. Figs. 17 (a), (b) and (c) show the curves at 10°，20°and 30° respectively with an air gap in between the displacement. By comparing the curves in Fig. 17, we understand that the performance of FODS is strongly depended on the fiber core size. The output power collected by receiving fiber is highest when the k1 and k2 values are set at 0.667 and 1, respectively. The inclination angle θ of two asymmetrical fiber core is also effected the sensor performance with the bigger inclined angle has a higher output sensitivity with a lower linearity range. Compared to the FODS with zero inclination angle, the sensitivity of the proposed sensor increased by 3.6, 8.5 and 16 times with the inclination angles of 10°, 20° and 30°, respectively. However, the corresponding linear range reduced by 67%, 55% and 33%, respectively. The performances of the proposed FODS are summarized as shown in Table 3.8. By using the k1 and k2 values of (0.667, 1), the maximum sensitivities of 0.2752 mV/mm, 0.3759 mV/mm and 0.7286 mV/mm are obtained at inclination angles of 10°, 20° and 30°, respectively. This sensitivity is higher compared to the previous work by Buchade and Shaligram (2006). The maximum linear ranges of 10.4mm, 7mm and 3mm are obtained at inclination angles of 10°, 20° and 30°, respectively for the FODS with k1 and k2 values of (0.667, 1).

Methods Front slopes

Sensitivity (mV/mm) Linearity Range (mm)

(k1, k2) 10° 20° 30° 10° 20° 30°

(0.5, 0.5) 0.1345 0.1838 0.4761 1.5-3.5 0.4-2.1 0-0.7

(0.667,1 ) 0.2752 0.3759 0.7286 1 – 5.2 0.1-2.8 0 - 1

(1, 1) 0.1671 0.2224 0.5528 1.2-4.8 0.2-2.8 0 - 1

(1.5, 1) 0.1885 0.2745 0.6371 1.2-4.8 0.1-2.9 0 - 1

(2, 2) 0.0645 0.1201 0.1904 1.4-3.5 0.2- 2 0-0.8

Back slopes

Sensitivity (mV/mm) Linearity Range (mm)

(k1, k2) 10° 20° 30° 10° 20° 30°

(0.5, 0.5) 0.0223 0.0479 0.1447 4.3-11.5 2.9-8.9 1.5-3.7

(0.667,1 ) 0.0675 0.1336 0.3224 6.6-17 3.2-10.2 1.3-4.3

(1, 1) 0.0472 0.0823 0.2296 5.6-14.8 3 - 9 1.2-4.3

(1.5, 1) 0.056 0.1155 0.2929 6.8-16 3.2-9.2 1.2-4.3

(2, 2) 0.0128 0.0389 0.0885 4.5-11.5 2.4-7.5 1 - 3

Table 6. The performances comparison of FODS with two asymmetrical inclined fibers


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(a) Inclination angle 10°

(b) Inclination angle 20°

(c) Inclination angle 30°

Fig. 17. The performance output of k1, k2: (0.667, 1), (1, 1), (1.5, 1), (0.5, 0.5) and (2,2) in inclined angle (a) 10° (b) 20° (c) 30°

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4. Applications of the FODS Beside displacement measurement, the FODS can also be used to sense many other parameters such as temperature, pressure, refractive index, strain, mass and etc (Bechwith, 2000; Chang et al., 2008; Arellano-Sotelo, 2008). In this section, two different applications of FODS will be described.

4.1 Liquid refractive index detection using a FODS

In this study, the sensor probe is containing two pieces of fiber optics, one set connected to a light source and termed the transmitting fiber, and the other set connected to a photo detector (photodiode) and known as the receiving fiber. These two groups of fibers are bundled into a common probe to be used in a FODS. The FODS has a capability to measure physical quantities such as the displacement, vibration, strain, pressure, etc (Yasin et al., 2008; Yasin et al., 2009; Rastogi, 1997). However, the use of FODS sensors for detection of environmental refractive index change has not been fully explored. Refractive index sensing is important for biological and chemical applications since a number of substances can be detected through measurements of the refractive index. In the development of a liquid refractive index sensor (LRIS) (Suhadolnik et al., 1995; Chaudari and Shaligram, 2002; Yang et al., 2009; Nath et al., 2008; Kleizal & Verkelis, 2007), an intensity modulation in conjunction with multimode plastic fiber is the most suitable technique because of its non-contact sensing and many advantage properties are inherited by the multimode plastic fiber such as efficient signal coupling and being able to receive the maximum reflected light from target.

A FODS based refractive index measurement using a bundle fiber is first introduced by Suhadolnik et al. in 1995. Later on Chaudhari & Shaligram reported on study of LRIS at various types of optical sources. In our earlier work, a FODS was proposed based on two asymmetrical fibers for liquid refractive index measurement (Yang et al., 2009). In this section, a new LRIS is studied and demonstrated by using pair type of fibers bundled at various inclination angles.

The structure of proposed LRIS is shown in Fig. 18, which consists of a pair of transmitting and receiving fiber. We assume that the transmitting and receiving fibers have inclination angles of θ1 and θ2, respectively against the y-axis. The image of transmitting fiber is located opposite of the mirror with same distance. The central of the receiving fiber and the image of transmitting fiber are pointed as O' and O, respectively in Fig. 18. From the geometrical analysis of Fig. 18, the angle )/(sin 1 nNA and 1

tanarz

. Therefore, the following

distances are given by;

2 2 1 11 1sin tan ( )

2d

a da

rAB z rz

,

1 1 2 2' 4 sin 2 sind dO C r x AB r ,

2 2 1 11 1cos tan ( )

2d

a da

rOA z rz


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Fig. 18. Structure of sensor probe for the sensing of liquid refractive index.

2 2cosdOC OA r

2 2' 'OO O C OC where the NA is numerical aperture of transmitting fiber, n is refractive index of liquid, r1 and r2 are the core radius of transmitting fiber and receiving fiber while the rd1and rd2 are the radius of transmitting fiber and receiving fiber, respectively and the x is the displacement between the sensor probe tip and reflector mirror.

Also from the geometrical analysis, the acceptance angle β of the light cone from the virtual point source O, is given by

1

1'( ) tan ( )

2O CzOC

(29)

The intensity of the light emitted from the transmitting fiber can be well described with Gaussian distribution (Chang, et al., 2008) and is given by;

2

2 22 2

( , ) exp( ) ( )EP r

I r zz z

(30)

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382

where r is the radial coordinate, z is the longitudinal coordinate. ( )z is the beam radius

and expressed as a function of z, 2

( ) 10z

zz R

. The waist radius0 and Rayleigh

range Rz are the important parameters in the Gaussian Beam function and the detailed

description can be found (Chang et al., 2008). Eq. (30) shows that the light intensity decays exponentially as it goes radially away from the center of the light circle. The radial coordinate r of Eq. (30) can be determined by;

'sinr OO (31)

The longitudinal coordinate is the distance between the sensor probe tip and the virtual laser source point O and it can be determined

' cosz OO (32) For points situated in the far-field Rz z , the beam radius of the virtual point source can be derived as

( )z z (33)

By the approximation,

1 tana ar z z (34) Based on the properties above, the power harnessed by the receiving fiber, P can be evaluated by integrating the Gaussian distribution function of Eq. (30) over the area of the of receiving fiber end surface, Sr

( , ) ( , )

r

rS

P r z I r z dS

(35)

where the core area of the receiving fiber is

2 2 2

1r aS r z (36) By combining and substituting Eqs. (31), (32), (33) and (36) into the Eq. (35), finally the proposed LRIS response can be summarized as;

22 2

( ) 2 2 2 2 2,22 2 2exp exp

( ) ( )a EE

rz rz PP r rP S

z z z z

(37)

This equation shows that the liquid refractive index response of sensor is a function of displacement x and refractive index n of surrounding medium while sensor probe is design of inclination angles of θ1 and θ2. Therefore, based on Eq. (37), the proposed LRIS can be used to detect the liquid refractive index where the sensor probe is immersed by the liquid to be measured.

The mathematical model of proposed LRIS is simulated by MATLAB programming. In the simulation, the wavelength of the laser source and numerical aperture NA is set at 594nm


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383

and 0.32, respectively. The fiber core radius r1 and r2 are set at 0.25mm and 0.5mm while the fiber diameters rd1 and rd2 are set at 0.5mm and 0.75mm, respectively. Fig. 19 shows the experimental set-up, which consists of a 594nm yellow He-Ne laser as a light source and a bundled fiber as a probe. The emitted laser light has an average output power of 3.0mW, beam diameter of 0.75mm and beam divergence of 0.92mRads. The external chopper modulates the light at a frequency of 200 Hz before it is launched into the transmitting fiber. The transmitting fiber transfers the modulated light from laser source to reflector mirror while the receiving fiber collects the reflection light to the detector. The sensor probe is mounted on the stage controlled by NewFocus Picomotor for the displacement measurement. Silicon detector measures the received light and converts it into electrical signal which is then denoised using the lock-in amplifier. During the measurement, the room temperature is keeping in 28 ̊ C to avoid the change of liquid refractive index.

Fig. 19. Experimental set-up of the proposed liquid refractive index sensor

The simulation results are illustrated in Fig. 20. This figure shows the relationship between the sensor responses and probe inclination angles θ1 and θ2 in the measurement of three different refractive indices 1, 1.3 and 1.6 based on the probe inclined with the same angles of 0, 10 and 20̊. In Fig. 5.8, the outputs powers are normalized in 1 and the displacements are simulated in mm. Three different group curves are showing three various sensors response based on the three various inclination angles. As seen in Fig. 20, it was found that the inclination angles θ1 and θ2 are reasonably affects the displacement curves profile and output power. The highest output power is almost 10 times of the lowest output power. The vertical dash lines are located in the displacements of 1.1 mm, 2.0 mm and 3.4 mm corresponding to the sensor probe inclination angles 20, 10 and 0, respectively. In those positions, the sensor responses have the biggest output differentiation in the increase of refractive indices from 1.0 to 1.5. At those positions, the sensor output intensity increases almost linearly with the increase of the refractive index of the medium. From Fig. 20, we can observe that the increase the inclination angles improves the performance of the LRIS. The larger the inclination angles of θ1 and θ2, the better the performance of liquid refractive index response.

Chopper

Piezoelectric

Flat mirror

He-Ne Laser

Modulator

Lock-In Amplifier

Silicon Detector

PCC

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384

Fig. 20. Simulation results for the displacement at various inclination angles and refractive indices. Inset shows the maximum normalized output against the refractive index for inclination angles of 0, 10 and 20.

Inset of Fig. 20 shows the maximum normalized output of the sensor as a function of refractive index for various inclination angles. The normalized outputs were taken at the sensor probe positions of 1.1 mm, 2.0 mm and 3.4 mm for inclination angles of 20, 10 and 0, respectively, which is indicated by vertical dash lines in Fig. 20. It was found that the sensitivities of the sensor increases with the increment of probe inclination angle. As shown in the inset of Fig. 20, the highest sensitivity of 0.8235 is achieved by the use of probe with inclination angle of 20 which is almost 13 times higher than that in zero inclination. Fig. 21 shows the simulation curves of the LRIS at various inclination angles for the receiving and transmitting fibers when the refractive index of liquid is set at 1.3. It is clearly seen that the inclination angle of receiving fiber θ1 has the stronger affect in the sensor output compared with the angle θ2. As shown in Fig. 21, the highest output power is achieved by the inclination angles; θ1 = 20 and θ2 = 10. The lowest output power is observed when the inclination angles of θ1 and θ2 are set at 0 and 10, respectively. These results show that the sensor sensitivity can be increased by increasing the inclination angle especially for θ1. However, increasing the inclination is very difficult to be implemented in the experiment unless we can control the position of both fibers very precisely.

In our experiment, three different liquids: isopropyl alcohol, water and methanol are used as the surrounding medium at two conditions; zero inclination for both fibers and the same inclination angles of 10 for both fibers. The refractive index values for isopropyl alcohol, water and methanol are 1.377, 1.333 and 1.329, respectively. The sensor performance with air surrounding medium is also carried out for comparison purpose. During the experiment operation, the sensor probe is mounted onto the stage and the tank is fixed in the experiment table. The liquid in the tank is changed without moving the tank to ensure the accuracy of the measurement. The room temperature was kept at 28° to ensure that the refractive index of the liquid is maintained and only displacement parameter is changed in the experiment.


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Fig. 21. The output power against displacement for various probe inclination angle combinations when the refractive index is set at 1.3.

Fig. 22 shows the displacement curve at various surrounding media when the inclination angles are set at 0° for both transmitting and receiving fibers. As shown in this figure, the normalized peak output power increases from 0.83 to 1.00 as the refractive index increases from 1.329 (methanol) to 1.377 (alcohol). It is also found that the displacement position for the peak output increases from 4.0 to 5.1 mm as the refractive index increases from 1.329 to 1.377. This is attributed the acceptance cone angle that increases with the refractive index increase. The larger acceptance angle provides a mean to collect more signal power. Fig. 23 shows the displacement curves when the inclination angles for both fibers are increased to 10°. As seen in Fig. 23, the peak power and its position increase with refractive index. The peak location of the curve also increases from 3.0 mm to 3.4 mm as the refractive index changes from 1.329 to 1.377. From these experimental results, it was found that the sensitivity of the sensor with 10° inclination of probe arrangement shows a higher sensitivity compared to that of the use of straight probe. The sensitivities are obtained at 0.11 mm-1 and 0.04 mm-1 for the sensors with 10° and 0° inclination angles respectively. This finding may be quite useful for chemical, pharmaceutical, biomedical and process control sensing applications.

4.2 The monitoring of liquid level using a FODS

Liquid level measurement is vital in many industry areas, finding the applications in such as fuel storage system, chemical processing, etc. Fiber based liquid level sensors are received in prefer of studies since they inherit many merits such as they are non-conductive, anti-erosion, and immune to electromagnetic interference. In past few years, several technical are employed to develop the fiber optic Liquid Level Sensors (LLSs). Fiber Bragg grating technology was carried out to sense the changing of liquid level by (Yun et al., 2007; Sohn & Shim, 2009; Dong & Zhao, 2010). Bending the multimode fiber for multi-points liquid level monitoring can be found in (Lomer et al., 2007). In these cases, the sensing elements are submerged inside the liquid to indicate the presence of liquid. These immersed sensing

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Fig. 22. Experiment results of the displacement curve at various liquid materials when the probe inclination angles are set at 0.

Fig. 23. Experiment results of the displacement curve at various liquid materials when the probe inclination angles for both transmitting and receiving fibers are set at 10

elements have some limitations in their applications, such as in harsh environments, acid solutions, etc. Thereby, the fiber as sensing element for non-contact the measurement liquid seems good way to explore. There is one proposed sensor (Sohn and Shim, 2009) which employed one fiber bragg grating embedded inside the cantilever beam to connect with a float for indicating the level changing. This case is limited in its cost, surrounded temperature, and sensitivities.

In this section, a simply and cost efficiency intensity based LLS is proposed. A straightforward displacement sensor is employed; its sensing probe is engaged with a float which is contact with the measurement liquid to conduct the information of liquid. This simply setup transfers the sensing of liquid level into the measurement of displacement. It breaks down the limitations of sensitivities and measurement range which can be conquest


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by the selection of displacement sensors. Thus, in this study two type of displacement sensor are used to supply a flexible and compatible sensing of liquid level.

The basic sensing principle is the sensor probe displacement moving that causes from the moving of float during the liquid level change. A schematic setup of proposed fiber optic LLS is illustrated in Fig.24. It composed a He-Ne laser source light wavelength in 594nm (average output power 3.0mW, beam diameter 0.75mm and beam divergence 0.92mRads) which coupling the light into the transmitting fiber (2m length); a external chopper modulates the light at a frequency of 200Hz before launched into the transmitting fiber; a concave mirror (or flat mirror) is located at end of transmitting fiber and reflects the guided light of transmitting fiber into a receiving fiber which is bundled with the transmitting fiber. The receiving fiber guides the light into the photo detector which converts the light power into voltage. Meantime, the bundled fiber as sensor probe is fastened by an L cantilever beam which connects a float on the other side. The float is contact with the measurement liquid to indicate the information of liquid levels. A model SR-510 lock-in amplifier is connected with the chopper and photo detector. It plays the function of experiment data-acquisition system, matches the phase between the modulation light and modulator chopper and removes the noise generated by laser source, photo detector and amplifier.

Fig. 24. Experiment setup of monitoring the liquid level

In the using of concave mirror as the reflector, the transfer function of proposed LLS is governed by parameters, namely focal length f and the diameter of the circular concave mirror D. The characteristic of the proposed sensor can be found the detailed theoretical analysis in paper (Yang et al., 2010). In consideration of the limited reflecting surface area and FL of the concave mirror, the transfer function of the proposed sensor can be given as

22

2 2 2

2 22 2

2 1 exp2( ) 8( ) exp

( ) ( )1 1

a Ea a

a a a a

a a

Dz Pu z zP u

u z f u z u z f u zu uu f z f u f z f

(38)

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388

where PE is the emitting power of laser source, u is the distance between the sensor probe tip and concave mirror, θ = sin-1(NA) and NA is the numerical aperture of the transmitting fiber. By using the flat mirror as the reflector, the transfer function of proposed sensor was analyzed in (Lim et al., 2009) detailed. It is given by

2 2

2 22( 1) 2(1 )( ) exp 1k kP z

(39)

The core radial ratio k is the ratio of the transmitting fiber core radius to the receiving fiber core radius, is the normalized displacement.

From the transfer functions of Eq. (38) and (39), it can be seen that the sensor response is only influenced by the focal length f and the diameter of the circular concave mirror D in the concave mirror as reflector and by the core radial ratio k for that of flat mirror. As such, the proposed LLS has the widely compatible for the variants type of FODSs, which can escapes the trade-off design in the selection of sensitivity and measurement range. Furthermore, according to the Eqs. (38) and (39), there are no more parameters can affect the sensor response, hence, the proposed LLS is independency for surrounded environments.

Fig. 25 depicts the experiment result when the liquid level is moving upward. In this experiment, the proposed LLS is composed two plastic fiber bundled parallel as sensor probe, a concave mirror with focal length 10mm and diameter 24mm as reflector. From Fig. 25, it can be seen that the sensor output intensity is modulated by the moving of liquid level in six variants slops. According to this, hence, the proposed sensor can achieve the continue monitoring of liquid level in the multi-points. Totally six monitoring points are shown in Fig. 25, each point is represented each slop which can be used as the each level of liquid measurement. These six monitoring points are located in the total measurement range of 25mm, which can provide variants sensitivities and measurement range. The performances of these monitoring points are summarized in table 7.

Fig. 25. The experiment result of proposed sensor for concave mirror as reflector and two plastic fibers bundled parallel


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The second experimental is carried out by the sensor probe consisted with two bundled parallel and a flat mirror as reflector. On the other hand, another experimental is repeated by two asymmetrical plastic fiber bundled parallel as sensor probe and a flat mirror as reflector. Both of responses are illustrated in Fig. 26, where each of the curves are displayed two various slops in the measurement range of 3mm and 3.5mm, respectively. From the observation of Fig. 26, two various monitoring points are given from both curves as shown. They are occupied the measurement ranges of 3mm and 3.5mm, respectively. In Fig. 26, the higher sensitivity is obtained from the curve of k=1 and the bigger measurement range is supplied by the curve of k=0.5. The output curves in Fig. 26 are shifted to the right along the further of liquid level as the value of k decreases. Therefore, the sensitivities of front slope and back slope of the curves are decreasing with decreasing the values of k. Mean times, the linearity ranges of curves are enlarged with decreasing the values of k. The sensitivities and linearity ranges of Fig. 26 are summarized in Table 7.

Fig. 26. The experiment results of various ratios of fibers bundled as sensor probe.

Table 7 tabulates the performances of LLS for three different experiments. From the results summarized in table 7, it can be seen that the highest sensitivity is 1.4533mV/mm when its measurement range is 0.84mm. The largest measurement range 4.8mm is obtained from the monitoring point 3 for the concave mirror as reflector. In the experiment of concave mirror as reflector, the monitoring points 1, 4, and 5 have the higher sensitivities and shorter measurement ranges than that of 2, 3, and 6. In the experiment of flat mirror as reflector, the sensitivities for k=0.5 are decreased by 52.5% and 14.8% for of front slope and back slope, respectively, if compared to k=1. The measurement ranges of front slope and back slope are increased by 46.2% and 18.8% if compared with the k=1. From the analysis, by using a particular combination of fiber core diameters and reflector, two different k values, ten different sensitivity and linear range selections can be achieved by using the proposed method. Thus, the proposed fiber optic LLS has wide compatibility for other type of displacement sensor to avoid the trade-off selection of sensitivity and measurement range which is always encountered in the design of conventional fiber optic LLS.

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Methods Monitoring point Sensitivity (mV/mm)

Measurement range (mm)

Concave mirror k=1

Point 1 1.4026 0-0.8 Point 2 0.2649 0.8-3.8 Point 3 0.1625 14-18.8 Point 4 1.030 18.9-20 Point 5 1.3015 20.1-20.9 Point 6 0.1821 21-25

Flat mirror k=0.5 Point 1 0.9529 0.1-1.4 Point 2 0.3443 1.4-3.5

Flat mirror k=1 Point 1 1.4533 0-0.84 Point 2 0.3951 0.84-3

Table 7. The Sensitivities and measurement ranges of proposed sensor

In the literature of (Sohn and Shim, 2009), the wavelength-shift sensitivity is around 0.15nm/cm and the resolution of intensity modulated the liquid level is approximated 0.1dB/cm are achieved. However, these sensitivities cannot satisfy the high precision measurements and the LLS proposed in (Sohn and Shim, 2009) is high costly. In this paper, the highest sensitivity is achieved by using a probe with two fibers bundled and flat mirror as reflector, it is around 100 times higher than that of (Sohn and Shim, 2009) and is 20 times lower in the cost.

5. Conclusion In this chapter, the performance of various FODSs is investigated theoretically and experimentally. A beam-through FODS has been demonstrated for sensing of lateral and axial displacements, which shows that the usage of smaller core normally shows a better sensitivity and resolution with the expense of the smaller linear range. Various reflective FODSs are then proposed to provide a possibility for the development of the cheap, simple, sensitive and wide dynamic range sensor systems. Compared with the conventional sensor with only one receiving fiber, FODS with two receiving fibers is observed to have a better linearity range. The performance of FODS with asymmetrical bundled fiber is also theoretically and experimentally demonstrated in this chapter. The effect of different core radial ratios (CRRs) on the performance of the sensor is investigated in terms of dynamic range and sensitivity. The experimental results are almost in agreement with the theoretical results. The location of the maximum output is shifted toward the right along the axis of displacement as the value of k increases. Besides, the linear range for both front and back slopes increases with the value of k. To improve the sensitivity, a FODS with two inclined asymmetrical fiber has been proposed. This FODS has been applied in liquid refractive index sensor. An extra-low cost, ultra-high sensitivity, and wide compatibility liquid level fiber optic sensor has been demonstrated as another application for the FODS. A float is touched with the measurement liquid to translate the information of liquid change to the sensor probe displacement moving where an L cantilever beam is fixed in between, the moving of sensor probe finally results the output intensity is modulated. All the theoretical and experimental results are capable of offering quantitative guidance for the design and implementation of a practical FODS.


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Lim, K.S.; Harun, S.W.; Yang, H.Z.; Dimyati, K.; Ahmad, H. (2009). Analytical and experimental studies on asymmetric bundle fiber displacement sensors. J. of Modern Optics, Vol. 56, pp.1838-1842.

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Lomer, M.; Arrue, J.; Jauregui, C.; Aiestaran, P.; Zubia, J.; Lopez-Higuera, J.M. (2007). Lateral polishing of bends in plastic optical fibers applied to a multipoint liquid level measurement sensor. Sensors and Actuators A, 137, 68-73.

