answer optical technology assignment

8/8/2019 Answer Optical Technology Assignment

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2010

Amity Institute Of InformationTechnology

Saket Kumar Pathak M.Sc. NT & M

Optical Technology An introductory detail of the topics – subtopics defined as well as designed throughout thesyllabus with possible explanatory diagrams and pictorial representation.



Optical Fiber Communication

Q. No. 1. Difference Between LEDs and Lasers

LEDs Lasers

LEDs are semiconductor diode that

converts applied voltage to light and is used

in lamps and digital displays.

Laser diode is formed from a p-n junctionand powered by injected electric current.

LEDs are made from compoundsemiconductor materials such as galliumarsenide (GaAs), gallium phosphide (GaP),gallium arsenide-phosphide (GaAsP).

The former devices are sometimes referredto as Injection Laser Diodes

A LED is basically just a specialized type of

P-N junction diode, made from a thin chipof fairly heavily doped semiconductormaterial.

Laser diodes form a subset of the largerclassification of semiconductor p-n junction diodes.

The color of the light emitted, inthe case is visible light.

The key parameter for a laser diode is thethreshold current, which is the forwardcurrent level where lasing actually begins tooccur.

LEDs are intended to operate only inforward conduction mode, and should not

be subjected to reverse voltage.

Another important parameter is the ratedlight output, which is the highestrecommended light output level for reliablecontinuous operation.

LEDs light output isn’t coherent, it is

comparatively high in noise and hence notsuitable for use as a carrier for datacommunications.

Laser’s light output is coherent; it is very low in noise and also more suitable for useas a carrier for data communications.

The bandwidth also tends to be widerthan Lasers, making them not suitablefor optical systems.

Although most of the laser diodes used inelectronic equipment are capable of causing damage to a human or animal eye,and particularly to its light-sensitive retina.

An LED is often small in area (less than

1 mm2), and integrated optical components

may be used to shape its radiation pattern.

Infra-red (IR) lasers are especially capableof causing eye damage, because their lightis not visible.

LEDs present many advantages as lower

energy consumption, longer lifetime,smaller size, faster switching, and greater

durability and reliability.

A laser diode is a laser where the activemedium is a semiconductor similar to thatfound in a light-emitting diode.

Light-emitting diodes or LEDs are now very widely used in almost every area of electronics, mainly as indicator and display devices in effect, solid state lamps.

A laser diode is formed by doping a very thin layer on the surface of a crystal wafer.

Amity Institute Of Technology

http://en.wikipedia.org/wiki/Led#Advantages

http://en.wikipedia.org/wiki/Energy_conservation


http://en.wikipedia.org/wiki/Service_life

http://en.wikipedia.org/wiki/Laser

http://en.wikipedia.org/wiki/Semiconductor

http://en.wikipedia.org/wiki/Light-emitting_diode

http://en.wikipedia.org/wiki/Doping_(semiconductors)

http://en.wikipedia.org/wiki/Led#Advantages



http://en.wikipedia.org/wiki/Service_life

http://en.wikipedia.org/wiki/Laser

http://en.wikipedia.org/wiki/Semiconductor

http://en.wikipedia.org/wiki/Light-emitting_diode

http://en.wikipedia.org/wiki/Doping_(semiconductors)




Q. No. 2.Structures of LEDs and Differences

LEDs (Light Emitting Diode)

To achieve a high radiance and high quantum efficiency, the LED structure must provide

a means of confining the charge carriers and the stimulated optical emission to the active

region of the “pn junction” where radiative recombination takes place.

To achieve carrier and optical confinement, LED configurations such as “Homojunctions”

and “Double Heterojunctions” have been widely investigated. The most effective of these

predominantly in use at this time; is the configuration referred as “Double

Heterojunctions”.

Homojunction LED:

Nonradiative recombination due to surface states

Self-absorption with a thick p-layer

Recombination not confirned

The processes occurring in a junction LED can be divided into three stages. The 1st is the

excitation or injection process, in which the energy of carrier is raised by forward bias

injection. Next is the recombination process, during which most of these carriers give up

their excess energy photons. Finally, the generated photons must leave the semiconductor

and provide the desired optical stimulus to the eye or produce a photocurrent in a





detector. This is the extraction process. These processes have a characteristics efficiency,

and the over all device efficiency, η(0), may then be expressed as

η (0) = η(in) η(r) η(e)

where;

η(0) is external conversion efficiency,

η(in) is injection,

η(r) is radiative recombination,

η(e) is extraction efficiency.

Double Heterojunctions:

Top n+ -GaAs and bottom p+ -GaAs for ohmic contacts

A well is etched into n+ -GaAs

Back metal contact reflects backward light.

This configuration evolved from earlier studies on laser diodes. By means of this

sandwich structure of differently composed alloy layers, both the carriers and the optical

field are confirmed in the central active layer. The band-gap differences of adjacent layers

confine the charge carriers (fig. b), while the differences in the indices of refraction of

adjoining layer confine the optical field to the central active layer (fig. c). This dully

confinement leads to both high efficiency and high radiance. Other parameter influencing

the device performance includes, optical absorption in the active region (self-absorption),

carrier recombination at the heterostructure interface.

Q. No. 3.Performance of LEDs (Distributed and Fabry-Perote)

Fabry-Perote Resonator:

In laser diode Fabry-Perot resonator, a pair of flat partially reflecting mirrors are directed

toward each other to enclose to cavity. The mirror facets are constructed by making twoparallel cleaves along natural cleavage planes of the semiconductor crystal. The purpose

of these mirrors is to provide strong optical feedback in the longitudinal direction,

thereby converting the device into an oscillator with a gain mechanism that compensates

for optical losses in the cavity. The laser cavity can have many resonant frequencies. The

device will oscillate (by emitting light) at those resonant frequencies for which the gain is

sufficient to overcome the losses. The sides of the cavity are simply formed by roughening

the edge of the device to reduce unwanted emissions in the directions.





Fabry-Perote Laser Diode

Distributed Feedback Laser Diode

Distributed Feed-Back:

Another laser diode type, commonly referred to as the distributed-feedback (DFB) laser,

the cleaved facets are not required for optical feedback. The fabrication is obtained from

Bragg reflectors (gratings) or periodic variations of refractive index called distributed-

Feedback corrugations which are incorporated into the multilayer structure along the

length of the diode.

Q. No. 4.Principle of Laser diodes (population inversion)

The term “laser” is an acronym for (L)ight (A)mplification by (S)timulated (E)mission of

(R)adiation. To understand the laser, one needs to understand the meaning of theseterms. The term “light” is generally accepted to be electromagnetic radiation ranging from1 nm to 1000 mm in wavelength. The visible spectrum (what we see) ranges fromapproximately 400 to 700 nm. The wavelength range from 700 nm to 10 mm isconsidered the near infrared (NIR), and anything beyond that is the far infrared (FIR).Conversely, 200 to 400 nm is called ultraviolet (UV); below 200 nm is the deepultraviolet (DUV).

Principles of Laser Diodes:





The GaAIAs Laser Diode consists of a double hetero junction formed by a Gal-YAIYAsactive layer surrounded by P-type and N-type Gal-, AlxAs cladding layers, where x y,

when a bias voltage is applied In the forward direction, electrons and holes are Injectedinto the active layer. Since the band gap energy is greater in the cladding layers than in

the active layer, the injected electrons and holes are prevented from diffusing across the junction by the potential barriers formed between the active layer and cladding layers.The electrons and holes confined to the active layer create a state of population Inversion,allowing the amplification of light by stimulated emission. The light amplification gainsacross the hetero action. The high refractive Index of the active layer, as compared to thecladding layers, serves to confine the emitted light to propagation within the active layer.The confinement of the charge carriers and the emitted light are the keys to highly efficient laser diodes.

Population Inversion:

Non equilibrium distribution of atoms among the various energy levelatomic system

To induce more atoms in E2, i.e. to create population inversion, a largeamount of energy is required to excite atoms to E2

The excitation process of atoms so N2 > N2 is called pumping

It is difficult to attain pumping when using two-level-system.