Miclos, S. & Zisu, T. (2001). Chalcogenide fibre displacement sensor. J. Optoelectron Adv. Matter., Vol. 3, pp. 373-376.

Murphy, A.M.; Gunther, M.F.; Vengsarkar, A.M.; Claus, O.R. (1991). Quadrature phase-shifted extrinsic Fabry-Perot optical fiber sensor. Opt. Lett ., 16, 273.

Murphy, M.M. & Jones, G.R. (1994). A variable range extrinsic optical fiber displacement sensor. Pure Appl. Opt., Vol. 3, pp. 361–369.

Nalwa, S. (2004). Polymer optical fibers. California: American Scientific Publishers. Nath, P.; Singh, H.K.; Datta, P.; Sarma, K.C. (2008). All-fiber optic sensor for measurement of

liquid refractive index. Sensor and Actuators A, Vol.148, pp.16- 18. Oiwa T. & Nishitani, H. (2004). Three dimensional touch probe using three fiber optic

displacement sensor. Meas. Sci. Technol., Vol. 15, pp. 84–90. Rastogi, P.K. (1997). Optical Measurement Techniques and Applications. Artech House, Inc.

Boston, London. Saunders, C. & Scully. (2007). Sensing applications for POF and hybrid fibres using a photon

counting OTDR. Meas. Sci. Technol., Vol. 18, pp. 615-622. Shimamoto, A. & Tanaka, K. (2001). Geometrical analysis of an optical fiber bundle

displacement sensor. Applied Optics, Vol. 35, Issue 34, pp. 6767-6774. Sohn, K.R. & Shim, J.H. (2009). Liquid level monitoring sensor system using fiber bragg

grating embedded in cantilever. Sensors and Actuators, A, 152, 248-251. Spooncer, R.C.; Butler, C.; Jones, B.E. (1992). Optical fibre displacement sensors for process

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Actuators B, Vol. 29, pp.428- 432. Van Etten W. & Van der Plaats J. (1991). Fundamentals of optical fiber communi-cations.

Prentice-Hall, London. Wang, D.X.; Karim, M.A.; Li, Y. (1997). Self referenced fiber optic sensor performance for

micro displacement measurement. Opt. Eng., Vol. 36, pp. 838–842. Yang, H.Z.; Lim, K.S.; Harun, S.W.; Dimyati, K.; Ahmad, H. (2010). Enhanced Bundle Fiber

Displacement Sensor Based on Concave mirror. Sensors and Actuators A, 162, 8-12. Yang, H.Z.; Harun, S.W.; Ahmad, H. (2009). Fiber Optic Displacement and Liquid Refractive

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Fabrication and Applications of Microfiber K. S. Lim1, S. W. Harun1,2, H. Arof2 and H. Ahmad1

1Photonic Research Center, University of Malaya, Kuala Lumpur, 2Department of Electrical Engineering, Faculty of Engineering, University of Malaya,

Kuala Lumpur, Malaysia

1. Introduction Microfibers have attracted growing interest recently especially in their fabrication methods and applications. This is due to a number of interesting optical properties of these devices, which can be used to develop low-cost, miniaturized and all-fiber based optical devices for various applications (Bilodeau et al., 1988; Birks and Li, 1992 ). For instance, many research efforts have focused on the development of microfiber based optical resonators that can serve as optical filters, which have many potential applications in optical communication and sensors. Of late, many microfiber structures have been reported such as microfiber loop resonator (MLR), microfiber coil resonator (MCR), microfiber knot resonator (MKR), reef knot microfiber resonator as an add/drop filter and etc. These devices are very sensitive to a change in the surrounding refractive index due to the large evanescent field that propagates outside the microfiber and thus they can find many applications in various optical sensors. The nonlinear properties of the micro/nanostructure inside the fiber can also be applied in fiber laser applications. This chapter thoroughly describes on the fabrication of microfibers and its structures such as MLR, MCR and MKR. A variety of applications of these structures will also be presented in this chapter.

2. Fabrication of microfiber 2.1 Flame brushing technique Flame brushing technique (Bilodeau et al., 1988 ) is commonly used for the fabrication of fiber couplers and tapered fibers. It is also chosen in this research due to its high flexibility in controlling the flame movement, fiber stretching length and speed. The dimension of the tapered fiber or microfiber can be fabricated with good accuracy and reproducibility. Most importantly, this technique enables fabrication of biconical tapered fibers which both ends of the tapered fiber are connected to single-mode fiber (SMF). These biconical tapered fibers can be used to fabricate low-loss microfiber based devices. Fig.1 shows a schematic illustration of tapered fiber fabrication based on flame brushing technique. As shown in Fig. 1, coating length of several cm is removed from the SMF prior to the fabrication of tapered fiber. Then the SMF is placed horizontally on the translation stage and held by two fiber holders. During the tapering, the torch moves and heats along the uncoated segment of fiber while it is being stretched. The moving torch provides a uniform heat to the fiber and the tapered fiber is produced with good uniformity along the

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heat region. To monitor the transmission spectrum of the microfiber during the fabrication, amplified spontaneous emission (ASE) source from an Erbium-doped fiber amplifier (EDFA) is injected into one end of the SMF while the other end is connected to the optical spectrum analyzer (OSA). Fig. 2(a) shows diameter variation of the biconical tapered fiber fabricated using the fiber tapering rig while Fig. 2(b) shows the optical microscope image of the tapered fiber with a waist diameter of 1.7µm. With proper tapering parameters, the taper waist diameter can be narrowed down to ~800nm as shown in Fig. 2(c).

Fig. 1. Tapered fiber fabrication using flame brushing technique.

Adiabaticity is one of the important criteria in fabricating good quality tapered fibers. It is commonly known that some tapered fibers suffer loss of power when the fundamental mode couples to the higher order modes. Some fraction of power from higher order modes that survives propagating through the tapered fiber may recombine and interfere with fundamental mode. This phenomenon can be seen as interference between fundamental mode HE11 and its closest higher order mode HE12. This results to a transmission spectrum with irregular fringes as shown by the dotted graph in Fig. 3 and the excess loss of the tapered fiber is ~0.6dB (Ding et al., 2010; Orucevic et al., 2007 ). This tapered fiber is not suitable to be used in the ensuing fabrication of microfiber devices. The solid curve in the same figure shows the transmission of a low loss tapered fiber with approximately more than 4mm transition length and the insertion loss lower than 0.3dB. Some analysis suggests that the coupling from fundamental mode to higher order modes can be minimized by optimizing shape of the tapers. In practice, adiabaticity can be easily achieved by using sufficiently slow diameter reduction rate when drawing tapered fibers or in other words manufacture tapered fibers with sufficiently long taper transition length. A detail discussion on the adiabatic criteria and optimal shapes for tapered fiber will be presented in the next section.


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(a)

(b)

(c)

Fig. 2. (a)The diameter variation of a biconical tapered fiber fabricated in the laboratory (b) Optical microscope image of tapered fiber with a waist diameter of 1.7 µm (c) SEM image of a ~700nm waist diameter tapered fiber.

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Fig. 3. Output spectra from a microfiber with 10 cm long and ~3µm waist diameter. Input spectrum from EDFA (dashed), adiabatic taper (solid) and non-adiabatic taper (dotted).

2.2 Adiabaticity criteria Tapered fiber is fabricated by stretching a heated conventional single-mode fiber (SMF) to form a structure of reducing core diameter. As shown in Fig. 4, the smallest diameter part of the tapered fiber is called waist. Between the uniform unstretched SMF and waist are the transition regions whose diameters of the cladding and core are decreasing from rated size of SMF down to the order micrometer or even nanometer. As the wave propagate through the transition regions, the field distribution varies with the change of core and cladding diameters along the way. Associated with the rate of diameter change of any local cross section, the propagating wave may experience certain level of energy transfer from the fundamental mode to a closest few higher order modes which are most likely to be lost. The accumulation of this energy transfer along the tapered fiber may result to a substantial loss of throughput. This excess loss can be minimized if the shape of the fabricated tapered fiber follows the adiabaticity criteria everywhere along the tapered fiber (Birks and Li, 1992; Love et al., 1991 ).

Fig. 4. Typical diameter profile of a tapered fiber.


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Fig. 5. Illustration of the taper transition.

Fig. 5 gives an illustration of a tapered fiber with decreasing radius where z denotes the position along the tapered fiber. Theoretically, an adiabatic tapered fiber is based on the condition that the beat length between fundamental mode LP01 and second local mode is smaller than the local taper length-scale zt.

b tz z (1)

Referring to illustration in Fig. 5, zt is given by

/ tantz (2)

where ρ= ρ(z) is the local core radius and Ω= Ω(z) is the local taper angle. The beat length between two modes is expressed as

1 2

2bz

(3)

where 1 1( )r and 2 2( )r are the propagation constants of fundamental mode and second local mode respectively. From the above equations, Inequality (1) can be derived to

1 2( )tan2

ddz

(4)

where ddz is the rate of change of local core radius and its magnitude is equivalent to tan .

For the convenience of usage and analysis, Inequality (4) is rewritten as a function of local cladding radius r=r(z),

1 2( )2

dr rdz

(5)

Based on this condition, adiabatic tapered fiber can be acquired by tapering a fiber at a smaller reduction rate in diameter but this will result to a longer transition length. Considering practical limitations in the fabrication of fiber couplers or microfiber based devices, long tapered fiber may aggravate the difficulty in fabrication. For the purpose of

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miniaturization, short tapered fiber is preferable. To achieve balance between taper length and diameter reduction rate, a factor f is introduced to Inequality (5) and yields

1 2( )2

frdrdz

(6)

where the value of f can be chosen between 0 to 1. Optimal profile is achieved when f = 1. Practically, tapered fiber with negligibly loss can be achieved with f = 0.5 but the transition length of the tapered fiber is 2 times longer than that of the optimal tapered fiber.

2.3 Shape of tapered fiber When a glass element is heated, there is a small increment in the volume under the effect of thermal expansion. However, the change in volume is negligibly small not to mention that the volume expansion wears off immediately after the heat is dissipated from the mass. It is reasonable to assume that the total volume of the heated fiber is conserved throughout the entire tapering process. Based on this explanation, when a heated glass fiber is stretched, the waist diameter of the fiber is reduced. The calculation of varying waist diameter and length of extension can be made based on the idea of ‘conservation of volume’. Birks and Li (1992) presented simple mathematical equations to describe the relationship between shapes of tapered fiber, elongation distance and hot-zone length. Any specific shape of tapered fiber can be controlled by manipulating these parameters in the tapering process. The differential equation that describes the shape of the taper is given by

2

dr rdx L

(7)

where L denotes the hot-zone length and r denotes the waist diameter. The function of radius profile is given by the integral

01( ) exp2

dxr x rL

(8)

To relate the varying hot-zone length L with the elongation distance x during the tapering process, L can be replaced with any function of x. Linear function

( ) oL x L x (9)

makes a convenient function for the integral in Eqn (8).

1/2

00

( ) 1 xr x rL

(10)

where r0 denotes the initial radius of the fiber. To express the taper profile as a function of z, distance along the tapered fiber is given as;

1/2

00

2( ) 1(1 )

zr z rL

(11)


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By manipulating the value of , several shapes of tapered fiber can be produced such as reciprocal curve, decaying-exponential, linear and concave curve. Several examples of calculated taper shape based on different values of α can be found in the literature of (Birks and Li, 1992 ). Consider the case of tapered fiber with decaying-exponential profile as shown in Fig. 6, the fabrication of such tapered fiber requires a constant hot-zone length (α=0). From the theoretical model presented above, the function for the decaying-exponential profile is given by

0 0( ) exp( / )r z r z L (12)

Based on this profile function, narrower taper waist can be achieved by using a small hot-zone length in the fabrication or drawing the taper for a longer elongation distance. Tapered fiber with a short transition length can be achieved from reciprocal curve profile based on positive value of α particularly with = 0.5.

Fig. 6. A tapered fiber with decaying-exponential profile fabricated using a constant hot-zone L0=10mm.

Fig. 7. Three linear taper profiles (a-c) with its smallest waist point at different positions on the tapered fibers. Profile (a) has its smallest waist point at the center of the tapered fiber.

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Linear taper profile can be produced using = -0.5 and the profile function is given by

00

2( ) 13

zr z rL

(13)

Fig. 7 shows typical examples of linear taper profiles. As shown in the figure, profile (a) has the smallest waist diameter, which located at the center of the tapered fiber. By doing some simple modification on the tapering process, the smallest waist point can be shifted away from the center to one side of the tapered fiber as shown by profiles (b) and (c) in Fig. 7. These profiles are found useful in the fabrication of wideband chirped fiber bragg grating, in which the grating is written on the transition of the tapered fiber. Long linear shape tapers make good candidates for the fabrication of such devices (Frazão and et al., 2005; Mora et al., 2004; Ngo et al., 2003; Zhang et al., 2003 ). On the other hand, linear profile tapers can be used for optical tweezing because of its capability to converge the optical wave to a high intensity at the taper tip (Liu et al., 2006; Xu et al., 2006 ). Microscopic objects are attracted to the high intensity field driven by the large gradient force at the taper tip. Fig. 8 gives a good example of such tapered fiber with 15cm linear taper profile. It was produced by using a long initial hot zone length Lo = 7cm and long elongation distance.

Fig. 8. The diameter of tapered fiber is linearly decreasing from ~128µm to ~10µm along the 15cm transition.

2.4 Throughput power of a degrading tapered fiber In the high humidity environment, the concentration of water molecules in the air is high and very ‘hazardous’ to tapered fibers/microfiber. The increasing deposition of particles (dust) and water molecules on the microfiber is one of the major factors which causes adsorption and scattering of light that lead to perpetual decay in transmission (Ding et al., 2010 ). In an unprotected environment, freestanding microfibers may sway in the air due to the air turbulence. A small mechanical strength induced can cause cracks in the glass structure which may result to an unrecoverable loss in the microfibers (Brambilla et al., 2006 ).


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Fig. 9. The throughput power of the 10 cm long and ~3um diameter tapered fiber degrades over time.

Fig. 9 shows the output spectra of a tapered fiber in an unprotected environment. After the tapered fiber was drawn, it was left hanging in an open air and transmission spectrum was scanned and recorded every 50mins as presented in Fig. 9. Over the time, the deposition of dust and water molecules on the taper waist accumulated and the insertion loss of the tapered fiber increased over time. The output power of the tapered fiber dropped monotonically and eventually the power has gone too low beyond detection after 350mins. The throughput of the tapered fiber can be recovered by flame-brushing again as suggested in (Brambilla et al., 2006 ) but there is a risk that the tapered fiber can be broken after several times of flame-brushing and this solution is not practical. Despite the fact that the experiment was carried out in an air-conditioned lab where the humidity was lower (40-60%) but it was still too high for the tapered fibers. Besides, free standing tapered fibers are vulnerable to air turbulence or any sharp objects. New strategies for handling these tapered fibers are crucial for the ensuing research and fabrication of microfiber devices. In order to achieve that, this research team has been motivated to devise a packaging method to address all the problems mentioned earlier which will be discussed in the next section.

3. Packaging of microfiber 3.1 Embedding microfiber photonic devices in the low-index material Besides the fast aging of bared microfiber in the air, the portability is another issue encountered when the microfiber is required at a different location. Moving the fabrication rig to the desired location is one way of solving the problem but it is not practical. Without a proper technique, it is risky to remove the tapered fiber from the fiber tapering rig and deliver it intact to another location. Xu and Brambilla (2007) proposed a packaging technique by embedding microfiber coil resonators in a low-index material named Teflon. Microfiber or microfiber device can be coated with or embedded in Teflon by applying some Teflon resin in solution on them and leave the solution to dry for several ten minutes. The resin is solidified after the solvent has finished evaporating from the solution, the optical properties and mechanical properties of the microfiber devices can be well preserved in the

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material for a very long time (Xu and Brambilla, 2007 ). Jung et al. (2010) had taken slightly different approach by embedding microfiber devices in a low-index UV-curable resin. The resin is solidified by curing it with UV-light. Here, the detail procedure of embedding a microfiber device in the low-index UV-curable resin is demonstrated.

Fig. 10. Illustration of microfiber device embedded in a low refractive index material and sandwiched between two glass plates.

First, the assembled microfiber device is laid on an earlier prepared glass plate with a thin and flat layer of low refractive index material (UV-Opti-clad 1.36RCM from OPTEM Inc.) as shown in Fig. 10. The material has a refractive index of 1.36 at 1550nm. The thickness of the low refractive index material is approximately 0.5mm which is thick enough to prevent leakage of optical power from the microfiber to the glass plate. Some uncured resin is also applied on surrounding the microfiber device before it is sandwiched by another glass plate with the same low refractive index resin layer from the top. It is essentially important to ensure that minimum air bubbles and impurity are trapped around the fiber area between the two plates. This is to prevent refractive index non-uniformity in the surrounding of microfiber that may introduce loss to the system. During the tapering, coiling and coating processes, we monitored both the output spectrum and the insertion loss of the device in real time using the ASE source in conjunction with the OSA. The uncured resin is solidified by the UV light exposure for 3 ~7 minutes and the optical properties of the microfiber device are stabilized. The image of the end product is as shown in Fig. 11.

Fig. 11. The image of the end product of an embedded MLR in the low-index resin.


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Fig. 12 shows recorded output spectra of the MKR at several intermediate times during the process of embedding it in a low-index UV-curable resin. The first spectrum in Fig. 12(a) was recorded right after an MKR was assembled. The fringes in the spectrum indicate that the resonance condition had been achieved in the MKR but the resonance extinction ratio remains appalling ~3dB. The MKR was benignly laid on an earlier prepared glass slide with thin layer of low refractive index material. After that, some low-index resin (refractive index ~1.36 ) in solution was applied onto the MKR by using a micropipette. Fig. 12(b) shows the stabilized output spectrum of the MKR and the improved resonance extinction ratio ~10dB. This phenomenon can be attributed to the reduction of index contrast; the mode field diameter (MFD) is the microfiber was expanded when it was immersed in the resin and the coupling efficiency of the MKR was altered. The changes of coupling coefficient and round-trip loss of the MKR may have induced critical coupling condition in the MKR and enhanced the resonance extinction ratio. At time = 4 minutes, UV-curing was initiated and a little bit of fluctuation is observed in the output power and extinction ratio (refer Fig. 12(c)) during the curing process. After UV-curing for 6 minutes, the output spectrum became very stable and the resin was finally solidified. Fig. 13 shows the optical microscope image of the embedded MKR in UV-curable resin.

Fig. 12. Embedding an MKR in a low-index material. The time in each graph indicates when the output spectrum of the MKR is recorded. a) MKR is freestanding in the air b) some low-index resin applied on the MKR c) UV curing is initiated and d) resin is solidified.

Fig. 13. Optical microscope image of an MKR embedded in UV-curable resin.

100µm

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3.2 Packaging tapered fiber in a perspex case Tapered fibers are susceptible to the air turbulence and the pollution of dust and moisture when exposed to air. It is very fragile when removed from the fiber tapering rig and maintaining the cleanliness of the tapered fiber in an unprotected condition is difficult. However, another simple packaging method had been devised to address all the difficulties mentioned. For the purpose of long term usage and ease of portability of the tapered fiber, a proper packaging process is essential. In the previous section, microfiber device is embedded in the low-index UV-curable resin to maintain the physical structure and resonance condition of the devices. Although, the refractive indices of the materials are in the range of 1.3~1.4 which is slightly lower than refractive index of silica tapered fiber and the mode can be still be confined within the tapered fiber but some optical properties such as numerical aperture (NA) and MFD will be altered due to the change in refractive index difference between silica microfiber and ambient medium when embedded in the low-index material. In the context of maintaining small confinement mode area and high optical nonlinearity, this method may not be a good idea. In this section, a new packaging method is proposed where the tapered fiber is kept in a perspex case. The taper waist is kept straight and surrounded by the air without having any physical contact with any substance or object thus maintaining its optical properties in the air. The following part of this section provides detail descriptions of this packaging method. First, an earlier prepared perspex tapered fiber case which was made of several small perspex pieces with a thickness of 2.5mm was used in housing the tapered fiber. The perspex case mainly comprises of a lower part and upper part. Both parts of the perspex case were specially prepared in such a way that the benches at both ends of the perspex case were positioned exactly at the untapered parts of the tapered fiber. After a fresh tapered fiber was drawn, the lower part of the perspex case was carefully placed at the bottom and in parallel with the tapered fiber. That can done with the assistance of an additional translation stage. Then, the perspex case was slowly elevated upward until both benches touch both untapered parts of the tapered fiber as shown in Fig. 14(a). After that, some UV-curable optical adhesive (Norland Product, Inc) was applied to the untapered fibers that laid on the benches before the upper part of the tapered fiber case covered the tapered fiber from the top as shown in Fig. 14(b). The UV-curable adhesive was used to adhere both the upper part and the lower part of fiber taper case. Despite that the refractive index of the optical adhesive(~1.54) is higher than silica glass (1.44) but the adhesive was only applied to untapered fiber and the light confined within the core of the fiber is unaffected. To cure the UV-curable adhesive, 9W mercury-vapour lamp that emits at ~254nm was used. Depending on the adhesive volume and its distance from the mercury-vapour lamp, the curing time takes for 2-8mins. After the adhesive was solidified and both case parts were strongly adhered to each other (Refer Fig. 14(c)). During the process illustrated in Fig. 14(a)-(c), the fiber taper was held by the two fiber holders in fiber taper rig and this helped to keep the fiber taper straight until the completion of the UV-curing process. After that the fiber taper and its case can be safely removed from the fiber holders. The fiber taper packaged inside the perspex case may remain straight permenantly. In the contrary, the fiber taper may suffer higher insertion loss if the taper fiber was bent during the packaging process. On the other hand, it is essential to prevent any physical contact between the taper waist with human hands or other objects.


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The dust or moisture on the fiber taper may introduce loss to the transmission. In the final step, the perspex case was sealed by wrapping it with a piece of plastic wrap. This can minimize the pollution of dust or air moisture in the perspex case for a very long period of time. This fiber taper can be kept in storage for a week and possibly a fortnight without having an increment of loss more than 1.5dB however it is subject to taper dimension and its usage in the experiment.

UV lamp

Plastic wrap

UV-curable adhesive

(b)

(c) (d)

Untapered fiber

Taper waist

Perspex case

(a)

Fig. 14. Schematic illustration for tapered fiber packaging process.

To observe the characteristic of the tapered fiber as well as the reliability of the tapered fiber case over time, an observation on the transmission spectrum was conducted on the packaged tapered fiber for 6 days. Figs. 16(a) and (b) show the 6 days output spectra and output power observation, respectively for the packaged tapered fiber. Unlike the monotonic decrease in throughput power observed in Fig. 9(a), the curve of every transmission spectrum is closely overlaid to each other with a small power variation <1.2dB in the graph. Refer to Fig. 16(b), the variation of the total output power is spontaneous which can be attributed to the fluctuation of power at the ASE source and change of ambient temperature. In comparison with the taper fiber without packaging, obviously Perspex case plays its role well in preserving the tapered fiber to a longer lifespan; it enables portability and allows integration with more complex optical fiber configurations away from the fiber tapering rig.

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(a)

(b)

Fig. 16. The 6 days comparison of (a) output spectrum and (b) output power variation of the 10cm long and ~3um diameter tapered fiber packaged in perspex case.