Require 3-level system instead

As the above fig.

In actual case, excite atoms from E1 to E3.

Exciting atoms from E1 to E3à optical pumping

Atoms from E3 decays rapidly to E2 emitting hυ 3

If E2 is a long lived state, atoms from E2 will not decay to E1 rapidly

Condition where there are a lot of atoms in E2 àpopulation inversion

achieved! i.e. between E2 and E1.





Q. No. 5.Power Launching and Coupling

Launching optical power from a source into a fiber entails considerations such as

numerical aperture, core size, refractive-index profile, and core cladding index difference

of the fiber, plus the size, radiance, and angular power distribution of the optical source.

A measure of the amount of optical power emitted from a source that can be coupled into

a fiber is usually given by the efficiency η defined as:

η =P(F)/P(S)

Here, P(F) is power coupled into fiber and P(S) is the power emitted from the light

source. The launching and coupling efficiency depends on the type of fiber that is

attached to the source and on the coupling process.

In practice, manysource suppliers offer deviceswith a short length optical fiber (1m or

less) already attached in a optimum power coupling configuration. This section of fiber is

generally referred as “Flylead” or “Pigtail”. The effect to be considered in this case

include:

• Types of fiber attached to source.

• Coupling process.

• Fiber misalignment.

• Different core-size.

• Numerical aperture.

• Core refractive-index profile.

In case of numerical aperture difference with the source beam profile,

B(θ) = Bo Cosθ

Where, θ = Direction away from the axis,

Bo = Intensity on the axis.

The power coupled into a step index fiber,

Pf = Ps (NA)² ;if r<a

(a/r)² Ps (NA)² ; if r>a





Where, r and a are the source and fiber radial dimensions.

When the size of the source is smaller than that of the fiber and is in close contact with

the fiber and the coupling efficiency becomes,

η =P(F)/P(S) = (NA)²

Q. No. 6. Types of photo detectors, Avalanche and Pin Photo-diode

Avalanche photodiodes (APDs):

APDs internally multiply the primary signal photocurrent before it enters the input

circuitry of the following amplifier. This increase receiver sensitivity, since the

photocurrent is multiplied before encountering the thermal noise associated with thereceiver circuit. In order for carrier multiplication to take place, the photo generated

carriers must traverse a region a photo generated electron or hole can gain enough energy

so that it ionizes bound electrons in the valence band upon colliding with them. This

carrier multiplication mechanism is known as “Impact Ionization”. The newly created

carriers are also accelerated by the high electric field, thus gaining enough energy to cause

further impact ionization. This phenomenon is the “Avalanche Effect”.

Reach-Through Avalanche Photo-Detector

The term “reach-through” arises from the photodiode operation. When a low reverce-bias voltage is applied, most of the potential drop is across the pn+ junction. The depletionlayer widens with increasing bias until a certain voltage is reached at which the peak electric field at the junction is about 5% to 10% below that needed to cause avalanche

breakdown. At this point depletion layer just “reaches through” to the nearly intrinsicregion.

pin – Photodiodes





The most common semiconductor photo detector is the pin photodiode. The device

structure consists of p and n regions separated by a very lightly n-doped intrinsic (i)

region. In normal operation a sufficiently large reverse-bias voltage is applied across the

device so that the intrinsic region is fully depleted of carriers. That is, the intrinsic n and p

carrier concentration are negligibly small in comparison with the impurity concentration

in this region.

When an incident photon has energy greater than or equal to the band-gap energy of the

semiconductor material, the photon can give up its energy and excite an electron from the

valence band to the conduction band. This process generates photon-carriers. The photo

detector is normally designed so that these carriers are generated mainly in the depletion

region where most of the incident light is absorbed. The high electric field present in the

depletion region causes the carriers to separate and be collected across the reverse-bias

junction. This gives rise to a current flow in an external circuit, with one electron flowing

for every carrier pair generated. This current flow is known as the photocurrent.

Q. No. 7. Optical Receiver (Design and Diagram)

The main component of an optical receiver is a photo detector, which convertslight into electricity using the photoelectric effect. The photo detector is typically

a semiconductor-based photodiode. Several types of photodiodes include p-n

photodiodes, a p-i-n photodiodes, and avalanche photodiodes. Metal-semiconductor-metal (MSM) photo detectors are also used due to their suitability

for circuit integration in regenerators and wavelength-division multiplexers.

Basic Optical Receiver Front-End





Requirements

General:

Low-noise electronics for optical to electrical signal conversion

Short to medium haul application

2+ Gbps data rate

Input side:

InGaAs Photodiode with junction capacitance ~ 100 ff’s

Optical powers ranging from -20 to +10 dBm, causing input currents from 10uA to 10mA

Output side:

Drive a capacitive load representing subsequent MOSFET gate

Digital signal RZ-type output

Photo detection is the process of conversion from optical to electrical domain. A block

diagram of the GMRT optical receiver is shown in following fig.. The basic detector is a

reverse biased p-n junction diode. In this bias condition a reverse leakage current (thedark current ) flows. The other important characteristic of a photo-detector is its

responsively . The responsively is a measure of the efficiency with which light is

converted to electrical current and it is related to the width of the depletion region of the

diode and to the spectral response of the receiver. A larger depletion region leads to a

better responsively. PIN diode detectors made of InGaAsP and grown on InP are popular

photo-detectors as they have low dark currents and high responsively.





Q. No. 8.Sonet (OC1, OC3, TDM, Frame size (9*9*8)), Sonet SDH

With the advent of fiber optic transmission lines, the next step in the evolution of the

TDM scheme which was the standard signal format called SONET (Synchronous Optical

Network) or SDH (Synchronous Digital Hierarchy).

Sonet frame are 2D structure consisting of 90 columns by 9 bytes, where one byte is 8

bits. Here, in standard SONET technology, a section connects adjacent pieces of

equipment, a line is a longer link that two SONET devices, and a path is a complete end-

to-end connection. The fundamental SONET frame has a 125 µs duration. Thus the

transmission bit rate of the basic SONET signal is;

STS-1 = (90 bytes/row) (9 rows/frame) (8 bits/byte)/(125 µs/frame)

= 51.84 Mb/s

90 Columns of Bytes

9 r o w s O f B y t e s

3 Columns1

Column

Sectionandline

overhead P a t h

o v e r h

e a d

Synchronous

payloadenvelope (SPE)

87 Columns





This is called an STS-1 signal, where STS stands for synchronous transport signal. All

other SONET signals are integer multiples of this rate, so that STS-N signal modulate an

optical source, the logical STS-N signal is first scrambled to avoid long string of ones and

zeros and to allow easier clock recovery at the receiver. After under going electrical-to-

optical conversion the resultant physical layer optical-signal is called OC-N, where OC

stands for Optical carrier.

In SDH the basic rate is equivalent to STS-3 or 155.52 mb/s, this is called “Synchronous

transport module – level-1”. Higher rates are designated by STS-M. Values of M

supported by IUT-T as, M = 1, 4, 16, 64. These are equivalent to SONET OC-N signals,

where N = 3M. Analogous to SONET, SDH first scrambles the logical signal. In contrast

to SONET, SDH doesn’t distinguish between a logical electrical signal and a physical

optical signal, so that both signal types are designated by STM-M. Following we have a

list of commonly used values.

SONET level Electrical level Line rate (Mb/s) SDH equivalent

OC-1 STS-1 51.84 -

OC-3 STS-3 155.52 STM-1

OC-12 STS-12 622.08 STM-4

OC-24 STS-24 1244.16 STM-8

OC-48 STS-48 2488.32 STM-16

OC-96 STS-96 4976.64 STM-32

OC-192 STS-192 9953.28 STM-64

Optical Interfaces:

The SONET/SDH specifications provide details for the optical source characteristics, the

receiver sensitivity and transmission distances for various types of fibers. 6 Transmission

ranges are defined as following, with different terminology for SONET/SDH.