4. Optical microfiber devices Optical microfiber devices have attracted growing interest recently especially in their simple fabrication methods. This is due to a number of interesting optical properties in this device, which can be used to develop low-cost, miniaturized and all-fiber based optical devices for various applications (Guo et al., 2007 ). For instance, many research efforts have been focused on the development of microfiber/nanofibers based optical resonators that can serve as optical filters, which has many potential applications in optical communication, laser systems (Harun et al., 2010 ), and sensors (Hou et al., 2010; Sumetsky et al., 2006 ). Many photonic devices that are conventionally fabricated into lithographic planar waveguides can also be assembled from microfibers. Recently, there are many microfiber devices have been reported such as MLR (Harun et al., 2010; Sumetsky et al., 2005 ), MCR (Sumetsky, 2008; Sumetsky et al., 2010; Xu and Brambilla, 2007; Xu et al., 2007 ), MKR (Jiang et al., 2006; Lim et al., 2011; Wu et al., 2009 ), reef knot microfiber resonator as an add/drop


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filter (Vienne et al., 2009 ) , microfiber mach-zehnder interferometer (MMZI) (Chen, 2010; Li and Tong, 2008 ) and etc. These microfiber based devices have the similar functionalities, characteristics and possibly the same miniaturizability with the lithographic planar waveguides. In future, these microfiber based devices may be used as building blocks for the larger and more complex photonic circuits. In this chapter, the transmission spectrum and the corresponding theoretical model of three microfiber based devices are presented, namely MLR, MKR and MMZI. In addition, some of the important optical properties of these devices will be reviewed and discussed.

4.1 Microfiber Loop Resonator (MLR) MLRs are assembled from a single mode microfiber, which is obtained by heating and stretching a single mode fiber. In the past, many MLRs have been demonstrated. For instance and Bachus (1989) assembled a 2mm diameter MLR from an 8.5µm tapered fiber where the coupling efficiency can be compromised by the large thickness of the microfiber. However the deficiency was compensated by embedding the MLR in a silicone rubber which has lower and near to the refractive index of silica microfiber. The transmission spectrum with a frequency spectral range (FSR) of 30GHz is observed from the MLR (Caspar and Bachus, 1989 ). Later on, Sumetsky et al. had demonstrated the fabrication of MLR from a ~1µm diameter waist microfiber which has the highest achieved loaded Q-factor as high as 120,000 (Sumetsky et al., 2006 ). Guo et al. demonstrated wrapping a ~2 µm diameter microfiber loop around copper wire which is a high-loss optical medium. By manipulating the input-output fiber cross angle, the loss induced and the coupling parameter in the resonator can be varied. In the condition when the coupling ratio is equivalent to the round-trip attenuation, the MLR has achieved critical coupling and the transmission of resonance wavelength is minimum. In their work, critical coupling condition have been achieved which resonance extinction ratio as high as 30dB had been demonstrated (Guo et al., 2007; Guo and Tong, 2008 ).

4.1.1 Fabrication of MLR Fig. 19 shows an example of ~3mm loop diameter MLR assembled from a ~2.0 µm waist diameter microfiber. Similar to other optical ring resonators, MLR has a ‘ring’ but manufactured from a single mode microfiber. This fabrication can be carried out with the assistance of two 3D translation stages as illustrated in Fig. 20. By aligning the three–axial position of each translation stage and twisting one of the pigtails, the microfiber is coiled into a loop. If the microfiber is sufficiently thin, the van der Waals attraction force between two adjacent microfibers is strong enough to withstand the elastic force from the bending microfiber and maintain the microfiber loop structure. The diameter of the loop can then be reduced by slowly pulling the two SMFs apart using the translation stages. Due the large evanescent field of the microfiber, a coupling region is established at the close contact between the two microfibers and a closed optical path is formed within the microfiber loop. Since the MLR is manufactured from an adiabatically stretched tapered fiber, it has smaller connection loss because microfiber based devices do not have the input-output coupling issue encountered in many lithographic planar waveguides. Despite the difference in the physical structure and fabrication technique between MLR and the conventional optical waveguide ring resonator, they share the same optical characteristics.

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Fig. 19. Optical microscope image of an MLR.

Fig. 20. Manufacture of MLR by using two three-dimensional stages.

4.1.2 Theory The microfiber guides light as a single mode waveguide, with the evanescent field extending outside the microfiber. This evanescent field depends on the wavelength of operation, the diameter of the fiber and the surrounding medium. If the microfiber is coiled onto itself, the modes in the two different sections can overlap and couple to create a resonator. On every round-trip of light in the loop, there are fractions of light energy exchange between the two adjacent microfibers at the coupling region, the input light is allowed to oscillate in the closed loop and the resonance is strongest when a positive interference condition is fulfilled which can be related to this equation.

nλR = L (14)

where L is the round-trip length, λR is the wavelength of the circulating waves and n is an integer. Positive interference occurs to those circulating waves and the wave intensity is building up within the microfiber loop. The relationship in Eqn (14) indicates that each


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wavelength is uniformly spaced and periodic in frequency, a well-known characteristic of an optical multichannel filter. The amplitude transfer function for MLR is given as (Sumetsky et al., 2006 );

exp( / 2)exp( ) sin1 exp( / 2)exp( )sin

L j L KTL j L K

(15)

sinK denotes the coupling parameter where K = κl, κ is the coupling coefficient and l is the coupling length. For every oscillation in the MLR, the circulating wave is experiencing some attenuation in intensity attributed to non-uniformity in microfiber diameter, material loss, impurity in the ambient of microfiber and bending loss along the microfiber loop. However, these losses can be combined and represented by a round-trip attenuation factor, exp(αL/2) in Eqn (15) while exp(jβL) represents phase increment in a single round-trip in the resonator. The intensity transfer function is obtained by taking the magnitude squared of the amplitude transfer function in Eqn (15).

2

22

exp( ) sin ( ) 2exp( / 2)sin( )cos( )1 exp( )sin ( ) 2exp( / 2)sin( )cos( )

L K L K LTL K L K L

(16)

The resonance condition occurs when

2L m (17)

where m is any integer. The critical coupling occurs when

sin exp( / 2)cK L (18)

The FSR is defined as the spacing between two adjacent resonance wavelengths in the transmission spectrum which is given by

FSR, 2

effn L (19)

or

2

effn D

(20)

where D is the diameter of the circular loop. In addition to the characteristic parameters mentioned earlier, Q-factor and finesse F are two important parameters that define the performance of the MLR. The Q-factor is defined as the ratio of resonance wavelength to the bandwidth of the resonance wavelength, the full wave at half maximum, FWHM (Refer Fig. 21). It is given as;

Q = FWHM

(21)

The finesse is defined as the FSR of the resonator divided by the FWHM;

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FSRFFWHM

(22)

Due to the narrow bandwidth at the resonance wavelengths, MLR also functions as a notch filter (Schwelb, 2004 ). The attenuation at the resonance wavelength can be used to filter out/drop the signal from specific channels in the WDM network by suppressing the signal power. In DWDM network, the spacing between two adjacent channels in the network is small therefore notch filter with narrow resonant bandwidth is preferable so that the signals from adjacent channels are unaffected by the attenuation in the drop channel. Based on the relationship in Eqn (22), narrow resonant bandwidth (FWHM) can be found in high finesse filter.

1539 1539.2 1539.4 1539.6 1539.8 1540 1540.2 1540.4 1540.6 1540.8 1541-12

-10

-8

-6

-4

-2

0

Wavelength (nm)

Tran

smis

sion

(dB

)

FSR

3dBFWHM

RER

Fig. 21. Typical transmission spectrum of an MLR. The labels in the graph indicates the terminology used in the chapter. RER is an abbreviation for resonance extinction ratio.

4.1.3 Transmission spectra of MLRs The typical transmission spectra of an MLR with different FSRs are shown in Fig. 22. For better clarity of viewing, the transmission spectra with different FSRs are presented in a increasing order from the top to the the bottom in the figure. These transmission spectra were recorded from a freestanding MLR in the air, started from a large loop diameter and the diameter is decreasing in step when the two microfiber arms of the MLR are stretched. Exploiting the van der Waals attraction force between the two microfibers in the coupling region, the resonance condition of the MLR can still be maintained during the stretching of microfiber. In the measurement, the loop diameters are at approximately 1.9mm, 1.4mm, 1.1mm, 0.8mm and 0.6mm which corresponds to FSR values of 0.275nm, 0.373nm, 0.493nm, 0.688nm and 0.925nm, respectively in the C-band region as shown in Fig. 22. These variations of FSR and loop diameter are very consistent with the reciprocal relationship expressed in Eqn (23). The loop diameter of an MLR is restricted by the microfiber elastic force, the smaller is the loop diameter the greater is the elastic force. Thus, it is difficult to keep the microfiber loop in shape when the loop diameter is very small and the MLR loses its resonance condition when the loop opens.


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Fig. 22. Transmission Spectra of an MLR with increasing FSR (from top to bottom).

(a)

(b)

Fig. 23. The fitting of experimental data (circles) with the characteristic equation (solid line). (a) Q-factor ~18,000 and finesse ~9.5 (b) Q-factor ~5700 and finesse ~3.8

Fig. 23(a) and Fig. 23(b) show the fitting of experimental data with the (intensity) analytical model based on characteristic equation in Eqn (19). The best-fit parameters for the transmission spectrum in Fig. 23(a) are L = 2.03mm, exp (-αL/2) = 0.8853 and sinK = 0.7354. The measured FSR ~0.805nm from transmission spectrum in Fig. 23(a) is in agreement with

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the calculated FSR ~ 0.807nm. The bandwidth at the resonance wavelength, FWHM is ~0.085nm indicates that the Q-factor and finesse of the MLR are 18,000 and ~9.5. The best fit parameter for Fig. 23(b) are L = 1.61mm, sin K = 0.5133 and exp (-αL/2) = 0.6961. The measured FSR and FWHM are ~1.02nm and ~0.27nm respectively which indicate the values of Q-factor and finesse are ~5700 and ~3.8. In the comparison between the two spectra, the finesse provides a good representation in weighting the bandwidth between passband and stopband. The higher is the finesse the narrower is the stop-band compared with pass-band.

4.2 Microfiber Knot Resonator (MKR) MKR is assembled by cutting a long and uniform tapered fiber into two. One tapered fiber is used for the fabrication of microfiber knot while the other one is used to collect the output power of the MKR by coupling the two tapered fiber ends and guides the output light back to an SMF. The fabrication of microfiber knot can be done by using tweezers. The coupling region of the MKR is enclosed by a dashed box in Fig. 24 where the two microfibers intertwisted and overlapped in the resonator. In comparison with MLR, MKR does not rely on van der Waals attraction force to maintain the coupling region yet it can achieve stronger coupling due to the rigid intertwisted microfibers structure at the coupling region. The knot structure can withstand strong elastic force of the microfiber and maintain a rigid resonator structure with a more stable resonance condition. Based on the same microfiber diameter, MKR of smaller knot diameter can be easily manufactured than that of MLR. However, MKR suffers a setback in a high insertion loss due to the cut-coil-couple process where the evanescent coupling between output microfiber and collector microfiber contributes a large fraction in the total insertion loss. The microfiber diameter in the range of 1~3µm is preferable because thinner microfiber is very fragile and it breaks easily in the fabrication of MKRs. Nonetheless, the operating principle of MKR is identical to MLR as it is based on self-touching configuration thus the same characteristic equation can be used to describe the transmission spectrum of MKR.

Fig. 24. Optical microscope image of an MKR.

4.2.1 Transmission spectra of MKRs MKR offers better capability in achieving smaller knot diameter. The knot can withstand the strong elastic force of microfiber and it can achieve a small knot diameter that cannot be achieved in the MLR. Fig. 25 shows transmission spectra of an MKR assembled in the


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laboratory. The transmission spectra are presented in an increasing order from the top to bottom of the figure and the values are 0.803nm, 1.030nm, 1.383nm, 1.693nm, 2.163nm and 2.660nm within the vicinity of 1530nm. The corresponding knot diameters are approximately 640µm, 500µm, 370µm, 310µm, 240µm and 190µm.

Fig. 25. Transmission Spectra of an MKR with increasing FSR (from top to bottom).

4.2.2 Resonance condition of microfiber knot resonator immersed in liquids Recently, microfiber resonators are suggested in numerous applications particularly in the sensing applications (Lim et al., 2011; Sumetsky et al., 2006 ). The operating principles of these sensors rely on the characteristics of the resonance, the variation of the position of resonance wavelength and the resonance extinction ratio with the sensing parameters, temperature, refractive index and etc. (Wu et al., 2009; Xu et al., 2008 ). The resonance condition of a resonator relies on the index contrast between microfiber and its ambient medium, evanescent field strength and distance between two microfibers in the coupling region. Large evanescent field which can be found in thinner microfibers is one of the solutions to achieving higher coupling in microfiber resonators. The large fraction of light intensity in the evanescent field allows stronger mode interaction between two microfibers and yields high coupling coefficient. Caspar et al. suggest embedding the microfiber resonator into a medium that has a slightly lower refractive index than that of silica. Due to the small index contrast, the microfiber has a larger evanescent field which yields stronger coupling in the resonator (Caspar and Bachus, 1989; Xu and Brambilla, 2007 ). Besides being used as a post-fabrication remedy for improving the resonance condition of the resonator, embedding also offers good protection from the fast aging process and enabling portability for the microfiber devices. Vienne at el. have reported that when a microfiber resonator is embedded in low-index polymer, the optimal resonance wavelength is down-shifted by ~20% (Vienne et al., 2007 ). However, there is very few literatures that provide mathematically analysis on the effect of embedding in low index contrast medium to the resonance condition of the resonator. In order to achieve a better understanding, an experiment on an MKR immersed in liquid solutions was carried out. MKR was used in the experiment due to its rigid knot structure and strong interfiber coupling. The knot structure and resonance condition could be easily maintained during the immersing process. Unlike MLR that exploits van der Waals attractive force to maintain the structure of the loop, MKR has a more rigid knot structure with interfiber twisted coupling. Nonetheless,

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both MLR and MKR share the same optical properties, the same transmission equation can be used to describe both structures. Fabrication of an MKR started with fiber tapering using heat and pull technique. After a single-mode biconical tapered fiber was drawn, it was cut at one third part of the waist which the longer section of tapered fiber was used for the fabrication of knot by using tweezers. Then the second section was used as collector fiber by evanescent coupling (Tong et al., 2003 ) with the output port of the MKR. Immediately after that, the transmission spectrum of the freestanding MKR in the air was recorded by an OSA. After that, the MKR was embedded in propan-2-ol solution that has a refractive index (RI) of 1.37. First, the MKR was slowly laid horizontally on an earlier prepared flat platform deposited with a thin layer of propan-2-ol. Using a micropipette, a small volume of propan-2-ol solution was dropped onto the MKR and had it entirely immersed in the solution. The structure of the microfiber knot was intact and the resonance was maintained. This is the crucial part that distinguishes MKR from MLR. It is very difficult to maintain the loop structure and resonance of MLR when immersed in the liquid.

Fig. 26. The transmission spectra of MKR in the air (solid) and propan-2-ol solution (dashed).

Fig. 26 shows the overlaid transmission spectra of the MKR in the air (solid) and solution (dashed). Refering to the peak powers of both spectra, it is easy to determine that the MKR had suffered an additional ~7dB excess loss after it was immersed in the solution. The drop in coupling efficiency at the evanescent coupling between MKR output microfiber and collector microfiber constituted a large fraction in this excess loss. On the other hand, the resonance extinction ratio of the MKR had improved from ~5dB to ~8dB. In the analysis of resonance characteristics, the coupling parameter, sinκℓ and round-trip attenuation factor of MKR, exp(-αL/2) can be extracted from the best-fit curves (lines) for the offset experimental data (circles) in Fig. 27(a) and Fig. 27(b) based on the transfer function in (18).


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Fig. 27. The offset experimental data (circles) with its best fit curve (solid line) (a) air, RI ~1.00 (b) propan-2-ol, RI ~1.37.

In the air, the best fit parameters for the transmission spectrum in Fig. 27(a) are sinκℓ = 0.6207 and exp(-αL/2)= 0.8547. When the MKR was immersed in the propan-2-ol solution, the best fit parameters for Fig. 27(b) are sinκℓ = 0.6762 and exp(-αL/2) = 0.8361. The reduction in round-trip attenuation factor can be attributed to the small index contrast between microfiber and ambient medium when immersed in the solution so that the bending loss at the microfiber knot is higher. The output - collector coupling loss is excluded from this analysis as it only affects the total output power (position in the vertical axis) and it can be eliminated in the offset spectrum. Based on Eqn (18), the resonance state of the resonator can be determined from the following expression

exp( / 2) sin1 exp( / 2)sin

L lL l

(23)

Smaller value of δ indicates that the state of resonance is closer to the critical coupling condition and it yields larger resonance extinction ratio. In fact, the resonance extinction ratio can be estimated by

10~ 20logRER (24)

Comparing the two spectra, the spectrum in Fig. 27(b) has higher coupling and smaller round-trip attenuation factor which give smaller value in δ = 0.3679 if compared with δ = 0.5060 obtained from the spectr sinκℓ and resulting the lower coupling value. The next experimental data may provide an example for such self-defeating scenario.The transmission spectra of an MKR in the air and low-index UV-curable resin (UV-Opti-clad 1.36RCM from OPTEM Inc.) with an RI of ~1.36 are as shown in Fig. 28(a) and Fig. 28(b) respectively. The coupling parameter, sinκℓ had dropped from 0.7132 to 0.6247 when it was immersed in the water. On the other hand, the round-trip attenuation factor, exp(-αL/2) suffers greater fall from 0.9432 to 0.7538. In spite of that, the resonance extinction ratio had

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increased from ~2dB to ~10dB. This is in agreement with the decreasing value of δ from 0.7027 to 0.2440 and the state of resonance is closer to critical coupling condition.

Fig. 28. The transmission spectra of MKR in different ambient mediums (a) air, RI ~1.00 (b) low-index resin, RI ~1.36.

Immersing MKR in a near-index medium do not always promise an improvement in the resonance condition or RER. There is a possibility that the changes in round-trip attenuation factor and coupling parameter yield larger value of δ and decreases the RER. Fig. 29 provides an example for this scenario. The best-fit parameter for the experimental data in the air (solid) are sinκℓ = 0.6235 and exp(-αL/2) = 0.8145 respectively. After the MKR is immersed in the water (dashed), the values have varied to sinκℓ = 0.7833 and exp(-αL/2) = 0.9339. In the air, the low value of round-trip attenuation factor can be attributed to the large amount of deposited dust on the microfiber surface which was introduced from the tweezers during the fabrication of microfiber knot. After it was immersed in the water, some portion of the dust might have been ‘washed’ away and that increases the round-trip attenuation factor. The value of δ has decreased from 0.3881 to 0.5609 which is an indication of the resonance state deviates from critical coupling.

Fig. 29. Example of an MKR with decreased resonance extinction ratio after it is immersed in the water (RI ~1.33).


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The purpose of using liquid solutions of different RIs is to investigate the influence of different index-contrasts to the resonance characteristics of the MKR. However, there was no significant indication observed in the experiment showing difference between the solutions. The only obvious changes were observed at the moment when the MKRs were immersed in the solutions. It is believed that the index contrasts induced by these three liquids are within a narrow range from 0.07 to 0.11 and the differences among them are too small to make significant impact on the characteristics of the MKR. On the other hand, we believe that that the microfiber waist diameter and orientation of the microfibers in the coupling region have an important relationship with the resonance of MKR. More investigations pertaining to those parameters are needed.

4.2.3 Polarization dependent characteristic Microfiber based resonator exhibits strong dependence on its input polarization state. Similar characteristic was reported by Caspar et al. (Caspar and Bachus, 1989 ) where the extinction ratio of the resonator varies with the change of input state of polarization.

Fig. 30. Experimental set-up to investigate the polarization dependent characteristic of the MLR (Lim et al., 2011 ). Polarized wideband source from EDFA is acquired with the aid of PBS (Dashed box), an unpolarized wideband source can be obtained by removing PBS from the setup.

In the investigation of polarization state dependent characteristics, a simple experimental setup as shown in the Fig. 30 is established. First, the unpolarized ASE source from an EDFA was linearly polarized by a PBS, followed by a PC for controlling the state of polarization (SOP) of the polarized source before it was fed into the MLR. Fig. 31 shows the transmission spectra of the MKR at various resonance conditions of the MLR, which was obtained at different input wave SOP. Both spectra (i) in Fig. 31(a) and Fig. 31(b) show the transmission of the MLR based on an unpolarized input wave (without PBS), which resonance condition was unaffected by the adjustment of the PC. In contrast, the resonance condition for the MLR with polarized input wave was sensitive to the PC adjustment. By carefully adjusting the PC, the resonance extinction ratio could be enhanced or reduced as depicted in spectra (ii) and (iii) of both Fig. 31(a) and Fig. 31(b). Nevertheless, the wavelength of each peak and the FSR remained unchanged regardless of input wave SOP. It is appropriate to attribute this phenomenon to the polarization dependent coupling in the MLR where the interfiber coupling, twisting and alignment of microfiber in the coupling region are accounted for the coupling coefficient difference between two orthogonal polarization states (Bricheno and Baker, 1985; Chen and Burns, 1982; Yang and Chang, 1998 ). Associated with the coupling coefficient, the resonance extinction ratio of a microfiber based resonator can be improved by using an optimized polarized input light. However, more efforts are required for more in-depth study and to explore the possible applications in many fields such as multi-wavelength laser generation and sensing.

MLR

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Fig. 31. The spectra of two MLRs for different input wave SOP. (a) FSR=0.162nm at 1530nm (b) FSR=0.71nm at 1530nm.

4.2.4 Thermal dependent characteristic Microfiber based devices are very sensitive to variation of ambient temperature due to the strong dependence of the microfiber dimension and refractive index on temperature. In a thermally unstable environment, these devices may experience thermal drift in the transmission spectrum and fluctuation in the transmission power. However, this problem can be alleviated by the placing the devices in a temperature controlled housing (Dong et al., 2005 ). Sumetsky et al. demonstrated a MLR based ultrafast sensor for measurement of gas temperature. Taking advantage of the close contact between the MLR and air, the change in gas temperature in the ambient of MLR can be determined from the transmission power at the resonance wavelengths within a short response time of several microseconds (Sumetsky et al., 2006 ). Besides, the positions of resonance wavelengths are found to be sensitive to temperature change. The spectral shift of an MLR can be expressed in a linear function of temperature. The property enables temperature measurement based resonance wavelength shift with higher accuracy (Wu et al., 2009; Zeng et al., 2009 ). MKRs exhibit the similar optical properties with MLRs. The free spectral range of MKR takes in the form of Eqn (22). Based on this equation, the variations in effective index neff and round-trip length L may lead to transmission spectral shift and their relationship can be expressed as

.

effres

res eff Temp

n Ln L

(25)

In relation with temperature, both terms on the right hand side of Eqn (25) expresses two linear thermal coefficients; thermal-optic coefficient (TOC) and thermal expansion coefficient (TEC) [13]. With this interpretation, Eqn (25) can be rewritten as


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( )resTOC TEC

resT

(26)

Fig. 32(a) shows the transmission spectra of MKR at temperatures of 30°C, 35°C and 40°C. The spectral shift is approximately 26pm for every temperature increment of 5°C and the linearity between wavelength shift and temperature change can be seen from the linear fitting of experimental data in Fig. 32(b).

(a) (b)

Fig. 32. (a) The output spectra of an MKR at temperature of 30°C(solid), 35°C(dashed) and 40°C (dotted) (b) The temperature response of the MKR has spectral sensitivity of 50.6pm/°C.

This characteristic has opened up new possibilities for temperature sensing and spectral control based on temperature manipulation. It provides a solution for stabilizing the spectrum of the device which often affected by the thermal drift. Dynamic spectral shift in optical filter can be realized by exploiting this characteristic. In addition, the insusceptibility of fiber-optic components to electrical noise has made these devices very attractive for many industrial sensing applications.