Transmission Distance SONET Technology SDH Terminology

≤ 2 km Short – reach Intra-office

15 km Intermediate reach Short – Haul

40 km at 1310 nm Long – reach Long Haul

80 km at 1550 nm

120 km at 1550 nm Very Long – Haul





160 km at 1550 nm Ultra long – Haul

The optical fiber generally fall into the following 3 categories and operation windows:

1. Graded – Index multimode in the 1310-nm window.

2. Conventional non-dispersion-shifted single-mode in 1310 nm and 1550 nm

window.

3. Dispersion-Shifted single – mode in the 1550 nm window.

SONET/SDH Networks:

An important SONET/SDH network element is the add/drop multiplexer (ADM). This

piece of equipment is a fully synchronous, byte-oriented multiplexer that is used to add

and drop sub channels within an OC-N signals.

As in the following figure, various OC-12 and OC-3 are multiplexed into an OC-48 stream.

Upon entering an ADM, these sub-channels can be individually dropped by the ADM and

others can be added. One OC-12 and two OC-3 channels enter the left-most ADM as part

of an OC-48 channel. The OC-12 is passed through and two OC-3 are dropped by the first

ADM. Then, two more OC-12 and one OC-3 are multiplexed together with the OC-12

channel that is passing through and the aggregate OC-48 is sent to another ADM node

downstream.

Q. No. 9. WDM, Types of coupling, Operational Principle, 2x2 coupler

(Splitting Ration), Start Coupler, Mach-Zehnder Interferometer

Multiplexers.





The technology of combining a number of wavelengths onto the same fiber is known as

“Wave-Length Division Multiplexing” or WDM. Conceptually, the WDM scheme is the

same as FDM used in microwave radio and satellite systems. The key system features of

WDM are as follows:

o Capacity Upgrade: - The classical application of WDM has been to upgrade the

capacity of existing point-to-point fiber optic transmission links that can increase

the capacity of fiber network dramatically.

o Transparency: - An important aspect of WDM is that, each optical channel can

carry any transmission format. Thus, by using different wavelengths, fast or slow

asynchronous or synchronous digital data and analog information can be sent

simultaneously and independently, over the same fiber channel.

o Wavelength routing: - wavelength sensitive optical routing devices using

multiple wavelengths to increase link capacity and flexibility, and treats

wavelength as another dimension in addition to time and space. Wavelength

routed networks use the actual wavelength of a signal as the intermediate or final

address.

o Wavelength Switching: - Wavelength routed networks are based on a rigid

fiber infrastructure, wavelength-switched architectures allow reconfigurations of

optical layers. Key components for implementing these networks include optical

add/drop multiplexer, optical cross connects and add/drop multiplexers.

Operational Principle

In standard point-to-point links a single fiber line has one optical source at itstransmitting end and one photo detector at the receiving end. Signal from different light

sources use separate and uniquely assigned optical fiber. Since an optical source has

narrow line width, these types of transmission make use of narrow portion of the

transmission bandwidth capacity of a fiber.





The above fig. depicts the attenuation of light in a silica fiber as a function of wavelength.

The curve shows that the two low-loss signal regions of a single mode fiber extends over

the wavelength ranging from about 1270nm to 1350nm (i.e. 14 THz, the 1310-nm second-

windows) and 1480nm – 1600nm (i.e. 15 THz, the 1550nm third-windows).

In the above fig. various type of optical amplifiers are used. At the transmitting end, thereare several independently modulated light sources each emitting signals at a unique

wavelength. Here a multiplexer is needed to combine these optical outputs into a serial

spectrum of closely spaced wavelength signals and couple them onto a single fiber. At the

receiving end, a de-multiplexer is required to separate the optical signals into appropriate

detection channels for signal processing.





The 2X2 fiber couplers

On discussing couplers and splitters, it is to refer in terms of the number of input and

output ports on the device. Hence a device with 2 inputs and 2 outputs would be called as

“2X2 coupler”, similarly N*M coupler has N inputs and M outputs.