4.2.5 Microfiber knot resonator based current sensor A variety of fiber optic based current sensors have been investigated in recent years using mainly a single mode fiber (SMF) of clad silica. They are typically divided into two categories, where one is based on Faraday Effect and the other is based on thermal effect. The former is capable to remotely measure electrical currents, but the device requires a long fiber due to the extremely small Verdet constant of silica. The latter needs a short length of fiber but requires complex manufacturing techniques to coat fibers with the metals. Recently, a resonant wavelength of the MLR has been experimentally reported to shift with electric current applied to the loop through a copper rod. An acceptable transmission loss is achieved despite the fact that copper is not a good low-index material to support the operation of such structure (Guo et al., 2007 ). This finding has opened up a way to enable dynamic and efficient spectrum control for optical filters by manipulating an electric current dependence spectral shift characteristic of the microfiber based resonator. Microfiber based devices have a strong dependence on temperature due to the thermal expansion characteristic and thermo-optic effect of silica glass. As discussed earlier in Section 12, the transmission spectrum of a microfiber based device shifts as the ambient temperature varies

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and the relationship between these two variables is well described by the linear equation in Eqn. (26). In this section, spectral tunable MKR is demonstrated based on the idea of thermally induced resonant wavelength shift. By manipulating the applying electric current through the microfiber knot wrapped copper wire, the copper wire acts as a heating element and induces temperature change in the MKR. The transmission spectrum of the MKR shifts corresponds to the temperature change. These modified MKRs can be used as low-cost and fast response tunable optical filters which are useful in the applications of optical signal processing, WDM communication and etc. On the other hand, this opto-electrical configuration may operate as a dynamic current sensor with strong immunity to electric noise. In addition, it has a dynamic operational range extending to the regime of extreme high temperature or pressure. Fabrication First, a ~2μm diameter silica microfiber is fabricated from a SMF using flame-brushing method (Graf et al., 2009 ). Then the microfiber is cut and separated into two unequal parts in which the longer one is used in the knot fabrication and the other one is used as a collector fiber to collect the transmitted light from the MKR (Jiang et al., 2006 ). During the fabrication of the knot, the copper wire is inserted into the knot which diameter is bigger than the diameter of the copper wire (Refer Fig. 33(a)). The light path from the knot resonator is completed by coupling the two microfiber ends. At least ~3 mm of coupling length between two microfibers is required to achieve strong van der Waal attraction force to keep them attached together. The microfiber knot diameter is then reduced and fastened on the copper wire by pulling microfibers from both arms of the microfiber knot as illustrated in Fig. 33(b).

(a) (b)

Fig. 33. Optical microscope image of MKR tied on a copper wire.

The optical characteristics of the resonator are strongly affected by the tensile strain on the microfiber arms of the MKR induced by the pulling on the microfibers arms. It is essential to reduce the tension on the both arms of the MKR by moving the fiber holders a bit closer to the microfiber knot after the knot is fastened. In spite of that, there is a very little change at the knot diameter and the resonance condition of the MKR remains good and stable after the tension is released. Theoretical Background By wrapping microfiber on a current loaded conductor rod, the conductor rod acts as a heating element. It generates heat and increases the temperature of the microfiber. As


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discussed earlier in Section 13, the transmission spectrum of a microfiber based device shifts as its ambient temperature varies and the relationship between these two variables is well described by the linear equation in Eqn. (26). Consider the linear relationship between the temperature change and heat energy generated by the conducting current, the relationship between wavelength shift and the conducting current, I can be expressed in the form of

2

res

res

IA

(27)

where and A represent the conductor resistivity and the cross sectional area of the conductor rod, respectively. The term / A in Eqn. (27) is equivalent to the resistance per unit length of the conductor material. The resistivity of the copper rod is 1.68×10−8 Ω·m. Current Response For optical characterization of the MKR, broadband source from amplified spontaneous emission is first launched into and guide along the SMF and then squeezed into the microfiber through the taper area. The light transmitted out from the MKR is collected by the collection fiber and measured by an OSA. The optical resonance is generated when light traversing the MKR When an alternating current flows through the copper wire, heat is produced in the wire to change temperature. Because the MKR is in contact with the copper wire, any temperature changes will influence the refractive index and the optical path length of the MKR. Fig. 34 shows the resonant spectral of the MKR tied on a copper wire with various current loadings. In the experiment, the applying current is uniformly increased from 0 to 2A.

Fig. 34. Resonant wavelength shift of the MKR tied on a copper rod loaded with different current. Inset shows unchanged FSR with the increasing current.

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In the spectrum, the resonant wavelength shifts to a longer wavelength the increasing of conducting current in the copper wire. The response time of the wavelength shift is approximately 3s and the spectrum comes to steady condition after 8s. Therefore, each spectrum is recorded at ~10s after the copper wire is loaded with an electric current. At loading current I = 1.0A, the resonant wavelength is shifted by ~30pm from 1530.56nm to 1530.59nm and at I = 2.0A, the resonant wavelength is further shifted to 1530.77nm, 210pm from the original wavelength. Inset of Fig. 34 shows the free spectral range of the transmitted spectrum against the applying currents. As shown in the inset, FSR of the MKR remains unchanged at 1.5 nm with the increasing current. The calculated Q factor and finesse of the MKR are ~4400 and 4.3 respectively. It is also observed that the transmission spectrum always shift towards the longer wavelength direction with increasing current regardless of the current flow direction, and the spectrum returns to original state once current supply is terminated.

Fig. 35. Schematic illustrations of microfiber knot tied on (a) single copper wire (b) two copper wires with identical wire diameter of ~200μm.

Fig. 35(a) and Fig. 35(b) give schematic illustrations of MKR wrapped on a single copper rod and two copper rods respectively. The measured FSR and knot diameter of the single-rod MKR are 1.7nm and ~317μm respectively while for the two-rod MKR, the measured FSR and knot diameter are 1.46nm and ~370μm. An direct current is applied through the copper wire and the resonant wavelength shift is investigated against the applying current. At small current of < 0.5A, no significant resonant wavelength shift is observed. Beginning at 0.6A, the resonant wavelength shifts gradually toward the longer wavelength. At applying current of 2.0A, a wavelength shift of 0.208nm and 0.09nm are achieved with single and two copper wires configurations of Fig. 35(a) and Fig. 35(b), respectively. Fig. 36 shows the resonant wavelength shift against a square of current (I2) for both configurations. The data set of each configuration can be well fit with a linear regression line with a correlation coefficient value r > 0.95. This justifies the linear relationship stated in Eqn. (27). In comparing the conductor wire cross-sectional area between the two configurations, the two-wire configuration is twice larger than the single-wire configuration. Based on the relation in Eqn. (27), the tuning slope of the wavelength shift with I2 of the two-wire configuration should be a half of the single-wire configuration. The slope of each linear line is 51.3pm/A2 (single rod) and 19.5pm/A2 (two rods) nonetheless it is reasonable to attribute the mismatch between the analysis and experiment to the different orientation and position of the rod(s) in the MKR. The tuning slope of the current sensor can be further increased by using different


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conductors with higher resistivity such as nichrome, constantan, graphite and etc which are commonly used as heating elements. However, the suitability in integration with microfiber or other opto-dielectric device remains uncertain.

Fig. 36. Current response of MKRs based on single-wire and two-wire configurations. The calculated resistance of the single copper wire and two copper wire are 0.53 Ω·m−1 and 0.26Ω·m−1 respectively.

4.3. Microfiber Coil Resonator (MCR) Microfiber coil resonators (MCRs) possess similar functionality with other microfiber resonators. It is useful for the applications of optical filtering, lasers and sensors. Additionally, it can be employed as an optical delay line for the optical communication network with small compactness. It is fabricated by winding a long microfiber on a low- index dielectric rod or a rod coated with low-index material (Sumetsky et al., 2010 ). The helical structure of microfiber coil enables propagation of light along the microfiber, across between the turns of microfiber in the forward and backward directions as illustrated in Fig. 37.

Fig. 37. Helical structure of an MCR and the propagation direction of the light in the resonator.

Fig. 38 shows the transmission spectra of an MCR of increasing number of microfiber turns. The microfiber was wound on a 0.5mm-diameter low-index coated rod. Basically, the optical

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characteristics a 1-turn MCR is exactly identical to that of MLR, for instance the interference fringes in the MCR transmission spectrum are equally spaced as shown in Fig. 38(a). From the spectrum, the measured FSR is ~0.8nm and the estimated diameter of the coil is ~0.6mm which is slightly larger than the rod diameter. When making additional turns to the coil, it is important to ensure overlapping or touching between turns to establish coupling between them. For every additional turn is made, the transmission spectrum of the MCR is altered. Figs. 38(b)-(d) show the transmission spectra of a 2-turn, 3-turn and 4-turn MCR fabricated in the laboratory. Consider the elastic force from the bent microfiber; it is very difficult to maintain the resonance condition and increasing the number of turns at the same. With the assistance of microscope, the ensuing coiling work can be alleviated. Nonetheless, the reproducibility of MCR was difficult and tedious. Compared with the other microfiber resonators, MCRs have more complicated light propagation properties (Hsu and Huang, 2005 ).

Fig. 38. Transmission spectra of a) 1 turn b) 2 turns c) 3 turns and d) 4 turns MCR.

5. Summary In the past, several fabrication techniques for tapered fibers/microfibers have been suggested. In this chapter, fabrication of microfiber based on flame brushing technique is reviewed. Flame brushing technique is commonly used for the fabrication of fiber couplers and tapered fibers. This technique enables fabrication of biconical tapered fibers which are important components for the manufacture of microfiber based devices. In order to achieve that, a fiber tapering rig was assembled. The heat source comes from an oxy-butane torch with a flame width of 1mm. Two stepper motors are incorporated in the rig to control the movement of the torch and translation stage. A biconical tapered fiber with a waist diameter as small as 700nm can be achieved with the rig. To achieve low loss tapered fibers, the shape of the taper should be fabricated according to adiabaticity criteria. In the laboratory, tapered fibers with linear and decaying-exponential profiles have been demonstrated. To provide protection to the tapered fibers, they are embedded in a low-index material or packaged into a perspex case. These protection measures can prolong the life span and stabilize the temperal performance of these tapered fibers and microfiber based devices. Three microfiber based devices have been reviewed in this chapter namely MLR, MKR and MMZI. In the first section, the fabrication of MLR is introduced. MLR is assembled from a


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single mode microfiber by coiling it into a loop. A closed optical path within the loop is established when the two microfibers are put in close contact with each other and form an evanescent coupling. Then, the theoretical model of the self-touching MLR is presented and used for curve-fitting with the experimental data. The important characteristic parameters of the transmission spectra can be extracted from the best-fit curve in the experimental data. In the next section, MKR is presented. Similar to MLR, MKR shares the similar transmission characteristics and the same theoretical model for the MLR can be applied to MKR. However, MKR outperforms MLR in several aspects for instance MKR has more stable and stronger coupling due to its small spacing between the two coupling microfibers in the coupling region. In addition, the structure of MKR is more rigid and robust with the interfiber twisted coupling in the resonator. Nonetheless, MKRs suffer a setback in higher insertion loss if compared with MLRs because of the cut-coil-couple process in the fabrication. The evanescent coupling between output microfiber and collector microfiber contributes a large fraction in the total insertion loss. When MKR is embedded into a medium of different refractive index, the coupling in the MKR varies and it alters the resonance state of the resonator. By curving the experimental data for transmission spectra with the theoretical model, the coupling coefficient and round-trip attenuation factor of the MKR can be extracted from the best-fit curve. The results indicate that the state of resonance of an embedded MKR has been altered and it is closer to critical coupling condition. Besides, microfiber based resonators exhibit an interesting polarization dependent characteristic in the investigation. The coupling coefficient in the resonator is dependent on input state of polarization and it can be manipulated by using a PBS and a PC. In relation with the coupling, the resonance extinction ratio (RER) of transmission spectrum can be varied by tuning the PC. In the experiment, the variation of RER between 5dB and 11dB was observed. In the past, several literatures have reviewed that the refractive index and dimension of silica microfiber have a strong dependence on temperature. The resonance extinction ratio and resonance wavelength can be expressed in functions of temperature. In order to gain insight into this characteristic, an investigation was conducted to study temperature response of an MKR. Both theoretical analysis and experimental result indicate that the spectral shift in the transmission of MKR is linearly proportional to the increment of temperature. This characteristic can be exploited in many applications particular in sector industrial sensors. By wrapping the microfiber knot on a conductor rod, MKR can perform as a current sensor. When an electric current is loaded through the conductor rod, the rod acts as a heating element and increases the temperature of the MKR. As a result, transmission spectrum of the MKR is shifted. This opto-electric configuration can be used a current sensor and an electric controlled optical filter. In the final section, the fabrication of MCR was demonstrated. An MCR of different number of turns was assembled and tested.

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Chen, C.-L., and W. K. Burns, (1982), Polarization characteristics of single-mode fiber couplers, Microwave Theory and Techniques, IEEE Transactions on 30, 1577-1588.

Chen, Yi-Huai; Wu, Yu; Rao, Yun-Jiang; Deng, Qiang; Gong, Yuan, (2010), Hybrid mach-zehnder interferometer and knot resonator based on silica microfibers, Optics Communications 283, 4.

Ding, Lu, Cherif Belacel, Sara Ducci, Giuseppe Leo, and Ivan Favero, (2010), Ultralow loss single-mode silica tapers manufactured by a microheater, Appl. Opt. 49, 2441-2445.

Dong, H., G. Zhu, Q. Wang, H. Sun, N. K. Dutta, J. Jaques, and A. B. Piccirilli, (2005), Multiwavelength fiber ring laser source based on a delayed interferometer, Photonics Technology Letters, IEEE 17, 303-305.

Frazão, O., and et al., (2005), Chirped bragg grating fabricated in fused fibre taper for strain–temperature discrimination, Measurement Science and Technology 16, 984.

Graf, J. C., S. A. Teston, P. V. de Barba, J. Dallmann, J. A. S. Lima, H. J. Kalinowski, and A. S. Paterno, 2009, Fiber taper rig using a simplified heat source and the flame-brush technique, Microwave and Optoelectronics Conference (IMOC), 2009 SBMO/IEEE MTT-S International.

Guo, Xin, Yuhang Li, Xiaoshun Jiang, and Limin Tong, (2007), Demonstration of critical coupling in microfiber loops wrapped around a copper rod, Applied Physics Letters 91, 073512-3.

Guo, Xin, and Limin Tong, (2008), Supported microfiber loops for optical sensing, Opt. Express 16, 14429-14434.

Harun, S., K. Lim, A. Jasim, and H. Ahmad, (2010), Fabrication of tapered fiber based ring resonator, Laser Physics 20, 1629-1631.

Harun, S. W., K. S. Lim, A. A. Jasim, and H. Ahmad, (2010), Dual wavelength erbium-doped fiber laser using a tapered fiber, Journal of Modern Optics 57, 2111 - 2113.

Hou, Changlun, Yu Wu, Xu Zeng, Shuangshuang Zhao, Qiaofen Zhou, and Guoguang Yang, (2010), Novel high sensitivity accelerometer based on a microfiber loop resonator, Optical Engineering 49, 014402-6.

Hsu, Shih-Hsin, and Yang-Tung Huang, (2005), Design and analysis of mach?Zehnder interferometer sensors based on dual strip antiresonant reflecting optical waveguide structures, Opt. Lett. 30, 2897-2899.

Jiang, Xiaoshun, Limin Tong, Guillaume Vienne, Xin Guo, Albert Tsao, Qing Yang, and Deren Yang, (2006), Demonstration of optical microfiber knot resonators, Applied Physics Letters 88, 223501-223501-3.

Jung, Y, G. S. Murugan, G. Brambilla, and D. J. Richardson, (2010), Embedded optical microfiber coil resonator with enhanced high-q, Photonics Technology Letters, IEEE 22, 1638-1640.

Li, Yuhang, and Limin Tong, (2008), Mach-zehnder interferometers assembled with optical microfibers or nanofibers, Opt. Lett. 33, 303-305.

Lim, K. S., S. W. Harun, S. S. A. Damanhuri, A. A. Jasim, C. K. Tio, and H. Ahmad, (2011), Current sensor based on microfiber knot resonator, Sensors and Actuators A: Physical 167, 377–381.


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Lim, K. S., S. W. Harun, A. A. Jasim, and H. Ahmad, (2011), Fabrication of microfiber loop resonator-based comb filter, Microwave and Optical Technology Letters 53, 1119-1121.

Liu, Zhihai, Chengkai Guo, Jun Yang, and Libo Yuan, (2006), Tapered fiber optical tweezers for microscopic particle trapping: Fabrication and application, Opt. Express 14, 12510-12516.

Love, J. D., W. M. Henry, W. J. Stewart, R. J. Black, S. Lacroix, and F. Gonthier, (1991), Tapered single-mode fibres and devices. I. Adiabaticity criteria, Optoelectronics, IEE Proceedings J 138, 343-354.

Mora, J., A. Diez, M. V. Andres, P. Y. Fonjallaz, and M. Popov, (2004), Tunable dispersion compensator based on a fiber bragg grating written in a tapered fiber, Photonics Technology Letters, IEEE 16, 2631-2633.

Ngo, N. Q., S. Y. Li, R. T. Zheng, S. C. Tjin, and P. Shum, (2003), Electrically tunable dispersion compensator with fixed center wavelength using fiber bragg grating, Lightwave Technology, Journal of 21, 1568-1575.

Orucevic, Fedja, Valérie Lefèvre-Seguin, and Jean Hare, (2007), Transmittance and near-field characterization of sub-wavelength tapered optical fibers, Opt. Express 15, 13624-13629.

Schwelb, O., (2004), Transmission, group delay, and dispersion in single-ring optical resonators and add/drop filters-a tutorial overview, Lightwave Technology, Journal of 22, 1380-1394.

Sumetsky, M., (2008), Basic elements for microfiber photonics: Micro/nanofibers and microfiber coil resonators, J. Lightwave Technol. 26, 21-27.

Sumetsky, M., Y. Dulashko, J. M. Fini, and A. Hale, (2005), Optical microfiber loop resonator, Applied Physics Letters 86, 161108-161108-3.

Sumetsky, M., Y. Dulashko, J. M. Fini, A. Hale, and D. J. DiGiovanni, (2006), The microfiber loop resonator: Theory, experiment, and application, Lightwave Technology, Journal of 24, 242-250.

Sumetsky, M., Y. Dulashko, and S. Ghalmi, (2010), Fabrication of miniature optical fiber and microfiber coils, Optics and Lasers in Engineering 48, 272-275.

Tong, Limin, Rafael R. Gattass, Jonathan B. Ashcom, Sailing He, Jingyi Lou, Mengyan Shen, Iva Maxwell, and Eric Mazur, (2003), Subwavelength-diameter silica wires for low-loss optical wave guiding, Nature 426, 816-819.

Vienne, G., Li Yuhang, and Tong Limin, (2007), Effect of host polymer on microfiber resonator, Photonics Technology Letters, IEEE 19, 1386-1388.

Vienne, Guillaume, Aurélien Coillet, Philippe Grelu, Mohammed El Amraoui, Jean-Charles Jules, Frédéric Smektala, and Limin Tong, (2009), Demonstration of a reef knot microfiber resonator, Opt. Express 17, 6224-6229.

Wu, Yu, Yun-Jiang Rao, Yi-huai Chen, and Yuan Gong, (2009), Miniature fiber-optic temperature sensors based on silica/polymer microfiber knot resonators, Opt. Express 17, 18142-18147.

Xu, Fei, and Gilberto Brambilla, (2007), Embedding optical microfiber coil resonators in teflon, Opt. Lett. 32, 2164-2166.

Xu, Fei, Gilberto Brambilla, and David J. Richardson, 2006, Adiabatic snom tips for optical tweezers.

Xu, Fei, Peter Horak, and Gilberto Brambilla, (2007), Optical microfiber coil resonator refractometric sensor, Opt. Express 15, 7888-7893.

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Xu, Fei, Valerio Pruneri, Vittoria Finazzi, and Gilberto Brambilla, (2008), An embedded optical nanowire loop resonator refractometric sensor, Opt. Express 16, 1062-1067.

Yang, Szu-Wen, and Hung-Chun Chang, (1998), Numerical modeling of weakly fused fiber-optic polarization beamsplitters. I. Accurate calculation of coupling coefficients and form birefringence, Lightwave Technology, Journal of 16, 685-690.

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Zhang, J., P. Shum, X. P. Cheng, N. Q. Ngo, and S. Y. Li, (2003), Analysis of linearly tapered fiber bragg grating for dispersion slope compensation, Photonics Technology Letters, IEEE 15, 1389-1391.

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7

Doped Fiber Amplifier Characteristic Under Internal and External Perturbation

Siamak Emami1, Hairul Azhar Abdul Rashid2, Seyed Edris Mirnia1, Arman Zarei1,

Sulaiman Wadi Harun1 and Harith Ahmad1 1University of Malaya Malaysia,

2Multimedia University Malaysia, Malaysia

1. Introduction Significant effort has been made in recent years to improve the Doped Fiber amplifier gain and noise figure. Extend the optical bandwidth of doped fiber amplifiers beyond the traditional 1550nm band, making the excellent EDFA characteristics available in a wider spectral region also was the main effort in optical amplifier fields. Several techniques have been developed to improve gain and shift the gain to the shorter wavelength region. In this chapter, the effects of external perturbation such as macro-bending and fiber length and internal perturbation such as transversal distribution profile and doped concentration on doped fiber performance have been demonstrated (S.D.Emami et al., 2010). A macro-bending approach is demonstrated to increase a gain and noise figure at a shorter wavelength region of EDFA. The conventional double-pass configuration is used for the EDFA to obtain a higher gain with a shorter length and lower pump power. The macro-bending suppresses the ASE at longer wavelength to achieve a higher population inversion at shorter wavelengths. Without the bending, the peak ASE at 1530nm, which is a few times higher than the ASE at the shorter wavelength, would deplete the population inversion and suppresses the gain in this region. Macro-bending is introduced as a new method to increase gain flatness and bandwidth of EDFA in C-band region. Varying the bending radius and doped fiber length leads to the optimized condition with flatter and broader gain profile. Under the optimized condition, gain at shorter wavelengths is increased due to increment of population inversion which results in gain reduction in the longer wavelength regions. The balance of these two effects in the optimized condition has a significant result in achieving a flattened and broadened gain profile. This technique is also capable to compensate the fluctuation in operating temperatures due to proportional temperature sensitivity of absorption cross section and bending loss of the aluminosilicate EDF . This new approach can be used to design a temperature insensitive EDFA for application in a real optical communication system which operates at different environments but still maintaining the gain characteristic regardless of temperature variations. The effect of macro-bending on high concentration EDFA using optimized

Doped Fiber Amplifier Characteristic Under Internal and External Perturbation

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Selected Topics on Optical Amplifiers in Present Scenario 126

bending radius and length of the doped fiber is demonstrated. This gain increment compensates the gain reduction of the EDF before applying macro-bending and result in a flat and broad gain spectrum. One of the many EDFA optimization parameters reported includes the Erbium Transversal Distribution Profile (TDP). The Erbium TDP is essential in determining the overlap factor, which affects the absorption and emission dynamics of the EDFA. At the end of this chapter, numerical models of different Erbium TDP is demonstrated and later verified by experiment. The model considers the overlap factor and absorption/ emission dynamics for different Erbium TDP. Results indicate a high performance EDFA is achievable with an optimized and yet realistic Erbium TDP.

2. Macrobending effects on doped fiber amplifier In the first part of this chapter, a macrobending approach is demonstrated to increase the gain and noise figure at a shorter wavelength region of EDFA. The conventional double pass configuration is used for the EDFA to obtain a higher gain with a shorter length and lower pump power. The macrobending suppresses the ASE at a longer wavelength to achieve a higher population inversion at shorter wavelengths. Without the bending, the peak ASE at 1530 nm, which is a few times higher than the ASE at the shorter wavelength, would deplete the population inversion and suppresses the gain in this region (Harun et al., 2008). The configuration of the EDFA is based on a standard double-pass configuration, where a circulator was used at the input and output ends of the EDF to couple light out of the amplifier and to allow the double propagation of light in the gain medium, respectively. The EDF is pumped by a 980-nm laser diode using a propagating pump scheme. The commercial EDF used is 15 m long with an erbium ion concentration of 440 ppm. A tunable laser source is used to characterize the amplifier in conjunction with an optical spectrum analyzer (OSA). The amplifier is characterized in the wavelength region between 1480 to 1560 nm in terms of the gain and noise figure under changes in the optical power. Before the amplifier experiment, the optical loss of the EDF was characterized for both cases with and without macrobending. The macrobending is obtained by winding the EDF in a bobbin with various radiuses between 0.35 and 0.50 mm (Daud, et al. 2008). The optical losses of EDF were measured against wavelengths at various radius of macro-bending and the result is compared to the straight EDF. Then the bending loss spectrum (dB/m) is obtained by taking the difference of the optical loss measurement between bent and straight EDF. Fig. 1 shows the bending loss spectrum at various bending radius between 0.35 to 0.50 mm. The experimental result is in agreement with the earlier reported theoretical prediction on bending loss in optical fiber (Thyagarajan & Kakkar, 2004), which uses a simple infinite cladding model. The theoretical result shows that the bending loss profile is almost exponential with respect to wavelength, with strong dependencies on fiber bending radius and refractive index profile. Bending of optical fiber, including EDF causes the propagating power of the guided modes to be transferred into cladding, which in turn resulted in loss of power and therefore the bending loss spectrum is obtained as shown in Fig. 2. The bending loss has a strong spectral variation because of the proportional changes of the mode field diameter with the signal wavelength. At bending radius of 0.40 mm, the experimental result shows that the bending loss is drastically increase (>10 dB/m) at the wavelengths above 1505 nm whereas the minimal loss is observed at the wavelengths below 1505 nm. This provides the ASE suppression of more than 270 dB at 1530 nm, which allows


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Doped Fiber Amplifier Characteristic Under Internal and External Perturbation 127

a higher attainable gain at a shorter wavelength region. This result shows that the distributed ASE filtering can be achieved by macro-bending of the fiber at an optimally chosen radius. This characteristic can be used in research of S-band EDFA and fiber lasers (Daud, et al. 2008).

Fig. 1. EDF bending loss profile (dB/m) against wavelength (nm) for different bending radius (3.5 mm, 4 mm and 5 mm).

Fig. 2. Gain (solid symbols) and noise figure (hollow symbols) spectra with and without the macro-bending effect. The input signal and pump power is fixed at -30dBm and 100mW, respectively.

Fig. 2 shows the variation of gain and noise figure across the input signal wavelength for the double-pass EDFA with and without the macro-bending. The input signal and 980nm pump powers is fixed at -30 dBm and 100 mW respectively. The bending radius is set at 4 mm in case of the amplifier with the macro-bending. As shown in the figure, the gain enhancements of about 12 ~ 14 dB are obtained with macro-bending at wavelength region between 1480 nm and 1530 nm. This enhancement is attributed to macro-bending effect

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which suppresses the ASE at the longer wavelength. This resulted in an increase of population inversion at shorter wavelength, which in turn improves the EDFA’s gain at the shorter wavelength as shown in Fig. 2. With the macro-bending, the positive gain is observed for input signal wavelength of 1516 nm and above. On the other hand, the macro-bending also reduces the noise figure of the EDFA at wavelengths shorter than 1525nm as shown in Fig. 2. Fig. 3 shows the gain and noise figure as a function of 980nm pump power with and without the macro-bending. In this experiment, the input signal power and wavelength is fixed at -30 and 1516nm, respectively. The bending radius is fixed at 4 mm. As shown in the figure, the macro-bending improves both gain and noise figure by approximately 6 dB and 3 dB, respectively. These improvements are due to the longer wavelength ASE suppression by the macro-bending effect in the EDF. With the macro-bending, the double-pass EDFA is able to achieve a positive gain with pump power of 90 mW and above. These results show that the bending effect can be used to increase the gain at a shorter wavelength, which has potential applications in S-band EDFA and fiber lasers. The operating wavelength of EDF fiber laser is expected can be tuned to a shorter wavelength region by the macro-bending.

Fig. 3. Gain (solid symbols) and noise figure (hollow symbols) against pump power for EDFAs with and without the macro-bending effect.

3. Application of Macro-Bending Effect on Gain-Flattened EDFA The configuration of the single pass Macro Bent EDFA used in this research is shown in Fig. 4, which consists of a piece of EDF, a wavelength division multiplexing (WDM) coupler, and a pump laser. An Aluminosilicate host EDF with 1100 ppm erbium ion concentration is used in the setup. Alumina in this fiber is to overcome the quenching effect for high ion concentration. A WDM coupler is used to combine the pump and input signal. Optical isolators are used to ensure unidirectional operation of the optical amplifier. Laser pump power at 980nm is used for providing sufficient pumping power. The EDF is spooled on a rod of 6.5 mm radius to achieve consistent macro-bending effect. The rod has equally spaced threads (8 threads per cm) where each thread houses one turn of EDF to achieve consistency in the desired bending radius. Tunable laser source (TLS) is used to characterize the amplifier in conjunction with the optical spectrum analyzer (OSA) (Hajireza, et al. 2010).

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Fig. 4. Configuration of the single-pass EDFA

Initially, the gain and noise figure of the single pass EDFA is characterized without any macro-bending at different EDF lengths. The input signal power is fixed at -30 dBm and the 980 nm pump power is fixed at 200 mW. The wavelength range is chosen between 1520 nm and 1570 nm which covering the entire C-band. It is important to note that using macro-bending to achieve gain flatness depend on suppression of longer wavelength gains. The EDF length used must be slightly longer than the conventional C-band EDFA to allow an energy transfer from C-band to L-band taking place. This will reduce the gain peak at 1530nm and increases the gain at longer wavelengths. The macro-bending provides a higher loss at the longer wavelengths and thus flattening the gain spectrum of the proposed C-band EDFA. The combination of appropriate EDF length and bending radius, leads to flat and broad gain profile across the C-band region. The bending loss spectrum of the EDF is measured across the wavelength region from 1530 nm to 1570 nm. Fig. 2 illustrates the bending loss profile at bending radius of 4.5 mm, 5.5 mm and 6.5 mm, which clearly show an exponential relationship between the bending loss and wavelength, with strong dependencies on the fiber bending radius. Bending the EDF causes the guided modes to partially couple into the cladding layer, which in turn results in losses as earlier reported. The bending loss has a strong spectral variation because of the proportional changes of the mode field diameter with signal wavelength (Giles et al., 1991) As shown in Fig. 5, the bending loss dramatically increases at wavelengths above 1550 nm. This result shows that the distributed ASE filtering can be achieved by macro bending the EDF at an optimally chosen radius. This provides high ASE or gain suppression around 1560 nm, which reduces the L-band gain. Besides this, lower level suppression of C-band population inversion reduces the effect of gain saturation, providing better C-band gain. Eventually, this characteristic is used to achieve C-band gain flattening in the EDFA (Hajireza, et al. 2010). The gain spectrum of the EDFA is then investigated when a short length of high concentration EDF spooled in different radius. Fig. 6 shows the gain spectrum of the EDFA with 3m long EDF at different spooling radius. The result was also compared with straight EDF. The input signal power and pump power are fixed at -30dBm and 200 mW respectively in the experiment. As shown in the figure, the original shape of the gain spectrum is maintained in the whole C-band region with the gain decreases exponentially at wavelengths higher than 1560nm. Without bending, the peak gain of 28dB is obtained at 1530 nm which is the reference point to find the optimized length. When the EDF was spooled at a rod with 4.5mm and 5.5mm radius, the shape of gain spectra are totally

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Fig. 5. Bending loss (dB/m) vs. wavelength (nm) for different bending radius

changed. Finally after trying different radius, 6.5 mm was the optimized radius for this amplifier. As shown in Fig. 5, bending loss at radius of 6.5 mm is low especially at wavelengths shorter than 1560nm and therefore the gain spectrum maintains the original shape of the gain spectrum for un-spooled EDF (Hajireza, et al. 2010)..

Fig. 6. Efficient length of EDF (3m) in different bending radius for -30dBm input signal]

Fig. 7 shows the gain spectra for the single pass EDFA with unspooled EDF at various EDF lengths. The input signal and 980nm pump powers are fixed at -30 dBm and 200 mW. As the length of the EDF increases, the gain spectrum moves to a longer wavelength region. The C-band photons are absorbed to emit photons at longer wavelength. The overall gain drops at the maximum length of 11m due to the insufficient pump power. Fig. 5 shows the gain spectra for the EDFA with the optimum spooling radius of 6.5 at various EDF lengths.


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Fig. 7. Gain for un-spooled EDF in different length with -30dBm input signal

To achieve a flatten gain spectrum, the EDFA must operate with insufficient 980nm pump, where the shorter wavelength ASE is absorbed by the un-pumped EDF to emit at the longer wavelength. This will shift the peak gain wavelength from 1530nm to around 1560nm. The macro-bending induces a wavelength dependent bending loss that results in higher loss at longer wavelength compared to the shorter wavelength as shown in Fig. 5. In relation to the EDFA, the macro-bending also suppresses the population inversion in C-band and thus reduces the gain saturation effect in C-band. With this reduced gain saturation, the C-band gain will increase. On the other hand, the L-band gain will reduce due to the suppression of L-band stimulated emission induced by macro-bending. The net effect of both phenomena will result in a flattened gain profile as shown in figure 8. Thus, the level of population inversion is dependent on different parameters such as length of fiber, bending radius and erbium ion concentration of the EDF. The same mechanism of distributed ASE filtering is used for S-band EDFA (Wysocki et al., 1998).

Fig. 8. Gain for 6.5mm spooled radius in different length with -30dBm input signal

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Fig. 8 shows a better flattening approach for 9m EDF, where the flattened gain profile is obtained by incremental gain enhancement of about 3 dB and 20 dB at wavelengths of 1550 nm and 1530 nm respectively. This enhancement is attributed to macro-bending effect in the EDF. This incremental gain compensates the incremental gain reduction of the EDFA before applying macro-bending resulting in a flat and broad gain spectrum. The gain variation to gain ratio ΔG/G is generally used to characterize the gain variation, where ΔG and G are the gain excursion and the average gain value, respectively (Wysocki et al., 1997). In order to define the gain flatness of EDFAs, the ΔG/G for the EDFA with and without macro bending is compared between 1530 and 1555 nm under the same condition. The gain variation ΔG/G for this macro-bent EDFA was 0.10 (2.8 dB / 27.84 dB), which is a 50% improvement compared to earlier reports (Uh-Chan et al., 2002). Besides this, we also observe a gain variation within ±1 dB over 25 nm bandwidth in C-band region (S.D.Emami et al., 2009). Fig. 9 compares the gain spectrum of the EDFA with and without macro bending EDF at various input signal power. The input signal power is varied for -10 dBm and -30 dBm. The input pump power is fixed at 200 mW. The EDF length and bending radius is fixed at 9m and 6.5 mm respectively. As shown in the figure, increasing the input signal power decreases the gain but improves the gain flatness. The macro bending also reduces the noise figure of EDFA at wavelength shorter than 1550 as shown in Fig. 10 (Hajireza, et al. 2010).

Fig. 9. EDFA gain spectra; with and without acro bending at various input signal power.


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Fig. 10. EDFS noise figure spectra; with and without macro bending at various input signal power.

4. Modelling of the macro-bent EDFA Macro-bending is defined as a smooth bend of fiber with a bending radius much larger than the fiber radius (Marcuse, 1982). Macro-bending modifies the field distribution in optical fibers and thus changes the spectrum of the wavelength dependent loss. Various mathematical models have been suggested to calculate the bending effects in optical waveguide. Earlier references for bending loss in single mode fibers with step index profiles was developed by Marcuse. According to Marcuse, the total loss of a macro bent fiber includes the pure bending loss and transition loss caused by mismatch between the quasi-mode of the bending fiber and the fundamental mode of the straight fiber (Marcuse, 1976). The analytical expression for a single meter fiber bend loss α can be expressed as follows (Marcuse, 1982):

(1)

where eν=2 , a is the radius of fiber core, R is the bending radius, βg is the propagation constant of the fundamental mode, K(υ-1)(γα) and K(υ+1)(γα) are the modified Bessel functions and V is the well-known normalized frequency, which is defined as (Agrawal, 1997):

(2)

The values of k and γ can be defined as follows (Gred & Keiser, 2000):

)()(

32exp

)(11

232

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aKaKRVe

Rk

vvv

g

NAaV .2

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(3)

(4)

For an optical fiber with length L, bending loss (α) is obtained by: Equation (3) agrees well with our experimental results for

macro-bent single-mode fiber. The macro-bent EDFA is modeled by considering the rate equations of a three level energy system. Fig.11 shows the absorption and emission transitions, respectively in the EDFA considering a three-level energy system with 980 nm pump. Level 1 is the ground level, level 2 is the metastable level characterized by a long lifetime, and level 3 the pump level (Armitage, 1988) . The main transition used for amplification is from the 4I13/2 to 4I15/2 energy levels. When the EDF is pumped with 980 nm laser, the ground state ions in the 4I15/2 energy level can be excited to the 4I11/2 energy level and then relaxed to the 4I13/2 energy level by non-radiative decay. The variables N1, N2 and N3 are used to represent population of ions in the 4I15/2, 4I13/2 and 4I11/2 energy levels respectively. According to Fig. 11 we can write the rate of population as follows (Desurvire, 1994):

(5)

(6)

(7)

(8)

where R13 is the pumping rate from level 1 to level 3 and R31 is the stimulated emission rate between level 3 and level 1. The radiative and non radiative decay from level i to s is represented by RAij and NRAij. The interaction of the electromagnetic field with the ions or the stimulated absorption and emission rate between level 1 and level 2 is represented by W12 and W21 .

Fig. 11. Three level energy system of EDF pump absorption, and signal transitions.

2221 gknk

222

2 kng

aLlL 68.8))2log(exp(10

2212211123311131 NANWNWNRNRdzdN R

3232212211122 NANANWNWdtdN NRR

313 1 31 3 32 3

NRdN R N R N A Ndz

321 NNNNT

4I15/2 N1

N2

N3

RA21

4I13/2

4I11/2

RA31 RA32

NRA21

NRA32

W21

W12 R31R13


127


The stimulated absorption, emission rate and pumping rate are calculated respectively as follows (Desurvire et al., 1990) :

(9)

(10)

(11)

where σPA is the 4I11/2 → 4I15/2 absorption cross sections of the 980 nm forward pumping. σSA and σSE are the stimulated absorption and the stimulated emission cross-section of input signal respectively. PASE is the amplified spontaneous emission (ASE) power and A is the effective area of the EDF. The light-wave propagation equations along the erbium-doped fiber can be established as follows (Parekhan et al., 1988):

(12)

(13)

(14)

Absorption and emission coefficient are essential parameters to know for any types of EDFA modelling. With aid of cutback method the absorption coefficient of fiber was measured experimentally (Hajireza, et al. 2010). For an EDF with uniform radial core doping it is preferred to use the MFD expression developed by (Myslinski et al.,1996). The absorption cross section and emission cross section in room temperature were calculated respectively as follows:

a nt (15)

0( ) ( )exp( )B

h Ea eK T

(16)

a is absorption Cross section that describes the chance of an erbium ion absorbing a

photon at wavelength λ. Cross section is given in terms of area because it represents the area is occupied by each erbium ion ready to absorb. Multiplying this by the number of ions, s, gives the total area of the fiber cross section that has erbium ready to absorb. The overlapping factors between each radiation and the fiber fundamental mode, Г (λ) can be expressed as (Desurvire, 1990):

ASEASESs

ssSA PPPAhv

W)(

12

ASEASESs

ssSE PPPAhv

W)(

21

Pp

PPPA PAhv

R

)(

2 1 2( )( ) ( )2A SEA SE se se A SE s se A SE

dP N N P h N Pdz

PPPAPEPP PPNNdzdP

))(( 12

sssasess PPNNdzdP

))(( 12

128


(17)

(18)

where ω0 is the mode field radius defined by equation (18), a is the core diameter, b is the Erbium ion-dopant radius and V is the normalized frequency. The absorption and emission cross section has shown in fig.12 (Michael & Digonnet, 1990). Background scattering loss and wavelength-dependent bending loss is represented by α (λ). Wavelength-dependent bending losses used in this numerical model for three different bending diameters as shown in Fig. 13. The bending loss spectral profile is obtained theoretically with help of Marcuse formula (Marcuse, 1982).. These bending radius values are chosen because significant bending losses can be observed in the L-band region. The bending loss profile indicates the total distributed loss for different bending radius associated with macro-bending at different EDF lengths. This information is important when choosing the appropriate bending radius to achieve sufficient suppression of the gain saturation effect in L-band region and reduces the energy transfer from C-band to the longer wavelength region (Giles & Digiovanni, 1990).

Fig. 12. Absorption and Emission Cross section.

In order to solve the population rate in steady state condition, the time derivatives of for pump and signal powers, equations are set to zero. All the equations are first order differential equations and the Runge-Kutta method is used to solve these equations. The variables used in the numerical calculation and their corresponding values are shown in Table 1.

20

22

1)( b

e

65.10

429.1237.1761.0VV

a


129


Fig. 13. Bending loss spectral for different bending radius

Parameter Unit ValueNA (typ) 0.22 -λcut-off (typ) 935 [nm]dcore (typ) 3.3 [µm]Doping density 1.6 [×1025 ions/m3]τ (Life time) 10 [ms]Saturation paramter (typ) 7.985 [×1015 /ms]λpump 980 [nm]MFDpump 3.7 [µm]A 1.633x10-11 m2

nclad 1.451 -ncore 1.469 -λsig 1550 [nm]MFDsig 5.3 [µm]Гsig 0.74 -Гpump 0.77 -σSA(λs) 2.9105x10-25 m2

σSE(λs) 4.1188x10-25 m2

σ PA(λp) 2.78x10-25 m2

σ PE(λp) 0.81056x10-25 m2

Δν 3100 GHz

Table 1. Numerical parameter used in the simulation

The bending loss spectrum of the EDF is measured across the wavelength region from 1530 nm to 1570 nm. Fig. 14 illustrates the bending loss profile at bending radius of 4.5 mm, 5.5 mm and 6.5 mm, which clearly show an exponential relationship between the bending loss and wavelength, with strong dependencies on the fiber bending radius. Bending the EDF causes the guided modes to partially couple into the cladding layer, which in turn results in

130


losses as earlier reported. The bending loss has a strong spectral variation because of the proportional changes of the mode field diameter with signal wavelength. As shown in Fig. 14, the bending loss dramatically increases at wavelengths above 1550 nm. This result shows that the distributed ASE filtering can be achieved by macro bending the EDF at an optimally chosen radius. It was important to analysis the bending loss in an optimized C-band amplifier before proceed to the next step. The results as shown in Fig. 15 indicate that 3 meter is optimized length for C- band amplifier. It was also seen that with decreasing length, S-band gain is increasing. This happened because of reduction of inversion in C-band region which allow a peak competition for S-band photons to increase. In general C-band always keeps the gain peak unless for longer lengths.

Fig. 14. Different length of unspooled EDFA for -30dBm input signal

Fig. 15. Efficient length of EDF (3m) in different bending radius for -30dBm input signal (Experimental).


131


The gain spectrum of the EDFA is then investigated when the optimized length of high concentration EDF spooled in different radius. Fig. 16 shows the gain spectrum of the EDFA with 3m long EDF at different spooling radius. The result was also compared with straight EDF. The input signal power and pump power are fixed at -30dBm and 200 mW respectively in the experiment. As shown in the figure, the original shape of the gain spectrum is maintained in the whole C-band region with the gain decreases exponentially at wavelengths higher than 1560nm. Without bending, the peak gain of 28dB is obtained at 1530 nm which is the reference point to find the optimized length. When the EDF was spooled at a rod with 4.5mm and 5.5mm radius, the shape of gain spectra are totally changed. Finally after trying different radius, 6.5 mm was the optimized radius for this amplifier.

Fig. 16. Gain profile of EDFA with and without macro bending at various input signal power.(Experimental)

As shown in Fig. 14, bending loss at radius of 6.5 mm is low especially at wavelengths shorter than 1560nm and therefore the gain spectrum maintains the original shape of the gain spectrum for un-spooled EDF. To achieve a flatten gain spectrum, the unbent EDFA must operate with insufficient 980nm pump, where the shorter wavelength ASE is absorbed by the un-pumped EDF to emit at the longer wavelength. This will shift the peak gain wavelength from 1530nm to around 1560nm. The macro-bending induces bending loss is dependent on wavelength with an exponential relationship and longer wavelength has a higher loss compared to the shorter wavelength. In relation to the EDFA, the macro-bending also increase the population inversion in C-band due to reduction of gain saturation effect in L-band. Since the L-band gain cannot improve more than a limited value due to exposure bending loss, less C band photons will be absorbed by un-pumped ions to emit at L-band. This effect reduced gain saturation in L-band, so the C-band gain will increase. This increment for peak is not more than the optimized C-band EDFA (3m) since at that level the inversion is in the maximum value. Full inversion for bent EDFA take place at longer length

132


due to limited energy transfer to longer wavelength. On the other hand, the L-band gain will reduce due to the suppression of L-band stimulated emission induced by macro-bending. The net effect of both phenomena will result in a flattened gain profile.

Fig. 17. Noise figure profile of EDFA with and with-out macro bending at various input signal power(Experimental)

Fig. 17 compares the gain spectrum of with and with-out macro bending EDF at various input signal power. The input signal power is varied for -10 dBm to -30 dBm. The input pump power is fixed at 200 mW. The EDF length and bending radius is fixed at 9 meter and 6.5 mm respectively. As shown in the figure, increasing the input signal power decreases the gain but improves the gain flatness. The macro bending also reduces the noise figure of EDFA at wavelength shorter than 1550 as shown in Fig. 18. Since keeping the amount of noise low depends on a high population inversion in the input end of the erbium-doped fiber (EDF), the backward ASE power P –ASE is reduced by the bending loss. Consecutively, the forward ASE power P +ASE can be reduced when the pump power P is large at this part of the EDF which is especially undesirable. This is attributed can be described numerically by the following equation (Harun et al., 2010)

1 2

ASEPNFG Gh

(19)

where G is the amplifier’s gain, PASE is the ASE power and hν is the photon energy. Fig 19 indicates the simulation of ASE for standard C-band EDFA (3m) and optimized gain flattened C-band EDFA (9m) after and before bending. ASE here represents population inversion. It clearly explains the gain shifting from longer wavelength to the shorter wavelength due to length increment. Besides effect of bending on gain flattening is explained. Fig 10 is the comparison between standard C-band EDFA with the flattened gain EDFA. We observe a gain variation within ±1 dB over 25 nm bandwidth in C-band region.


133


Fig. 18. Amplified Spontaneous emission (Simulation)

Fig. 19. Comparison of the standard C-band EDFA with the flattened gain EDFA for -30 dB input power(Experimental)

5. Temperature insensitive broad and flat gain EDFA based on macro-bending Recently, macro-bent EDF is used to achieve amplification in S-band region. In this paper, a gain-flattened C-band EDFA is proposed using a macro-bent EDF. This technique is able to compensate the EDFA gain spectrum to achieve a flat and broad gain characteristic based on distributed filtering using a simple and low cost method. This technique is also capable to compensate the fluctuation in operating temperatures due to proportional temperature sensitivity of absorption cross section and bending loss of the aluminosilicate EDF. This new approach can be used to design a temperature insensitive EDFA for application in a real optical communication (Hajireza, et al. 2010).