In following fig. a common construction is the fused fiber coupler. This is fabricated by

twisting together, melting and pulling two single mode fibers so they get fused together

over a uniform section of length ‘W’. Each input and output fiber has a long tapered

section of length ‘L’, since the transverse dimensions are gradually reduced down to that

of coupling region when the fibers are pulled to fusion process. The total draw length is

L+W. This device is known as fused bi-conical tapered coupler. Here P0 is the input

power, P1 is the throughput power and P2 is the power coupled into the second fiber. The

parameter P3 and P4 are extremely low signal level resulting from backward reflections

and scattering due to bending in and packaging of the device.

Splitting Ratio: To specify the performance of optical coupler one usually indicates the

percentage division of optical power between the output ports by means of ‘splitting

ratio’.

i.e. Splitting Ratio = (P2 / (P1 + P2)) * 100%

Excess Loss: It is defined as the ratio of input power to the total output power.

i.e. Excess Loss = 10 log (P0 / (P1 + P2))

Insertion Loss: It refers to the loss for a particular port-to-port path. If the path of

input has port ‘Pi’ to the path of output has port ‘Pj’, then





Insertion Loss = 10 log (Pi / Pj)

Crosstalk: It measures the degree of isolation between the input at one port and the

optical power scattered or reflected back into other the other input port. Hence theoptical power level P3 as;

Crosstalk = 10 log (P3 / P0)

Star Coupler:

The principle role of star coupler is to combine the powers from N inputs and divide them

equally among M output ports. The fiber-fusion technique has been a popular

construction method for N X N star couplers. Following fig. shows, 8 X 8 device formed

by using 12, 2 X 2 couplers. This device could be made from either fused fiber or

integrated optic components. Similarly as to construct 16 X 16 coupler we need 24, 2 X 2interconnected couplers.

Mach-Zehnder Interferometer Multiplexers:

Wavelength-dependent multiplexers can also be made using Mach-Zehnder

interferometer technique. These devices either passive or active. As the following fig. of

passive 2 X 2 MZI consists of three stages: an initial 3Db directional coupler which splitsthe input signals, a central signal where one of the waveguides is longer by ∆L to give a

wavelength dependent phase shift between the two arms, and another 3Db coupler, which

recombines the signal at the output. The function of this arrangement is that, by splitting

the input beam and introducing a phase shift in one of the paths, the recombined signals

with interfere constructively at one output and destructively at the other. The signals then

finally emerge from only one output port.





Q. No. 10. Optical Attenuators, Optical Power meter and OTDR

Optical Attenuators:

Whenever a high optical signal level may need to be measured and the level is so high

such that an strong output coming from the amplifier needs to be precisely attenuated

before being measured. This is done to prevent instrument damage and to avoid overload

distortion in measurements.

Optical attenuators allow a user to reduce an optical signal level up to 60 dB in presice

steps at a specified wavelength, which is usually 1310 nm or 1550 nm.

Optical Power meter:

Optical power measurement is the most basic function in fiber optic metrology. Hand-held instruments come in wide variety of types in different levels of capabilities. Multi

wavelength optical power meters that use photo-detectors are the most common

instrument for measuring optical signal power level.

In this versatile instrument various photo-detector heads that have different performance

characteristics are available. As example, GE detector allows a measuring range of + 18

dBm to -60 dBm in 780 nm to 1600 nm wavelength band whereas an InGaAs detector

allows a measuring range of + 3dBm to – 73 dBm in 840 nm to 1650 nm wavelength

band.

Optical Time-Domain Reflectometer (OTDR):

The long term workshop instrument in fiber optic system is the OTDR. In addition to

locating faults, within an optical link, these instrument measures parameter such as

attenuation, length, connector, splice losses and reflectance level. A typical OTDR

consists of optical source and receiver, a data acquisition module, a CPU, an information





storage unit for retaining data either in the internal memory or on an external disk and a

display.

An OTDR is fundamentally optical radar. It operates by periodically launching narrow

laser pulses into one end of fiber under test by using either a directional coupler or a

beam splitter. The properties of optical fiber link are then determined by analyzing theamplitude and temporal characteristics of waveform of the back scattering light.