134


The bending loss profile of the erbium-doped fiber (EDF) for various bending radius is firstly investigated by conducting a simple loss- test measurement. In order to isolate the bending loss, the profile is obtained by taking the difference between the loss profile of the same EDF with and without macro-bending across the desired wavelength range. A one meter EDF is used in conjunction with a tunable laser source (TLS) and optical power meter to characterize the bending loss for bending radius of 6.5 mm at wavelength region between 1530 nm and 1570 nm. The bending loss profile indicates the total distributed loss for different bending radius associated with macro-bending at different EDF lengths. This information is important when choosing the optimized bending radius to achieve sufficient suppression of the gain. Fig. 20 illustrates the bending loss profile at bending radius of 6.5 mm at different temperatures, which clearly show an exponential relationship between the bending loss and wavelength. It is also shown that the bending loss in L-band is reduced by increasing the temperature. Bending the EDF causes the guided modes to partially couple into the cladding layer, which in turn results in losses as earlier reported. The bending loss has a strong spectral variation because of the proportional changes of the mode field diameter with signal wavelength. As shown in Fig. 20, the bending loss dramatically increases at wavelengths above 1550 nm. This result shows that the distributed ASE filtering can be achieved by macro bending the EDF at an optimally chosen radius.

Fig. 20. Loss spectrum of the bent EDF with 6.5 mm bending radius at different temperatures.

Initially, the gain of the single pass EDFA is characterized without any macro-bending at different EDF lengths as shown in Fig. 21. The input signal power is fixed at −30 dBm and the 980 nm pump power is fixed at 200 mW. The wavelength range is chosen between 1520 nm and 1570 nm which cover the entire C-band region. To achieve a flattened gain spectrum, the unbent EDFA must operate with insufficient 980 nm pump, where the shorter wavelength ASE is absorbed by the un- pumped EDF to emit at the longer


135


wavelength. This will shift the peak gain wavelength from 1530 nm to around 1560 nm. Therefore The EDF length used must be slightly longer than the conventional C- band EDFA to allow an energy transfer from C-band to L-band taking place. This will reduce the gain peak at 1530 nm and increases the gain at longer wavelengths. As shown in Fig. 21, the optimum C-band operation is successfully achieved using only one meter of this high erbium ion concentration EDF. It is also shown that for the lengths longer than 2m gain shifts to longer wavelengths. Figure 22 shows the gain spectrum of the C-band EDFA, which is characterized with macro-bending at different EDF lengths. In the experiment, the input signal power is fixed at -30dBm and the 980nm pump power is fixed at 200mW. These lengths are chosen due to their gain shift characteristics as depicted in Fig. 21. It is important to note that using macro-bending to achieve gain flatness depend on suppression of longer wavelength gains. The macro-bending provides a higher loss at the longer wavelengths and thus flattening the gain spectrum of the proposed C-band EDFA. The combination of appropriate EDF length and bending radius, leads to flat and broad (Hajireza, et al. 2010).

Fig. 21. Gain spectrum of the C-band EDFA

6. Effects of erbium transversal distribution profiles on EDFA performance Over the past years, Erbium-doped fiber amplifiers (EDFAs) have received great attention due to their characteristics of high gains, bandwidths, low noises and high efficiencies. As a key device, EDFA configures wavelength division multiplexing systems (WDMs) in optical telecommunications, finding a variety of applications in traveling-wave fiber amplifiers, nonlinear optical devices and optical switches. The EDFA uses a fiber whose core is doped with trivalent erbium ions as the gain medium to absorb light at pump wavelengths of 980 nm or 1480 nm and emit at a signal wavelength band around 1500 nm through stimulated

136


emission. Theoretical study on optimization of rare-earth doped fibers, such as fiber length and pump power has grown along with their increased use and greater demand for more efficient amplifiers (Emamai et al., 2010). Previously, one of the most important issues in improving fiber optic amplifier performance is optimization of the rare-earth dopant distribution profile in the core of the fiber. Earlier approaches to numerical modeling of EDFA performance have assumed that the Erbium Transversal Distributions Profile (TDF) follow a step profile. Only the portion of the optical mode which overlaps with the erbium ion distribution will stimulate absorption or emission from erbium transitions. The overlap factor equation is defined by (Desurvire, 1982):

(20)

Ψ(r, υ) is the LP01 fiber optic mode envelope, which is almost Gaussian and is defined as :

(21)

where J0 and K0 are the respective Bessel and modified Bessel functions and uk and wk are the transverse propagation constants of the LP01 mode. NT is total dopant concentration per unit per length which is defined by:

(22)

Various profiles of erbium transversal distributions can be used for describing mathematical function of EDFA. Two main requirement on choosing erbium transversal distribution functions are; flexibility to be adapted to a collection of profile as broad as possible and dependence on a number of the parameters as low as possible. The optimum transversal distribution function should be (Yun et al., 1999):

(23)

where nT,maz is the value of the maximum erbium concentration per unit volume. β, θ and δ are distribution profile parameters which construct the profile shapes. θ and β are defined as dopant radius and the roll-off factor of the profile respectively. In practical, it would seem difficult to maintain a high concentration of Erbium in the center of the core, due to diffusion of erbium ions during the fabrication. Modifying the δ value, low ion concentration at the core center can be achieved. The Erbium distribution profiles of EDFA with different values β, θ and δ are depicted in Fig. 22. Figure 22(a) shows several of β values with fixed values of θ=1 and δ=1.5. Figure 22(b) shows the effect of θ values with fix values of δ=1.5 and β=1 while figure 22(c) shows several values of δ changes with fixed values of θ=1 and β =1.5.

drrrnrN TT

)(),(2)(0

ararwK

wKuj

ararujr

kk

k

k

)/()()(

)/(),( 2

020

20

20

drrrnN TT

)(20

,max( ) exp{ [| | / ] }T Tn r n r


137


(a)

(b)

(c)

Fig. 22. The Erbium distribution profile of EDFA (a) at different values of β when, θ=1 and δ=1.5 (b) at different values of when δ=1.5 and β=1. (c) at different values of δ when θ=1 and β =1.5.

138


Figs. 23(a) and (b) demonstrate the gain and noise figure trends of EDFA, respectively at different and values of fiber. The input signal power and input pump power is fixed at -30 dBm and 100 mW respectively while the EDF length is fixed at 14m.

(a)

(b)

Fig. 23. (a) Gain profile of EDFA at =0 (b) Noise figure of EDFA =0

Fig. 24 shows the gain trends of EDFA at different and δ values of fiber. The input signal power and input pump power is fixed -30 dBm and 100 mW respectively. The EDF length is 14m long. By comparison between overlap factor and gain results, it is intuitive that the gain result follows the overlap factor values of the fiber. In the low values of the fiber in the same EDF concentration the gain decrease by decreasing the as depicted on figure 24, this is the results of high erbium intensity at the core and the quenching effect on the fiber amplifier.


139


Fig. 24. (a) Gain profile of EDFA at =2 (b) Noise figure of EDFA =2

The effect of Erbium transversal distribution profile on the performance of an EDFA is investigated. The EDFA uses a 14m long EDF as the gain medium, which is pumped by a 980 nm laser diode via a WDM coupler. An optical isolator is incorporated in both ends of optical amplifier to ensure unidirectional operation. Two types of EDF with the same fiber structure and doping concentration but different on distribution profile are used in the experiment. Fig. 25 shows the Erbium transversal distribution profile of both fibers, which have a doping radius of 2 μm and 4 μm as shown in Figs. 25(a) and (b), respectively. In the experiment, the input signal power and 980nm pump power are fixed at -30dBm and 100 mW respectively.

140


Fig. 25. Erbium TDP. (a) 2μm doping radius (b) 4μm doping

Fig. 26 compares the experimental and numerical results on the gain characteristics for both EDFAs with the 2μm and 4μm doping radius. As expected from the theoretical analysis, the amplifier’s gain is higher with 2μm doping radius compared to that of 4μm doping radius at the 1550 nm wavelength region. In the simulation, fiber distribution profile parameters are set as θ=2, β=1.5 and δ=0 for 2μm doping radius which for 4μm doping radius, fiber distribution profile parameters are set as θ=4, β=4 and δ=0.8. The numerical gain is observed to be slightly higher than the experimental one. This is most probably due to splicing or additional loss in the cavity, which reduces the attainable gain. In the case of 4 μm dopant radius, the overlap factor is higher since the overlap happens throughout the core region. However, the high overlap factor will affect the erbium absorption of both the pump and signal. If one considers the near Gaussian profile of the LP01 mode, the erbium in the outer radius of the core tend to be less excited due to the lower pump intensity. The remaining Erbium ions absorption capacity in the outer radius of the core will be channeled to absorbing the signal instead. In the case of 2 μm dopant radius, the overlap factor is lower since the overlap happens only in the central part of the core region. If one considers the near Gaussian profile of the LP01 mode, the erbium in the inner radius of the core tend to be more excited due to the higher pump intensity. Since the outer radius of the core is not doped with Erbium, the lower intensity pump in the outer radius will not be absorbed. The advantage of reduced doping region is that the Erbium absorption only takes place in the central part of the core. Since, the pump intensity is the highest here; the Erbium population can be totally inverted, thus contributing to higher gain. Furthermore, the signal in the outer radius will no longer be absorbed. Hence, the signal will receive a net emission from the erbium which then contributes to higher gain.


141


Fig. 26. Numerical and experimental gain comparison of 2μm and 4μm doping radius EDFA

7. Conclusion In this reserch work a macro-bending approach is demonstrated to increase a gain and noise figure at a shorter wavelength region of EDFA. In the conventional double-pass EDFA configuration , macro-bending improves both gain and noise figure by approximately 6 dB and 3 dB, respectively. These improvements are due to the longer wavelength ASE suppression by the macro-bending effect in the EDF. A new approach is proposed at secound section to achieve flat gain in C-band EDFA with the assistance of macro-bending. The gain flatness is optimum when the bending radius and fiber length are 6.5 mm and 9 meter respectively. This simple approach is able to achieve ±1 dB gain flatness over 25 nm. This cost effective method, which improves the gain variation to gain ratio to 0.1, does not require any additional optical components to flatten the gain, thus reducing the system complexity. The proposed design achieves temperature insensitivity over a range of temperature variation. The gain flatness is optimized when the bending radius and fiber length are 6.5mm and 2.5m respectively. This simple approach is able to achieve 0.5 dB gain flatness over 35nm with no dependency on temperature variations. It is a cost effective method which needs 100mW pump power and does not require any additional optical components to flatten the gain, thus reducing the system complexity. At the end the effect of ETP on the performance of the EDFA is theoretically and experimentally investigated. The ETP can be used to optimize the overlap factor, which affects the absorption and emission dynamics of the EDFA and thus improves the gain and noise figure characteristics of the amplifier. It is experimentally observed that the 1550 nm gain is improved by 3 dB as the doping radius is reduced from 4μm to 2μm. This is attributed to the Erbium absorption

142


takes place in the central part of the core where the pump intensity is the highest and thus increases the population inversion.

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7

Ultra-Wideband Multiwavelength Light Source Utilizing Rare Earth

Doped Femtosecond Fiber Oscillator Nurul Shahrizan Shahabuddin1,

Marinah Othman2 and Sulaiman Wadi Harun1 1Photonics Research Centre, University of Malaya,

2Multimedia University, Malaysia

1. Introduction Multiwavelength sources are expected to play a major role in future photonic networks, where optical time-division multiplexing (OTDM) and wavelength division multiplexing (WDM) are employed (Teh et al., 2002; Ye et al., 2010; Liu et al., 2006; Parvizi et al., 2011; Harun et al., 2010; Zhong et al., 2010). Various approaches have been taken to develop this source such as by exploiting the nonlinear effects in an optical fiber as well as spectral slicing of a supercontinuum source.

Stimulated Brillouin scattering (SBS) and four wave mixing (FWM) effects are normally used to realize a multiwavelength output whereby the frequency shifts are determined by both the optical fiber structure and pump signal (Chen et al., 2010; Shahi et al., 2009a,2009b; Shahabuddin et al., 2008). A multiwavelength Brillouin fiber laser exhibits wavelength spacing of approximately 0.08 nm depending on the type of material used while FWM frequency shift is known to be dependent on the spacing of the pump signals (Johari et al., 2009; Shahi et al., 2009b; Ahmad et al., 2008). The bandwidth of operation however, is relatively narrow due to the limitation imposed by the erbium doped fiber. Previously, multiwavelength sources also have been demonstrated using superstructure Bragg grating and Fabry perot filter, both have limited range of multiwavelength region and spacing (Teh et al., 2002).

A technique known as spectral slicing can be realized using an arrayed waveguide grating or sagnac loop mirror as the wavelength selective component to slice a broad emission spectrum of amplified spontaneous emission or supercontinuum. It has been shown in many earlier works that slicing of a broadband continuum spectra is capable of generating a multiwavelength laser comb with both spacing tunability and a very wide spectral range (Nan et al., 2004). The supercontinuum light can be produced from a pulsed laser by using the interaction of multiple nonlinear effects such as self-phase modulation (SPM), four-wave mixing (FWM) and stimulated Raman scattering (SRS) in a highly nonlinear optical fiber (Buczynski et al., 2009, 2010; Kurkov et al., 2011; Genty et al., 2004; Lehtonen et al., 2003; Chen et al., 2011; Gu et al., 2010; Akozbek et al., 2006). Spectral slicing allows the channel spacing to be adjusted by changing the length of polarization maintaining fiber (PMF) used in the loop mirror (Ahmad et al., 2009). Hence, spectral slicing provides better spacing

Ultra-Wideband Multiwavelength Light Source Utilizing Rare Earth Doped Femtosecond Fiber Oscillator

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Semiconductor Laser Diode Technology and Applications 120

tunability as compared to other multiwavelength techniques such as stimulated brillouin scattering as the spacing of the latter highly depends on the waveguide material and structure i.e. the spacing is fixed.

In this chapter, we report a new multiwavelength source using supercontinuum slicing technique. This source use rare earth doped femtosecond fiber oscillator to generate supercontinuum, with multiwavelength operation achieved via spectral slicing of the supercontinuum. Through this technique, the multiwavelength region obtained is the broadest ever to date.

2. Development of mode-locked fiber laser Mode-locked lasers have found widespread uses in many areas of research, medicine, and industry. The potential of making compact, rugged laser systems with low power consumption at a relatively low cost makes such fiber laser systems very promising candidates for those applications. The stability of the laser is a crucial factor for its applicability. In negative dispersion regime, a soliton pulse is maintained by the interaction of dispersion and nonlinear effect. The bandwidths of both gain fiber and mode-locked filter are neglected (Wise et al., 2008). However, in a normal dispersion fiber laser, the dissipative soliton is maintained by the effects of dispersion, nonlinearity, gain, and spectral filtering (Zhao et al., 2007; Chong et al., 2007). Accordingly, the gain-bandwidth and mode-locked filter play important roles for pulse reshaping and stability (Chong et al., 2007). A stable mode-locked fiber laser can be achieved with the use of polarization maintaining (PM) fibers and a semiconductor saturable-absorber mirror (SESAM) as the mode-locking mechanism. A setup requiring no free-space optics is highly attractive, as it increases stability, and reduces both costs, and the need for maintenance. The fiber mode-locked lasers reported to date obtains mode-locking based on the use of either SESAM or nonlinear polarization rotation (NPR) mechanism (Huan et al., 2006; Harun et al., 2011).

A saturable absorber absorbs the incoming light linearly up to a certain threshold intensity, after which it will saturate and becomes transparent. The recovery time of a semiconductor saturable absorber limits the laser repetition rate. Carbon nanotube (CNT) based saturable absorbers exhibit sub-picosecond recovery times, broadband operation, compatibility with fibers, a small footprint and is simple to fabricate besides being operable in either a transmission or a reflection mode, making them preferable over the more established SESAMs for commercial applications.

Various techniques to achieve stabilized laser sources such as by suppressing the supermode noise, starting the pulse in a laser, and compensating the small distortion caused by the gain fiber, have been reported (Li et al., 1998; Y. Li et al., 2001). In this section, an environmentally stable mode-locked fiber laser based on both NPR and a single wall carbon nanotube saturable absorber (SWCNT-SA) with zero use of free-space optics is demonstrated. The soliton pulse in the cavity can be reshaped and maintained by combining both these mode-locking mechanisms NPR and SWCNT-SA that are based on power-dependent filter. The configuration of our proposed fiber laser is shown in Figure 1. A 10 m long erbium-doped fiber (EDF), which is pumped by a 980-nm laser diode through a 980/1550 nm wavelength division multiplexer (WDM) is used as the gain medium. The EDF has a mode field diameter of 10.4 μm with maximum peak absorption of approximately -10


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Ultra-Wideband Multiwavelength Light Source Utilizing Rare Earth Doped Femtosecond Fiber Oscillator 121

dB/m at 1550 nm. The group velocity dispersion (GVD) is estimated to be around -42 ps/nm.km at 1550 nm. The other part of the ring cavity uses a 20 m long standard single mode fiber (SMF-28) with a dispersion of 17 ps/nm.km at λ=1545 nm. The net cavity GVD is negative, which enables soliton shaping in the laser. A standard isolator is used to ensure unidirectional operation and to act as a polarizer. A squeezed fiber type polarisation controller (PC) is used to control the polarization states within the cavity. In this experiment, the SWCNT-SA film sandwiched between two FC/PC fiber ferrules to form the fiber-integrated saturable absorber is used for the initiation and stabilization of mode-locking at around 1561 nm region. The SWCNT-SA is fabricated by chemical vapor deposition (CVD) method with an average diameter of 0.8~0.9 nm. The output optical spectrum of the EDFL is monitored by an optical spectrum analyzer (OSA), while the output pulse train and pulse duration are measured using an oscilloscope and an autocorrelator, respectively.

Fig. 1. Experimental setup of the mode-locked fiber laser.

To achieve mode-locking operation, the polarization state of the light within the cavity should be adjusted by using the PC. The pump power threshold for continuous wave laser operation is around 11 mW and a stable self-starting fundamental mode-locked (ML) operation commences at around 140 mW pump power. By altering the polarization state inside the cavity, the optical spectrum can be broadened to initiate Q-switching from which mode-locking operation is obtained. Figure 2 shows the pulse train observed at the output coupler after making both a careful adjustment to the imposed loss accompanied by the variation of the net birefringence of the cavity using a PC. As shown in the figure, a stable fundamental soliton pulse is obtained with a repetition rate of 142 MHz.

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Fig. 2. Pulse train of the proposed soliton EDFL.

The spectral and temporal outputs of the proposed soliton fiber laser are also characterized using an OSA and an autocorrelator. The output spectrum of the mode-locked EDFL obtained from the output port of the coupler is shown in Figure 3. It is clearly seen that the output pulse has a 3 dB bandwidth of 4.5 nm centered at 1558 nm. Kelly sidebands are also evident indicating that the output laser is soliton pulses. These sidebands are a kind of resonant coupling, which for some optical frequencies occur when the relative phase of soliton and dispersive wave changes by an integer multiple of 2π per resonator round trip. The strong Kelly sidebands obtained suggest that the pulse duration is near the minimum possible value. Figure 4 shows an autocorrelation trace of the output pulse from the mode-locked EDFL. The pulse duration is 0.79 ps assuming that the pulse shape follows a sech2 profile. For the purpose of testing the stability of this laser, we kept the laser in the fundamental mode-locked operation and pumped the power at 140 mW without disturbance for a few hours. It was observed that there was no significant change in the spectrum, central wavelength, 3 dB spectrum width and pulse width, output power and repetition rate. This indicates that a stable soliton fiber laser can be achieved using mode-locking based on the combination of both SWNT-SA and NPR.

Passively mode-locked fiber laser can be constructed based on nonlinear polarization rotation without the saturable absorber. This makes a simpler configuration.In this section, a simple mode-locked Bismuth-based Erbium-doped fiber laser (Bi-EDFL) is achieved by using a simple ring cavity structure incorporating a Bismuth-based erbium-doped fiber (EDF), an isolator and a polarisation controller. The short Bi-EDF making up the gain cavity allows the generation of both stable and clean pulses with an increase in the repetition frequency, while its high nonlinearity allows better suppression of the supermode noise. To date, this is the first demonstration of a passively mode-locked fiber laser, which uses such a short length of Bi-EDF together with the nonlinear polarisation rotation (NPR) method.


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Fig. 3. Optical spectrum of the output of a soliton fiber laser, exhibiting Kelly sidebands.

Fig. 4. Autocorrelation trace of the output pulse from the mode-locked EDFL

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The experimental set-up of the proposed system for mode-locked Bi-EDFL is illustrated in Figure 5. The Bi-EDFL employs a piece of 49 cm long Bi-EDF, a wavelength division multiplexing (WDM) coupler, an isolator, a polarization controller (PC) and a 3 dB output coupler. The Bi-EDFL cavity consists of a 49 cm long Bi-EDF, and a 2.0 m long SMF-28 which is used in the cavity that is composed of a coupler, polarisation controller, isolator and 980/1550 nm WDM coupler. The Bi-EDF has a nonlinear coefficient of ~60 (W/km)−1 at 1550 nm, an Erbium concentration of 3250 ppm, a cut-off wavelength of 1440 nm, a pump absorption rate of 130 dB/m at 1480 nm and a dispersion parameter of 130 ps/km.nm at λ=1550 nm. It is bi-directionally pumped using a 1480 nm laser diode via the WDM to provide an amplification in the C-band region. The other part of the ring cavity uses a standard single mode fiber (SMF-28) with a dispersion of 17 ps/nm.km at λ=1545 nm. A standard polarisation-independent isolator is used to ensure a unidirectional operation and acts as a polarizer. A PC is used to rotate the polarization state and to allow continuous adjustments of the birefringence within the cavity to balance both the gain and loss for the generation of the laser pulses.

A fraction of the stretched laser pulse operating at 1560 nm is extracted through the 50% output of the coupler. The pulse width and repetition rate of the laser pulse are measured to be around 131 fs and 42 MHz, respectively. The output power and spectrum are measured by a power meter and an optical spectrum analyzer (OSA), respectively. The entire experimental setup is fusion-spliced together.

Fig. 5. Experimental set-up for mode-locked fiber laser.

To achieve mode-locking operation, the polarization state of the light within the cavity should be adjusted by using the PC. By altering the polarization state inside the cavity, the optical spectrum can be broadened to initiate Q-switching from which mode-locking operation is obtained. When a linearly polarized light is incident to a piece of weakly birefringent fiber such as a Bi-EDF, the polarization of the light will generally become elliptically polarized in the fiber. The orientation and ellipticity of the resulting polarization of the light is fully determined by the fiber length and its birefringence. However, if the intensity of the light is strong, the nonlinear optical Kerr effect in the fiber must be considered, thus introducing extra changes to the polarization of the light. As the polarization change introduced by the optical Kerr effect depends on the light intensity, if a polarizer or isolator is placed behind the fiber, the transmission of the light through the


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polarizer will become light intensity dependent. Upon selecting the appropriate orientation of the polarizer, an artificial saturable absorber with an ultra-fast response could then be achieved in such a system, where light of higher intensity experiences less absorption loss on the polarizer. The proposed laser makes use of this artificial saturable absorption to achieve passive mode locking. Once a mode-locked pulse is formed, the nonlinearity of the fiber further shapes the pulse into the ultrashort stretched-pulse.