Q. No. 11. Fiber to Fiber joints and types, Fiber end Face

Preparation, Fiber-Related Losses

Significant factor in any fiber optic system installation is the requirement to interconnect

fibers in low – loss manner. A particular technique selected for joining fibers depends on

whether a permanent bond or easily demounted connection is desired. A permanent bond

is generally referred as to as a splice, whereas the demounted joint is known as a

connector. These connections occurs at the following cases of interconnection:

o At the optical source.

o At the photo-detector.

o At the intermediate point within a cable, where two fibers are joined.

o At intermediate point within a link, where two cables are connected.

Every joining technique is subject to a certain conditions which can cause various amount

of optical power loss at the joint. The parameters of these losses are:

o Input power distribution to joint.

o Length of fiber between optical source and joints.

o The geometric and waveguide characteristics of two fiber end at the joints.

o The fiber end faces quality.

Mechanical alignment is a major problem when joining two fibers, owing to their

microscopic size. Radiation losses results from mechanical misalignment, because the

radiation cone of the emitting fiber doesn’t match the acceptance cone of the receiving





fiber. The magnitude of the radiation loss depends on the degree of misalignment. The

three fundamental type of misalignment between fiber are:

o Longitudinal separation, when the fibers have same axis and the gaps

between them.

o Angular Alignment, when the two axis forms an angle so that the fiber end

faces are no longer parallel.

o Axial displacement, when the axis of two fibers is separated by a distance. This

is also known as lateral displacement.

Fiber End-Face Preparation:

Before fibers are connected or spliced to each-other, it is necessary to prepare fiber end

faces properly. In order not to have light deflected or scattered at the joint the fiber end

must be flat, perpendicular to fiber axis and smooth. This end-face preparation technique

that have been extensively used, include:

o Sawing

o Gridding and Polishing

o Controlled fracture.

Conventional Gridding and Polishing techniques can produce a very smooth surface that

is perpendicular to the fiber axis. This procedure employed is to use successively finer

abrasives to polish the fiber. Each successive abrasive polish the end face of the fiber,

until the scratches created by previous abrasive material are replaced by finer scratches of

present abrasive. The number of abrasive used depends on the degree of smoothness thatis desired.

Controlled fracture techniques are based on score-and-break methods for cleaving fibers.

As in the following fig. in this operation, the fiber to be cleaved is first scattered to create

stress concentration at the surface. The fiber is then bent over the curve form, while

tension is simultaneously applied. This action is produces a stress distribution across the





fiber. The maximum stress occurs at the scratch point so that a crack starts to propagate

through the fiber. If the stress distribution across the crack is not properly controlled, the

fracture propagating across the fiber can fork into several cracks and can produce

following action:

o LIP: This is a sharp protrusion from the edge of cleaved fiber, that prevents thecore from coming in close contact.

o Roll-Off: This rounding off of the edge of a fiber is opposite condition to lipping.

o Chip: A chip is a localized fracture or break at the end of a cleaved fiber.

o Hackle: This serve as irregularities across a fiber end face.

o Mist: This is similar to hackle but much less serve.

o Spiral or step: these are abrupt changes in the end-face surface topology.

o Shattering: This is the result of an uncontrolled fracture and has no definable

cleavage or surface characteristics.

Q. No. 12. Fiber Splicing, Fiber Connectors

Fiber Splicing

A fiber splice is a permanent or semi permanent joints between two fibers. These are

typically used to create long optical links or in situation where frequent connection and

disconnection is not needed. In making and evaluating such splices, one must considerfollowing factors:

o The geometrical distances between two fibers.

o Fiber misalignment at the joint.

o The mechanical strength of the splice.





Splicing Techniques include:

o The fusion splice

o The V-Groove mechanical splice.

o The elastic tube splice.

The fusion splice are made by thermal bonding together prepared fiber ends, as in

following fig.. Here the fiber ends are first pre-aligned and butted together, by microscope

and micro-manipulators. The Butt-joint is heated with an electric arc or a laser pulse, so

that a fiber laser are momentarily melted and hence bounded together. This technique

can produce very low splice losses (less than 0.06dB).