In the experiment, the 1480 nm pump powers were both fixed at 125 mW. The output spectrum of the mode-locked EDFL obtained after the 3 dB coupler is shown in Figure 6. A broad spectrum with a 3dB bandwidth of 21.7 nm is obtained at the optimum polarization state, which indicates that the output laser has a stretched pulse characteristic. The spectrum has a peak wavelength at 1560 nm. Q-switching operation mode is observed by an oscilloscope in the form of an unstable pulse train with periodic variation in its pulse amplitude. Further adjustment of polarization produces a more stable mode-locked pulse train as shown in Figure 7. The mode-locked pulse train has a constant spacing of 24 ps, which translates to a repetition rate of 42 MHz. The high repetition rate pulse trains are produced from a harmonically mode-locked laser, where multiple pulses circulate within the cavity. The multiple pulses are generated passively in the laser due to the phenomenon known as the soliton energy quantization. The pulse characteristic of the mode-locked EDFL at the 3 dB coupler is also investigated by an autocorrelator. Figure 8 shows the autocorrelator trace of the pulse, which shows the sech2 pulse profile with a full width half maximum (FWHM) of 131 fs. The output of the femtosecond pulses is also observed to be very stable at room temperature. The operation of the Bi-EDFL can be tuned by incorporating a tunable band-pass filter in the ring cavity. By optimizing the length of the Bi-EDF, a wideband tunable operation is expected to be achieved reaching up to the extended L-band region.

Fig. 6. Optical spectrum of the mode-locked laser.

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Fig. 7. Pulse train of the passive mode-locked laser with a repetition rate of 42 MHz.

Fig. 8. Autocorrelator pulse trace with FWHM of 131 fs and Sech2 pulse shape.

3. Supercontinuum generation Supercontinuum generation is the formation of an ultrabroad spectral broadening induced by the coupling of a high peak power sub picosecond pulse laser with a nonlinear optical fiber of adequate length. The supercontinuum spectrum is determined by the sequence of events in the


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supercontinuum generation, and is dependent on both the pump pulse and the fiber characteristics namely the pump pulse wavelength, power, and pulse duration. The dominant nonlinear effects occurring in the generation of an SC includes self-phase modulation, stimulated Raman scattering and soliton effects. Since supercontinuum was first discovered in 1970 by Alfano (Alfano & Shapiro, 1970), many works have been performed to understand the phenomenon as well as its implementation in practical devices where it has found applications in areas of semiconductor, biology, chemistry, optical coherent tomography, sensing and optical communication, femtosecond carrier-envelope phase stabilization, ultrafast pulse compression, time and frequency metrology, and atmospheric science (Hartl et al., 2001; Kano & Hamaguchi, 2003; Mori, 2003; Alfano, 2006). These included the study of primary events in photosynthesis, nonradiative processes in photoexcited chemicals, excitation of optical phonons, carrier dynamics of semiconductor, frequency clocks and broad spectrum LIDAR.

Specialty fibers such as photonic crystal fibers (PCFs) have high nonlinearity with a managed dispersion profile, and thus, can be used to generate supercontinuum (Parvizi et al., 2010; Russell, 2003). The first supercontinuum generation in a microstructured fiber was reported in 2000 by Ranka et al. Zero dispersion and anomalous dispersion regions could have contributed in higher order soliton generation, pulse compression and ultrabroadband continuum extending from the ultraviolet to the infrared spectral regions. In addition, the pulse broadening in PCF is of great interest for its coherence, brightness and low pulse energy required to generate supercontinuum. The most widely used type of PCF consists of pure silica core surrounded by periodic arrays of air holes, where genuine photonic band-gap guidance can occur. PCFs of this type have attracted much interest because of their potential for lossless and distortion-free transmission, particle trapping, optical sensing, and for novel applications in nonlinear optics (Russell, 2000; Wiederhecker et al., 2007; Benabid et al., 2005).

The supercontinuum can be generated using a picosecond to nanosecond pulses, or even a continuous wave pump where spectral broadening is initiated in the so-called “long pulse” regime (Harun et al., 2011b; Wang et al., 2006; Gorbach et al., 2007). Research is now shifting towards supercontinuum using a more robust and cheaper mode-locked laser, achievable with some innovative designs, as well as the study of the supercontinuum process with picosecond or nanosecond pulsed lasers. As were shown in Figure 1 and Figure 5, we experimentally demonstrated two fiber-based mode-locked lasers using two methods for mode-locking; fiber-based nonlinear polarization rotation (NPR) and saturable absorber. These mode-locked fiber lasers could then be implemented in the experiment to generate supercontinuum.

Figure 9 shows the experimental setup used to generate supercontinuum. A high power amplifier is used to increase the seed pulse peak power to a maximum power of 30 dBm to allow apectral broadening. At the output of the amplifier, the pulse enters a PCF for supercontinuum generation. The supercontinuum spectrum is measured by an optical spectrum analyzer.

Fig. 9. Schematic diagram of the supercontinuum generation experiment.

Optical Spectrum Analyzer

Amplifier Mode Locked Fiber Laser

Photonic Crystal Fiber

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Figure 10 shows an SC generation in a 100 m long PCF which is pumped by an amplified 1560 nm mode locked fiber laser at 30 dBm amplified pulse laser power. The mode locked fiber laser used in the experiment has a pulse width of 24 ps with a repetition rate of 42 MHz. The PCF used in the experiment has a zero and -1.5 ps/(km.nm) dispersion at wavelength of 1550 nm and 1580 nm respectively. The PCF length is fixed at the optimised length of 100 m. The nonlinearity coefficient of the PCF is around 11 W-1km-1.

As shown in Figure 10, we observe a supercontinuum starting from 600 nm up to 2100 nm at the maximum amplified pulse pump power of 30 dBm. The broad supercontinuum is obtained due to the injection of 1560 nm pulse laser in the anomalous-dispersion regime of the PCF. The pulse initially begins to self-Raman shift to longer wavelengths thus causing asymmetry in the supercontinuum spectrum. When these higher-order solitons break up, parametric four-wave mixing generates frequencies at wavelengths shorter than the zero-dispersion wavelength. With increase in the pump power, there is widening of the spectrum as well as marked improvement of the flatness characteristic.

Fig. 10. Supercontinuum generation with 100 m long PCF at fixed average pump power of 30 dBm.

4. Supercontinuum slicing The supercontinuum generated in the PCF is injected into the loop mirror as shown in Figure 11. In the loop mirror, the supercontinuum source is splitted into two by a 3-dB coupler, where one of the light beams travels in a clockwise direction and the other travels in the opposite direction of the polarization maintaining fiber (PMF). Spectral slicing occurs when the beams interfere constructively and destructively due to the phase differences encountered by the two propagating beams in the loop (Ahmad et al., 2009). Both beams are combined at the end of the fiber coupler to act as a comb filter. Figure 12 shows the multiwavelength spectrum measured by a optical spectrum analyzer through slicing the supercontinuum at pulse pump powers of 30 dBm. The spacing, ∆λ is related to the length, L of the PMF used by the following equation


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λ

Δλ =BL

(1)

where B and λ are the birefringence and wavelength, respectively. It is also observed that the comb spectrum is flatter at higher pump power. The spacing of the comb increases as the operating wavelength increases, which agree well with the above equation.

Fig. 11. Loop mirror configuration to slice the supercontinuum source.

Fig. 12. Multiwavelength comb.

Figure 13 shows the superimposed multiwavelength comb spectrum at different wavelength regions for the supercontinuum with 30 dBm pump pulse power. The multiwavelength comb spectra obtained have an average channel spacings of 2.22 nm and 3.03 nm obtained at 1440 nm and 1750 nm regions, respectively. The spacing of the comb increases as the operating wavelength increases, which agree well with the above equation. In addition, the best signal to noise ratio (SNR) obtained is about 20 dB.

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Fig. 13. Sliced spectrum at wavelength regions 1490 nm to 1540 nm and 1750 nm to 1800 nm.

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Chapter 3

Passive Q-switched and Mode-lockedFiber Lasers Using Carbon-basedSaturable Absorbers

Mohd Afiq Ismail, Sulaiman Wadi Harun, Harith Ahmad andMukul Chandra Paul

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61703

Abstract

This chapter aims to familiarize readers with general knowledge of passive Q-switched and mode-locked fiber lasers. It emphasizes on carbon-based saturable ab‐sorbers, namely graphene and carbon nanotubes (CNTs); their unique electronicband structures and optical characteristics. The methods of incorporating these car‐bon-based saturable absorbers into fiber laser cavity will also be discussed. Lastly,several examples of experiments where carbon-based saturable absorbers wereused in generating passive Q-switched and mode-locked fiber lasers are demon‐strated.

Keywords: Fiber laser, passive Q-switch, passive mode-lock, graphene, carbon nanotube

1. Introduction

Graphene and carbon nanotubes are carbon allotropes that have a lot of interesting opticalproperties, which are useful for fiber laser applications. For instance, both allotropes havebroadband operating wavelength, fast recovery time, are easy to fabricate, and can be inte‐grated into fiber laser cavity. As a result, they can function as saturable absorber for generatingQ-switching and mode-locking pulses. There are several techniques of incorporating thesecarbon-based saturable absorbers into fiber laser cavity. This chapter will discuss the advan‐tages and disadvantages of most of the techniques that have been used.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative CommonsAttribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,and reproduction in any medium, provided the original work is properly cited.

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2. Introduction to passive Q-switched and mode-locked fiber laser

2.1. Q-switched fiber lasers

A laser could emit short pulses if the loss of an optical resonator is rapidly switched from ahigh to a low value. By controlling the Q-factor (quality factor) of a laser resonator, Q-switchingallows the generation of laser pulses of short duration (from nanosecond to picosecond range)and high peak power. The Q-factor (dimensionless) is given by:

02 fQPp e

= (1)

where fo is the resonant frequency, ε is the stored energy in the cavity, and P = − dEdt is the power

dissipated. If the Q-factor of a laser’s cavity is abruptly changed from a low value to a highvalue, the laser will emit a pulse of light that is much more intense than the lasers’ continuesoutput. This technique is called Q-switching. There are two types of Q-switching; active andpassive.

Active Q-switching uses modulation devices that change the cavity losses in accordance withan external control signal. They can be divided into three categories: mechanical, electro-optical, and acousto-optics. They inhibit laser action during pump cycle.

In passive Q-switching, the laser consists of gain medium and saturable absorber. Thesaturable absorber absorbs light at low intensity and transmits them at high intensity. As thegain medium is pumped, it builds up stored energy and emits photons. After many round-trips, the photon flux begins to see gain, fixed loss, and saturable loss in the absorber. If thegain medium saturates before the saturable absorber, the photon flux may build, but the laserwill not emit a short and intense pulse. On the contrary, if the photon flux builds up to a levelthat saturates the absorber before the gain medium saturates, the laser resonator will see arapid reduction in the intracavity loss and the laser Q-switches and therefore, will emit a shortand intense pulse of light [1].

2.2. Mode-locked fiber laser

Mode-locking is a technique of generating an ultra-short pulse laser with pulse duration rangesfrom picoseconds (10-12 s) to femtoseconds (10-15 s). An ultra-short pulse can be generated whenall the longitudinal modes have a fixed phase relationship. The fixed phase superpositionbetween all the modes oscillating inside a laser cavity causes the cw laser to be transformedinto a train of mode-locking pulse. The number of longitudinal mode that can simultaneouslylase is dependent on the gain linewidth, Δvg and the frequency separation between modes.Under sufficiently strong pumping, we can expect that the number of modes oscillating in thecavity is given by:

2/ 2

gg

v LM vc L cD

= = D (2)

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where c is the speed of light and L is the length of a linear cavity. The shortest pulse durationthat we can expect to obtain by a given gain line width is:

2 1min M

g

LcM v

t t= = = (3)

From Eq. (3), we can conclude that the shortest pulse that can be obtained is a reciprocal ofgain line width (in Hz) [2]. Depending on fiber laser cavity type, the fundamental repetitionrate of a mode-lock fiber laser is determined by its cavity length, as shown in the equationsbelow:

( ) 2

cRepetition rate for linear cavityLn

= (4)

( ) cRepetition rate for ring cavityLn

= (5)

where -L, c and n denotes the length of the cavity, speed of light, and refractive index respec‐tively. As the round-trip time, TR, is the inverse of repetition rate, therefore,

( ) ( ) 2 R

Ln ring cavity or Ln linear cavityT

c= (6)

Under certain conditions, the repetition rate can be some integer multiple of the fundamentalrepetition rate. In this case, it is called harmonic mode-locking.

Mode-locking techniques can be divided into three categories; active, passive and hybrid.Active mode-locking can be achieved by using active modulator, e.g., acousto-optic or electro-optic, Mach-Zehnder integrated-optic modulator or semiconductor electro-absorptionmodulator. Passive mode-locking incorporates saturable absorber (SA) into the laser cavity.An artificial saturable absorber action can also be induced artificially by using NonlinearPolarization Rotation (NPR) technique or by using another technique called NonlinearAmplifying Loop Mirror (NALM). In comparison, the loss modulation of an active mode-locking is significantly slower due to its sinusoidal loss modulation. As with active mode-locking, a passive mode-locked pulse is much shorter than the cavity round- trip time. Hybridmode-locking combines active and passive mode-locking. Hybrid mode-locking uses activemodulator to start mode-locking while passive mode-locking is utilized for pulse shaping.

Many nonlinear systems exhibit an instability that result in modulation of the steady state.This is due to the interplay between the nonlinear and dispersive effects. This phenomenon isreferred to as the modulation instability. In the context of fiber optics, modulation instability

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requires anomalous dispersion and reveals itself as breakup of the cw or quasi-cw radiationinto a train of ultrashort pulses [3-5]. The instability leads to a spontaneous temporal modu‐lation of the cw beam and transforms it into pulse train [6].

There are many types of passive mode-locking pulses. However, for the sake of brevity, in thissection, we will only discuss the soliton mode-locking pulse. It refers to a special kind of wavepackets that can propagate undistorted over long distances. Soliton phenomena is formed bythe interplay between the dispersive and nonlinear effects in a fiber laser cavity. Soliton mode-locking implies that the pulse shaping is solely done by soliton formation; the balance of groupvelocity dispersion (GVD) and self-phase modulation (SPM) at steady state. The mode-lockingmechanism is not critically dependent on cavity design and no critical cavity stability regimeis required. Soliton mode-locking basically works over the full cavity stability range.

In soliton mode-locking, an additional loss mechanism such as saturable absorber is essentialto start the mode-locking process as well as stabilize the soliton pulse-forming process. Insoliton mode-locking, the net gain window can remain open for more than 10 times longerthan the ultrashort pulse, depending on the specific laser parameter [7, 8]. A stable solitonpulse is formed for all Group Delay Dispersion (GDD) values as long as the continuum loss(energy loss) is larger than the soliton loss [8] or the pulses break up into two or more pulses [9].

Soliton mode-locking can be expressed by using the following Haus master equation formal‐ism [8, 10, 11]:

( ) ( ) ( ) ( )2 22

2 2 , , , , 0i L gi

A iD i A T t A T t g l D q T t A T tt t

dé ù é ù¶ ¶

D = - + + - + - =ê ú ê ú¶ ¶ë û ë û

å (7)

Here, A(T, t) is the slowly varying field envelope, D is the intra-cavity GDD, Dg = g / Ωg2 is the

gain dispersion and Ωg is the Half-Width of Half-Maximum (HWHM) of gain bandwidth. TheSPM coefficient δ is given by δ =(2π /λ0AL )n2ℓL , where n2 is the intensity dependent refractiveindex of the gain medium, λ0 is the center wavelength of the pulse, and AL and ℓL is theeffective mode area in the gain medium and length of light path through the gain mediumwithin one round-trip, respectively. g is the saturated gain and l is the round-trip losses. q(T,t) is the response of the saturable absorber due to an ultrashort pulse.

This soliton pulse propagates without distortion through a medium with negative GVD andpositive SPM. The positive effect of SPM cancels the negative effect of dispersion. Kellysidebands can usually be found in the optical spectrum of a soliton mode-locked fiber laser.A pronounced Kelly sidebands is an indicator that the mode-locked fiber laser is operating inits optimal pulse duration [12, 13].

3. Carbon-based saturable absorbers

Graphene and CNT have been used in Q-switching and mode-locking fiber lasers since 2003[14-16] and 2009 [17], respectively. Compared to SESAM (Semiconductor Saturable Absorber


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Mirror), CNT and graphene holds several advantages, e.g., broadband operating bandwidth,simple and low-cost fabrication process, and moderate damage threshold [18]. In this section,both electronic and optical characteristics of graphene and CNT are discussed in detail.

3.1. Graphene

3.1.1. Electronic and band structure of graphene

Graphene is the name we gave to a one-atom thick sp2 hybridized carbon. It has a honeycomb-like structure. The sp2 hybridization between s, px and py atomic orbitals create a strongcovalent sp2 bonds. The pz orbital overlaps with other carbons to create a band of filled πorbitals. These bands have a filled shell and, therefore, form a deep valence band. On the otherhand, the empty π* orbitals are called the conduction band [19, 20].

Further observation of the band structure of graphene reveals three electronics properties thatsparked such interest; the vanishing carrier density at Dirac point, the existence of pseudo-spin, and the relativistic nature of its carriers. The valance and conduction bands meet at highsymmetry K points. Because the conduction and valence band meet at a symmetry K point,graphene is considered as zero-gap semiconductors (or zero-overlap semimetals) [21]. Inintrinsic graphene, each carbon atom contributes one electron completely filling the valanceband and leaving the conduction band empty. Therefore, the Fermi level, EF, is situatedprecisely where the conduction and valence bands meet. These are known as the Dirac orcharge neutrality points.

As mentioned before, due to this unique band structure of graphene, the following are threeimportant features which to a large extent define the nature of electron transport of thismaterial, namely, the zero-gap semiconductor, the existence of pseudo-spin, and the lineardispersion relation.

3.1.2. Optical properties of graphene

Graphene has three types of optical properties. They are linear optical absorption, saturableabsorption, and luminescence. For generating passive Q-switch and mode-locked fiber lasersusing graphene saturable absorber, our main interest is in the optical absorption and saturableabsorption properties that graphene has.

Graphene saturable absorber has an ultrawide band operating wavelength as a result of itslinear dispersion relation. Graphene only reflects <0.1% of the incident light in the visibleregion, increasing to ~2% for ten layers. Therefore, we can assume that the optical absorptionof graphene layers is relative to the number of layers, for each layer absorbing ≈2.3% over thevisible spectrum [22].

The saturable absorption property of graphene is the result of Pauli blocking. Inter-bandexcitation by ultrafast optical pulses produces a nonequilibrium carrier population in thevalence and conduction bands. In time-resolved measurements [23], two relaxation timescalesare observed; a faster one of ~100 fs and a slower one, on picosecond timescale. The faster


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relaxation time is related to the carrier—carrier intra-band collisions and phonon emission.The slower relation time corresponds to electron inter-band relaxation and cooling of hotphonons [24, 25]. For generating mode-lock laser, a saturable absorber with relaxation time inthe timescale of ~ps is necessary. In principle, single-layer graphene can provide the highestsaturable absorption [26-28].

Graphene can be made luminescent by inducing a bandgap through two techniques to modifythe electronic structure of graphene. One technique is by cutting it into ribbons and quantumdots [29] and the other is by chemical or physical treatments [30, 31], to reduce the connectivityof the π-electron network. A mild oxygen plasma treatment can make individual grapheneflakes luminescent. The combination of photo-luminescent and conductive layers could beused in sandwich light-emitting diodes. Luminescent graphene-based material has been madeto cover the infrared, visible, and blue spectral ranges [32-35].

3.2. Carbon nanotube

3.2.1. Electronic and band structure of carbon nanotube

The band structure of CNT can be assumed under a simple tight-binding model of [36]. In themodel, CNT is considered as a roll of graphene layers. A very small change in diameter of thecurvature of the fiber can affect the hybridization of sp3 orbitals in which the electronic structurewill also be affected. Therefore, the electronic structure depends on the geometry of the fiberand the fiber diameter.

The chiral vector, Ch→

≡ (n, m), determines whether the CNT is metallic or semiconducting withbandgap. If n −m =3k (k is integer), the CNT is metallic, while when n −m ≠3k , it shows thatthe CNT is semiconducting. At first, it was thought that the electrical and optical bandgaps ofsemiconducting CNTs were identical based on single particle model; however, the opticalbandgap is actually smaller [37].

3.2.2. Optical properties of carbon nanotube

Just like graphene, CNT has several interesting optical properties. They are: optical absorption,saturable absorption, and electroluminescence and photoluminescence. The semiconductingCNTs have peak absorption wavelength depending on the optical bandgaps. Typical CNTwith diameter d, of 7-15 nm has a bandgap energy of 1.2-0.6 eV, corresponds to the opticalwavelength of 1—2 µm. Thus, the peak absorption can be tuned by choosing the appropriatediameter. However, since CNTs are a mixture of several or many kinds of semiconducting andmetallic CNT as well as different diameter distribution, the absorption peak is determined bythe mean tube diameter and the absorption bandwidth depends on the tube diameter distri‐bution. Although CNT is essentially a rolled-up graphene, the absorption of CNT is nonlinear.The optical absorption in CNT is anisotropic because CNT only absorbs the light whosepolarization is parallel to the axial direction of the tube; therefore, an aligned CNT sample ispolarization-dependent [38]. In spite of this, since we use a randomly oriented CNT samples,the CNT is polarization-independent [39].


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Single-walled carbon nanotubes (SWCNTs) have been utilized as saturable absorber (SA) formode-locking fiber laser long before graphene. The first publication of SWCNT SA can betraced as early as 2003 [15]. CNT can saturate with high-intensity light when the states ofconduction band become full and the states at valence band become empty. Furthermore, therecovery time τ is observed to be very fast. In semiconducting CNT, the recovery time of E11

transition is an order of 1 ps, and the transition of E22 is in the order of 100 fs. Slower recoveryin an order of several ps is also seen in the E11 transition. Several mechanisms believed to beresponsible for the fast relaxation have been proposed. These mechanisms multi-phononemission [40], tube—tube interaction [41], and exciton—exciton annihilation [42, 43].

The direct bandgap that exists in CNTs suggests that they can be efficient light absorbers andemitters. Studies regarding electroluminescence properties of SWCNT—polymer compositeshave been performed [44, 45]. Electron and hole carriers in semiconductors can recombine bydifferent sorts of mechanisms. In most of the cases, the energy will be released as heat.Nevertheless, a fraction of the recombination events may involve the emission of a photon.The process is called electroluminescence and is responsible for producing solid-state lightsources such as light-emitting diodes (LED). The direct bandgap of a semiconducting CNT isresponsible for the photoluminescence phenomena in CNT. An electron in a CNT absorbsexcitation light via transition from v2 to c2 and creates another excitation [46]. Electron and holerapidly relax from c2 to c1 and from v2 to v1 states, respectively. Luminescence can only beobserved in isolated semiconducting CNTs because the bundled CNTs have rapid transferprocess from semiconducting to metallic CNTs [47]. Additionally, a semiconducting CNT canfunction as a nanoscale photodetector that converts light into current or voltage [48].

4. Preparation and fabrication of carbon-based saturable absorber

4.1. Optical deposition technique

In optical deposition technique, an intense light is injected into a CNT/ graphene-dispersedsolution from a fiber-optic end to attract the CNT/graphene particle onto the ferrule tip [49-54].There are generally two types of effects that lead to self-channeling of light in fluid suspension;the optical gradient force and thermal effects relying on (weak) absorption. When the particlesize is smaller than the optical wavelength, the optical gradient force is weak; therefore, ahigher particle density is required to cause significant change in the refractive index to trapnarrow beams. With higher particle density, multiple scattering dominates; in-turn thedirection of scattered light becomes random. This effect pushes the particles toward the beamcenter. Self-trap through very narrow beam is difficult because it requires high particledensities and this, in turn, involves multiple scattering, which acts as effective loss. Thermaleffect can lead to a significant refractive index change, but in liquid, the refractive indextypically decreases with increasing temperature [55].