The V-Groove mechanical splice technique, the prepared fiber ends are first butted

together in a V-shaped groove, as shown in following fig.. They are then bounded together

by means of a cover plate. The V-shaped channel can be either a grooved silicon, plastic

ceramic or metal sub-striate. The splice loss depends on the fiber size and electricity.

The elastic tube splice is a unique device that automatically performs lateral,

longitudinal, and angular alignment. This mechanism is basically elastic material. The

central hole diameter is slightly smaller than that of fiber to be spliced and is tapered in

each end for easy fiber insertion. When a fiber is inserted, it expands the whole diameterso that the elastic material exerts a symmetrical force on the fiber. This symmetry feature

allows an accurate and automatic alignment of axes of the two fibers to be joined. Thus

the fiber to be spliced does not have to be equal in diameter, since each fiber moves into

position independently and relative to the tube axis.





Fiber Connectors

A wide variety of optical fiber connectors has evolved for numerous different applications.

Some of the principle requirements of a good connector design are as:

o Low coupling losses: The connector assembly must maintain stringent

alignment tolerance to assure low mating losses. These low losses may not change

frequently.

o Interchangeability: Connectors of the same type must be from onemanufacturer to another.

o Easy to assembly: A service technician should readily be able to install the

connectors in field environment.

o Low Environmental sensitivity: Conditions such as temperature, dust, and

moisture, should have a small effect on connector loss variation.

o Low cost and reliable connection: the connector must have a precision

suitable to the application, but its cost must not be major factor in the fiber

system.

o Easy to connect: One should be able to connect or disconnect the connectors

simply by hand.

Connector are available in screw–on, bayonet-mount and push pull configuration. The

basic coupling mechanisms used in these connectors belong to either “the butt-joint” or

“the expanded-beam classes”.

Butt-joint connectors employ a metal, ceramic or molded plastic ferrule for each fiber

and precision sleeve into which the ferrule fit. The fiber epoxies into a precision hole which has been drilled into the ferrule. As in the following fig. two popular butt joined

alignment designed in both multimode and single mode fiber systems. These are “straight

sleeve” and “tapered sleeve” mechanism.

o In fig.(a) straight-sleeve connector the length of the sleeve and the guide ring

of the ferrule determine the end separation of the fibers.





o The tapered sleeve or bi-conical connector i.e. fig.(b) uses a tapered sleeve

to accept or guide tapered ferrules. Again the sleeve length and the guide rings

maintain a given fiber end separation.

An expanded-beam connector as in the following fig., employs lens on the end of fiber.

These lenses focus the expanded beam onto the core of receiving fiber. The fiber-to-lens

distance is equal to the focal length of the lens. The advantage is that, since the beam

collimated, the separation of fiber ends may take place within the connector.

Q. No. 13. Eye Pattern

Eye Patterns are used to detect and access data handling capacity of digital

transmission system. they are made in time domain and allows the effect of waveform

distortion to be shown immediately on the oscilloscope the key features of eye pattern

including the following opening and width of eye 20 to 80 % rise and all times over shoot

(spike) on logic 1's and 0's under shoot on a logic 0 and jitter in the eye pattern pseudo

random generator means that the signals are generated 1's and 0's at a uniform rate but

in a random manner. This sequence of generated 1's and 0's are repeated randomly for

test purposes.

Width of eye: width of an eye defines the time interval over which the received signal

can be sampled without error

Height of eye: it is the vertical distance between the top of the eye opening and the

maximum signal level. it is because of amplitude distortion.





Noise Margin: it is the percentage ratio of the peak signal voltage V1 for the alternative

bit sequence as defined by the height of the eye opening to V2 so V1/V2 X 100%. V2 is the

maximum threshold level time jitter or phase distortion. It arises from noise in the

receiver and pulse distortion in the optical fiber. It is given by

∆T/Tb*100%

Where, the ∆T is the amount of jitter time and Tb time taken by bit signal.

Rise time: defined as the time interval between the 10% of the final amplitude to the

time taken by the pulse to reach 90% of the final amplitude.


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