Thermophoresis is a thermal mechanism that is often observed in colloidal suspension, whichuses a strong reaction of the suspended particles to temperature gradients. Also known as theSoret effect, thermophoresis describes the ability of a macromolecule or particle to drift along


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a temperature gradient [55]. Although various mechanisms are capable of depositing carbon-based saturable absorber to the fiber core, [52] has considered thermophoresis as the mostlikely process that is responsible for creating carbon-based saturable absorber via opticaldeposition. When the laser is turned on with the fiber in the solution, a strong convectioncurrent centered at the tips is observed. The induced current moves the carbon-based particlesupward toward the fiber tip.

When fabricating a carbon-based saturable absorber using optical deposition, one must startby dispersing the carbon-nanotube/graphene bundles through ultrasonification process.Centrifugation follows to separate the macroscopic flakes and the agglomerated carbonnanotube/graphene. Only the homogeneous part of the solution is used for optical depositionprocess [49]. For depositing the carbon-based particles to the fiber core, precise optical poweris essential. Lower optical power will not make the carbon-based particles adhere to the fibercore, while higher optical power will concentrate the deposition to the area around the fibercore. The difference between optimal and higher optical power is 1 dB [53]. The optimumoptical power is influenced by other factors such as the size of particles, solution temperature,concentration levels of the solution, and the optical wavelength used in the deposition process[49, 53, 54]. For monitoring the optical deposition process, [50] has devised a setup that involvesoptical circulator and power meters. For controlling the insertion loss, the duration of thedeposition has to be observed. Long deposition duration would create higher insertion loss.

Optical deposition technique is highly efficient as it only uses a small/required amount ofcarbon nanotube/graphene as saturable absorber. However, the disadvantages of this techni‐que are large scattering loss [39], and the process itself is quite tedious as many factors caninfluence the required optical power in-order to make the carbon nanotube/graphene particlesadhere to the fiber core. Furthermore, the success margin is small as only 1 dB can differentiatebetween optimal and high optical power.

4.2. Drop cast technique

Drop cast technique is simple and straightforward. Drip a graphene/SWCNT solution onto afiber ferrule and let it dry to make an SA. This technique has been utilized in [56-58]. The SAinsertion loss can be controlled depending on the concentration of the solution and the numberof times this process is repeated. The process can also be repeated until the desired insertionloss is achieved.

Although the insertion loss is high, this can be overcome by increasing the pulse energy. Asthe pulse energy depends on the power and frequency, it can be altered by increasing pumppower and/or lengthen the fiber laser cavity. The drawback of this technique is that in thecourse of increasing the pulse energy, we inevitably change the repetition rate and pulse widthof the mode-locked fiber laser. Scattering loss is also an issue for this technique.

4.3. Mechanical exfoliation technique

Mechanical exfoliation uses scotch tape to repeatedly peel the graphene layers from a HighlyOrdered Pyrolitic Graphite (HOPG) or graphitic flakes and transferring the layers to the


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surface of the fiber ferrule. A fiberscope is normally used to examine that the graphene istransferred directly onto the core of the fiber ferrule. Mechanically exfoliated graphene SA hasbeen demonstrated by [59-62] and [63]. The advantage of this technique is that it yields thebest quality graphene SA. However, the drawback of this technique is that it is time-consum‐ing. In addition, it could be difficult to control the desired graphene layer(s) that need to betransferred onto the core of the fiber ferrule.

4.4. Thin film and polymer composite

SWCNT and graphene thin films have been reported in many literatures [15, 17, 64]. Forinstance, [65] sprayed a liquid with dispersed CNTs onto a fiber end surface that acts as asubstrate, while [15] sandwiched a thin layer of purified SWCNTs between two quartzsubstrates. CNTs and graphene polymer composite have also been demonstrated in numerouspublications [26, 66-69]. Many kinds of polymer materials can be used as a host to grapheneand CNTs, e.g., polymethylmethacrylate (PMMA), polymide, and polycarbonate. The mainadvantages of using polymer composite as a host are that it reduces scattering and facilitateshomogeneous dispersion of CNTs and graphene. It is thin enough to be sandwiched betweenfiber ferrules and has higher damage threshold compared to pure CNT/graphene layer. In spiteof this, in terms of the amount used, it is less efficient than optical deposition technique andinvolves extra processing.

Other types of SA are also available, including tapered fiber [51, 70], D-shaped fiber [71], aswell as CNT/graphene solutions embedded in photonic crystal fiber [72, 73].

5. Passive Q-switch and mode-lock experiments using carbon-basedsaturable absorbers

In this subsection, three pulsed Erbium-doped fiber lasers (EDFLs) are demonstrated using acomparatively simple and cost-effective carbon-based saturable absorber.

5.1. Passive Q-switch and mode-lock generation using Single-Walled Carbon Nanotubes(SWCNTs) saturable absorber via drop cast technique

Figure 1(a) shows the experimental setup for the proposed Q-switched EDFL, using a com‐paratively simple and cost-effective alternative technique based on SWCNTs SA. It consists ofa 4 m long Erbium-doped fiber (EDF), two 1480/1550nm wavelength division multiplexers(WDM), an SWCNTs-based SA, an optical isolator, and a 20 dB output coupler in a ringconfiguration. The EDF is a commercial fiber with Erbium ion concentration of 2000 ppm, cutoff wavelength of 920 nm, and numerical aperture of 0.24. It is backward pumped by a 1480nm laser diode (LD) with the maximum output power of 129 mW via the WDM. Another1480/1550 WDM is used after the gain medium to dispose excess power from the LD. Anisolator is used to ensure unidirectional propagation of light inside the cavity. The homemadeSWCNT SA is placed between the isolator and the WDM to act as a Q-switcher. The SWCNTSA was fabricated using the drop cast method.


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The output of the laser is extracted from the cavity using the 1% output port of the opticalcoupler. An optical spectrum analyzer (OSA, AQ6317B) is utilized for the spectral analysis ofthe Q-switched EDFL, which has the spectral resolution set to 0.02 nm, whereas an oscilloscope(OSC, Tektronix, TDS 3052C) is used to monitor the pulse train of the Q-switched operationvia coupling the oscilloscope with a 6 GHz bandwidth photo-detector. Total cavity length is23 m. Except for the gain medium, the rest of the cavity uses a standard SMF-28 fiber. The totalcavity length of the ring resonator is measured to be around 23 m. Furthermore, all opticalcomponents are polarization-independent.

Figure 1. (a) Experimental setup for the proposed SWCNTs-based Q-switched EDFL. (b) Output spectra of the Q-switched EDFL at three different pump powers: 30 mw, 80 mW, and 129 mW. (c) Repetition rate and pulse durationrelationship with pump power. (d) Pulse energy relationships with pump power.

Stable and self-starting Q-switching operation is obtained just by increasing the pump powerover 30 mW. Figure 1(b) compares the output spectra of the EDFL at three different pumppowers; 30 mW, 80 mW, and 129 mW. As shown in the figure, the laser operates at centerwavelength of around 1571.6 nm. Spectral broadening is observed in the spectrum especiallyat a pump power of 80 mW, which corresponds to the minimum pulse width region. This isattributed to the Self-Phase Modulation (SPM) effect in the laser cavity [74]. The maximum


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Full-Width at Half-Maximum (FWHM) of 0.6 nm is obtained when the pump power wasincreased to maximum (129 mW).

Figure 1(c) shows the relationship between repetition rates and pulse durations with pumppower. As pump power increases from 30 mW to 129 mW, the repetition rate increases linearlyfrom 10.25 kHz to 41.87 kHz. As pump power increases, more gain is provided to saturate theSA and thus increases repetition rate. In contrast, pulse duration decreases from 17.6 µs to10.92 µs as the pump power increases. However, the lowest pulse duration of 10.42 µs isachieved at 80 mW pump power. After the pump power increases from 80 mW to 129 mW,the pulse durations increase slightly before decreasing back at 129 mW. Hence, the minimumattainable pulse duration is 10.24 µs, which is related to modulation depth of the saturableabsorber [75, 76]. Based on the minimum attainable pulse duration, the modulation depth ofthe SWCNT SA is calculated to be around 3.7%. Figure 1(d) shows the relationship betweenpulse energy and pump power in the proposed Q-switched EDFL. As the pump powerincreases, the average output power also increases, which gives rise to pulse energy. It isobtained that the pulse energy can be increased from 2.23 nJ to 4.94 nJ by tuning the pumppower from 30 to 80 mW, and from 4.94 nJ to 5.19 nJ when the pump power increases from 80mW to 129 mW. The calculated average slope efficiency is 12% when the pump power increasesfrom 30 mW to 80 mW. From 80 mW to 129 mW pump power, the calculated average slopeefficiency is 16%. The pulse energy is saturated as the pump power is further increased above80 mW.

5.2. Mode-locked erbium-doped fiber laser using Single-Walled Carbon Nanotubes(SWCNTs) saturable absorber via drop cast technique

In order to saturate the SA in a single-pass, the laser cavity is slightly changed compared tothe previous subsection. The experimental setup for proposed SWCNTs-based mode-lockedEDFL is shown in Figure 2(a). Compared to the previous setup, a 200 m long SMF is added inthe mode-locked setup to reduce the repetition rate of the output pulse and thus increase thepulse energy in the cavity. This gives total cavity length of ~223 meter and a total GroupVelocity Dispersion (GVD) of 3.6364 ps nm-1. Therefore, the fiber laser is operating in theanomalous dispersion regime. For pulse duration measurement, an autocorrelator with 25 fsresolution was used. The SNR is measured using Anritsu MS2667C Radio Frequency SpectrumAnalyzer (RFSA).

Soliton mode-locking operation self-starts at 56.75 mW without Q-switching instabilities. It isobserved that the pulse state diminishes into continuous-wave (CW) when the pump poweris below 30 mW. The resultant repetition rate is 907 kHz, which corresponds to 1.1 µs round-trip time. Figure 2(b) shows output spectrum of the proposed mode-locked EDFL. As shownin the figure, the laser operates at a central wavelength, λC, of 1570.5 nm with 3-dB bandwidthof 1.080 nm. Compared to the Q-switched laser, the mode-locked laser operates at a shorterwavelength due to the incorporation of 200 m long SMF in the cavity, which increases thecavity loss. The operating wavelength shifts to shorter wavelength to acquire more gain tocompensate the loss.


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Figure 2(c) shows the typical pulse train of the mode-locked EDFL at pump power of 129 mW.Figure 2(d) shows the corresponding autocorrelation trace of the mode-locked pulse showingthe pulse duration, TFWHM, of 2.52 ps. The RF spectrum of the mode-locked laser is alsoinvestigated using a RF spectrum analyzer. Figure 2(e) shows the result, which indicates a

Figure 2. (a) Experimental setup for soliton mode-lock operation. (b) Output spectrum of the proposed soliton mode-locked EDFL when the pump power is fixed at 129 mW. (c) OSC trace of mode-locked fiber laser (d) Autocorrelationtrace of mode-locked fiber laser at 129 mW. (e) RFSA trace of soliton mode-locked fiber laser at 129 mW.


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strong mode-locked pulse at frequency of 907 kHz. Figure 2(e), SNR is obtained at 53.42 dB,which is limited by the available pump power. The average output power of the soliton mode-locked fiber laser is measured to be -6.54 dBm. Based on the 3 dB bandwidth of the outputspectrum, a Time Bandwidth Product (TBP) of laser is calculated to be around 0.331, whichshows that the soliton pulse is slightly chirped.

Referring to Figure 2(d), the autocorrelation trace does not follow exactly the sech2 fitting.When mode-locked pulse self-starts at 56.75 mW, the pulse shape does follow exactly the shapeof the sech2 fitting. However, in the RFSA, the SNR value is below the threshold value requiredto be qualified as mode-locked pulse. This is due to the 20 dB output coupler used in theexperimental setup. Only 1% of the total energy that is circulating inside the cavity is extractedfor measurement purpose.

Therefore, the pump power is increased to 129 mW to achieve a satisfactory SNR value.However, this, in turn, increases the pulse intensity in the autocorrelator. As a result, the pulseshape in the autocorrelator does not follow the sech2 fitting.

5.3. Passive Q-switched fiber laser generation using graphene saturable absorber

Graphene was first produced by a mechanical exfoliation method in 2004 [77]. In this work, afresh surface of a layered crystal was rubbed against another surface, which left a variety offlakes attached to it. Among the resulting flakes, a single layer flake can be found. Despitethere being other methods to produce graphene, mechanical exfoliation still gives the bestsamples in terms of purity, defects, electron mobility, and optoelectronic properties. A single-layer graphene saturable absorber has an ultrafast relaxation time, lower scattering loss, andit performs better than multilayer graphene saturable absorber in terms of pulse-shapingability, pulse stability, and output energy [28]. However, this method has disadvantages interms of yield and throughput, and thus it is impractical for large-scale production. Graphenecan be optically distinguished, regardless of being one-atom thick and its transmittance (T)can be expressed in terms of the fine-structure constant. Due to some properties of graphenesuch as linear dispersion of the Dirac electrons and Pauli blocking, it makes broadbandapplications and saturable absorption possible [35]. In this section, the preparation of a single-layer graphene SA (GSA) based on mechanical exfoliation technique, also known as the “scotchtape” method, is demonstrated. The position of graphene SA on the fiber core can easily berecognized by using a fiber probe.

The material is a commercially available, highly ordered pyrolytic graphite (HOPG). An HOPGflake was inserted on a strip of scotch tape and then were pressed and peeled off repeatedlyin order to reduce the graphene layers to a single layer. The resultant graphene layers werethen pressed against the end facet of an optical fiber ferrule in order to transfer it. The scotchtape was slowly peeled off and subsequently, a few graphene layers stick on the optical fiberferrule. Graphical presentation of this technique is explained in detail by [60]. The result isinspected using EXFO’s FIP-400 fiber inspection probe to ensure that the graphene sheets lieon top of the fiber core. The microscope image of the end surface of the ferrule after coatingthe graphene is illustrated in Figure 3.


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The setup of the proposed Q-switched EDFL with the newly developed GSA is similar to theprevious section, except for the gain medium, SA, and 200 m long fiber. It is based on unidir‐ectional ring cavity configuration consisting of two wavelength division multiplexer (WDM)coupler, 49 cm long bismuth-based Erbium-doped fiber (Bi-EDF) as gain medium. Since thereis no polarizer in the laser cavity, the graphene is the sole responsible mechanism for creatingsaturable absorption.

In the experiment, the continuous wave (CW) lasing threshold was about 30 mW. When thepump power was increased to about 80 mW, the Q-switched pulses were observed byintroducing physical disturbance to the cavity. Then, the pump power is further increased tothe maximum pump power of 130 mW and observed the Q-switched operation. Figure 4(a)shows the output spectrum of the Q-switched EDFL at 130 mW pump power. A slight spectralbroadening is also observed in the optical spectrum, which is caused by Self-phase Modulationeffect (SPM). Correspondingly, the typical Q-switched pulse train is presented in Figure 4(b).As shown in the figure, the peak-to-peak pulse interval is measured to be around 43.3 µs, whichcan be translated into repetition rate of 26 kHz. At 130 mW pump power, the Q-switched laserhas an average output power of 0.5656 mW, which corresponds to pulse energy of 24.399 nJ.

Figure 4(c) represents the pulse repetition rate and the pulse energy of the Q-switched fiberlaser as a function of the pump power. The repetition rate can be tuned from 16.7 kHz to 23.1kHz by increasing the pump power. The attainable energy is lower than the previous Q-switched laser with optical deposition technique based GSA. This is attributed to the largemodulation depth of the single-layer graphene SA. A large modulation depth implies a largechange in absorption for the incident light. Therefore, a lower repetition rate and pulsewidthare achieved with higher modulation depth [78].

Figure 3. Raman Spectrum of the GSA. Fiberscope image of fiber ferrule with graphene (inset).


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5.4. Passive mode-locked fiber laser using graphene saturable absorber

Figure 5(a) shows the modified configuration where a longer EDF is used in conjunction withan additional 200 m long SMF-28 to reduce the repetition rate and increase the pulse energy.The length of total cavity is about 207 m, including 1.6 m EDF, 205.4 m SMF-28 fiber from theWDM, isolator, coupler, and additional spool of SMF. The net GVD in the cavity was calculatedto be 3.457 ps/nm, confirming that the laser was operating at an anomalous dispersion regime.

The mode-locking operation is not self-started in the proposed setup. Stable mode-lockedpulses were observed as shown in Figure 5(b) by introducing physical disturbance to the SAafter increasing the 1480 nm pump power to the maximum (130 mW). The mode-locking pulsethen disappears when pump power falls below 84.5 mW. As shown in Figure 5(b), the cavityround-trip time is measured to be 1.034 µs, which corresponds to repetition rate of 967 kHz.

Figure 5(c) shows the measured optical spectrum of the soliton pulses at the launched pumppower of 130 mW. Although the resultant pulse fiber laser is a soliton mode-locked fiber laser,Kelly sidebands are less prominent due to excessive nonlinearity caused by high pump powerand cavity length [12]. Figure 5(d) shows the measured interference autocorrelation trace of

Figure 4. (a) Optical spectrum of the Q-switched laser at pump power of 130 mW. (b) Pulse train of the Q-switchedlaser at 130 mW pump power. (c) Repetition rate and pulse energy as a function of pump power. (d) Output powerand pulse width as a function of pump power.


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the mode-locked pulses at a scanning range of 40 ps. As shown in Figure 5(d), the pulse wasvery well fitted by a sech2 pulse profile, and the pulse duration was measured to be 3.41 ps.Consequently, the TBP was calculated to be 0.38, which is almost 1.2 times larger than the idealTBP value (0.315). This is most probably due to the large GVD in the laser cavity and highpump power. Figure 5(e) shows the RF spectrum of the output at the launched pump powerof 130 mW. The SNR of 62.45 dB indicated that the oscillator operated at stable mode-lockingregime.

Figure 5. (a) Configuration of the mode-locked EDFL with GSA. (b) Typical mode-locking pulse train on oscilloscope.(c) OSA trace of the mode-locked EDFL. (d) Autocorrelation trace of the mode-locking pulse at launched pump powerof 130 mW. (e) RF spectrum of the mode-locked pulse train.


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5.5. Passive mode-locked fiber laser using nonconductive graphene oxide paper

Figure 6 shows the result of Raman spectroscopy on graphene oxide paper. The spectroscopywas performed using a 532 nm laser with only 10% power and exposure time was set to 20 s.From the result, there are two distinctive peaks that can be observed; at 1349 cm-1 and 1588cm-1. These two peaks are D-band and G-band, respectively [79]. The excitation at D-band iscaused by the hybridized vibrational mode related to graphene edges, and it also shows adisorder in the graphene structure. The graphite or tangential band (G-band) exists due to theenergy in the sp2 bonded carbon in planar sheets. The in-plane optical vibration of the bondresulted in Raman spectrum at the mentioned frequency [80]. A small peak at 2700 cm-1, whichis also known as G’ or 2D band, is barely observable because the laser power is low. Thegraphene layers can be indicated by the ratio of G’ and G bands. Because the intensity of G’band is lower than the G band, it also shows that the GO paper is more than one layer.

Figure 6. Raman spectroscopy result of graphene oxide paper.

The schematic of the proposed mode-locked EDFL is shown in Figure 7(a). It was constructedusing a simple ring cavity, in which a 1.6 m long EDF with an Erbium ion concentration of2000 ppm was used for the active medium and the GO paper SA was used as a mode-locker.

The SA was fabricated by cutting a small piece (2×2 mm2) of a commercially nonconductivegraphene oxide paper [81] and sandwiching it between two FC/PC fiber connectors, afterdepositing index-matching gel onto the fiber ends. The thickness of the GO paper is 10 µm,while the measured insertion loss of the SA to be 1.0 dB at 1550 nm. The total length of thelaser cavity was measured to be approximately 12.6 meter. The resultant total GVD for thismode-locked fiber laser was calculated to be 0.1513 ps/nm. This indicated that the proposedmode-locked fiber laser operated in the anomalous dispersion regime and thus it could beclassified as a soliton fiber laser.


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The mode-locked fiber laser had a low self-starting threshold; approximately at 17.5 mW.Before all the modes were locked, multiple pulsing could be seen to occur at pump power aslow as 10 mW. Figure 7(b) shows the spectral profile where the presence of soliton is confirmed.The presence of Kelly sidebands confirms that this mode-lock fiber laser is operating inanomalous dispersion regime. Figure 7(c) shows the pulse train of the passive mode-lockedfiber. It has a cavity round-trip time of 64 ns, corresponding to a pulse repetition rate 15.6 MHz.

Figure 7(d) shows the autocorrelation trace with measured pulsewidth of 680 fs at its FWHM.The sech² fitting, which indicates the generation of the soliton pulse, is also included in thefigure. The autocorrelation trace reveals that the experimental result follows the sech² fittingclosely. A TBP of 0.315 is calculated from the 3-dB bandwidth of the optical spectrum and theacquired pulsewidth. This shows that the pulse is a transform-limited pulse. Since the pulsingthreshold is low, the output power for this fiber laser is 0.134 mW. Consequently, the resultantpulse energy and peak power are 0.0085 nJ and 11.85 W, respectively.

As the data measurements are performed, it is observed that the pulse increasingly expandedand the spectral profile gradually changed from soliton to laser. Therefore, it is suspected thatthe pulse gradually is destroying the SA. Moreover, at approximately 24.4 mW, the outputpower was attenuated. Thus, it is concluded that a 1-layer GO paper SA is only effective in a

Figure 7. (a) Experimental setup. (b) Spectral characteristic of mode-locked fiber laser using GO paper. (c) Mode-locked pulse train with cavity round-trip time. (d) Autocorrelation trace.


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short period of time and has low damage threshold. For the same reason, a satisfactory SNRdata using RFSA is unable to be acquired. Together with low pulsing and damage thresholds,combined with the 5% of the intracavity energy taken out for performing measurements, itseems that the SNR is unable to extend more than 30 dB.

It is found that CNT and graphene saturable absorber may have inconsistent properties whenprepared by different groups of researchers; despite repeating the same process. Currently,scientists are moving forward toward finding new materials that can be utilized as saturableabsorber. The discovery of 2D material such as graphene has sparked interest in the potentialof other 2D material. In recent developments, other 2D materials have been incorporated intofiber laser cavity to generate Q-switched and mode-locked fiber lasers. Topological insulator[19-22], transition metal dichalcogenides [23-25], and black phosphorus [26, 27] are among therecent materials being developed as saturable absorber.

6. Conclusion

We have discussed graphene and CNT saturable absorbers and their applications in generatingpassive Q-switched and mode-locked fiber lasers. We have demonstrated several examples ofexperiments where carbon-based saturable absorbers were used in generating Q-switched andmode-locked pulse in EDFL cavity. Currently, scientists are also moving forward toward newsaturable absorber materials such as topological insulator and black phosphorus.

Acknowledgements

The authors acknowledge funding from the University of Malaya (Project Number: UM.C/625/1/HIR/MOE/ENG/09 and SF014-2014)

Author details

Mohd Afiq Ismail1*, Sulaiman Wadi Harun2, Harith Ahmad1 and Mukul Chandra Paul3

*Address all correspondence to: [email protected]

1 Photonics Research Centre, University of Malaya, Kuala Lumpur, Malaysia

2 Dept. of Electrical Engineering, Fac. of Engineering, University of Malaya, Kuala Lumpur,Malaysia

3 Fibre Optics Division, Central Glass and Ceramic Research Institute, Kolkata, India


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