CALIFORNIA STATE UNIVERSITY, NORTHRIDGE

A 100Mbps Free Space Optical Infrared Link: Transmitter

A graduate project submitted in partial fulfillment of the requirements

For the degree of Master of Science in

Electrical Engineering

By

Rohit A Kulkarni

December 2012

ii

The graduate project of Rohit Kulkarni is approved:

________________________ ________________________

Dr. Kourosh Sedgisigarchi Date

________________________ ________________________

Prof. James Flynn Date

________________________ ________________________

Dr. Sharlene Katz, Chair Date

California State University, Northridge

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ACKNOWLEDGEMENTS

I wish to express my appreciation to those who have served on my graduate project.

Firstly, I would like to thank Dr. Sharlene Katz for her valuable advice and guidance throughout

my project. Her constant support and encouragement has helped me learn a great deal all through

the project. She was instrumental in providing not only all the guidance but also inspiration that I

needed.

I also wish to acknowledge special appreciation to Professor Flynn and Dr. Kourosh

Sedgisigarchi for their valuable comments on my work.

Finally, the patience and support from my parents, family and friends has been enormously

important to me while I have been engaged in the graduate project. Thanks to all of you.

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Table of Contents

Signature Page……………………………………………………………………………………ii

Acknowledgement………………………………………………………………………………..iii

List of figures……………………………………………………………………………………..V

Abstract…………………………………………………………………………………………..VI

CHAPTER 1 Introduction.………………………………………………………………………...1

1.1 Background……………………………………………………………………………......1

1.2 Comparison of Fiber Optic and Free Space Optical Communication………………….....2

1.3 Overview………………………………………………………………………………......3

1.4 Applications of free space optics system………………………………………………….4

CHAPTER 2 Link Equation and Transmitter Circuit…………………………………………......6

2.1 Link budget……………………………………………………………………………......6

2.2 Transmitter Circuit………..……………….……………………………………………....8

CHAPTER3. Design of Transmitter……………………………………………………………..11

3.1 LED………………………………………………………………………………………11

3.2 LED Driver………………………………………………………………………………13

CHAPTER4. Performance and testing…………………………………………………………..18

CHAPTER5. Summary and Conclusion………………………………………………………....29

Bibliography……………………………………………………………………………………..30

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List of Figures

Figure 1 Range and data rate of networking technologie…………………………………………3

Figure 2 Transmission properties of atmosphere………………………………………………….7

Figure 3 Block diagram FSO transmitter………………………………………………………….8

Figure 4 Block diagram of free space optic system……………………………………………….9

Figure 5 Relationship of light rays to right and left focal points in lens………………………...10

Figure 6 Switching speed calculation …………………………………………………………...11

Figure 7 Explanation of divergence……………………………………………………………...12

Figure 8 Half and full viewing angle…………………………………………………………….12

Figure 9 Generation of photon …………………………………………………………………..13

Figure 10 Typical operating circuit using MAX2967A………………………………………….14

Figure 11 Pin configurations …………………………………………………………………….14

Figure 12 Evaluation kit operating circuit ………………………………………………………15

Figure 13 Block diagram MAX3967A…………………………………………………………..16

Figure 14 Input square wave……………………………………………………………………..18

Figure 15 Actual transmitter PCB………………………………………………………………..19

Figure 16 Actual setup of FSO system…………………………………………………………..20

Figure 17 Emitting strength versus Forward current…………………………………………….21

Figure 18 Divergence of the beam……………………………………………………………….22

Figure 19 Power measurement with optical meter………………………………………………23

Figure 20 Final output waveform image…………………………………………………………26

Figure 21 Final output captured on the oscilloscope…………………………………………….27

Figure 22 Final output at 100Mbps ……………………………………………………………...28

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ABSTRACT

A 100Mbps Free Space Optical Infrared Link Transmitter

By

Rohit Kulkarni

Master of Science in Electrical Engineering

The Free Space Optical (FSO) LED link has the ability to connect two devices at high-speed

while taking advantage of the high bandwidth of optical communication and the low cost and

low power consumption of LEDs. This link will provide an alternative to traditional RF wireless

communication that is currently approaching its bandwidth limitations. As the speed increases

for data transmitted over a wire, it is necessary that wireless communication continues limitless.

The FSO link also outperforms USB 2.0 and Bluetooth allowing for an additional market and

perhaps a new standard for data transmission. This will become necessary as file sizes increase

and multimedia dominates the business world.

A key advantage to the LED system will be cost. The FSO link will consist of a transmitter and a

receiver. The transmitter will have a very less cost and will operate on two AA batteries. The

receiver will also be the same. Overall the cost of this system is significantly less than a

comparable LASER optical link and draws less power.

This project aims at designing a free space optic transmitter in the FSO system. The details of the

receiver are discussed in the project "Free Space Optic Receiver". This work was performed as

separate project but in cooperation with this project. The result of the project after testing,

debugging, and data analysis was a working link at 13 meter with a speed of 100 Mbps. The

target market for this project is the typical consumer who has a need for high-speed and low cost

data transmission.

1

CHAPTER 1

Introduction

The computer and the internet are the two growing technologies that came into existence around

the same time and then both progressed simultaneously. This revolution advanced smoothly,

even though the software was complex; it was still user friendly and powerful, which along with

the developing hardware contributed to their growth.

The telecommunications industry faced various problems. The internet bandwidth requirement

started increasing drastically and the existing telecommunication infrastructures were not able to

support the exponentially increasing demand. As a result, telecom providers faced difficulty in

satisfying their customers. In order to address this problem two technologies have become

increasingly important - fiber based communications and free space optics communications.

The remainder of this section provides background of optical communication and an overview of

the project. Section 2 provides an overview of a free space optical transmitter and section 3

provides the details of the transmitter design. Section 4 presents the performance and testing

results. Section 5 is the summary and conclusion.

1.1. Background:

1.1.1 Fiber-Optic Cable

A fiber optic cable is the most commonly used medium, preferred between free space and fiber

optic cable. A fiber-optic cable carries a light signal from a source to a destination. The light

sources are the devices that generate the light in optical networks. For a digital signal, that is, a

pattern of 0's and 1's, modulation can be achieved by simply turning the light source on and off

in response to an electrical 1 or 0. The propagation medium for this optical signal is a fiber optic

cable.

Two types of light sources can be used.

1) Light-emitting diodes (LEDs).

2) Laser diodes.

2

1.1.2 Free Space Optical communication (Transmission through air or free space)

The name itself implies that free space optical communication (FSO) is transmission through air.

For this type of communication link air is used as medium for propagation of an optical signal.

FSO is a simple concept and is similar to the optical transmission using fiber-optic cables. The

only difference is the medium. The most interesting fact is that, light travels faster through air (at

approximately 300,000 km/s) than it does through glass (approximately 200,000 km/s), so free-

space optical communication is faster than fiber-optic communication as it takes place at the

speed of light but its range is limited to line of sight. Free-space optical systems operate in the

infrared (IR) spectrum range, which is 800-1550 nm with corresponding frequencies around 200

THz. As this frequency is above 300 GHz, this range is not regulated by Federal

Communications Commission (FCC). [Ref 1]

1.2 Comparison of Fiber Optic and Free Space Optical Communication

The basic difference between the two technologies is the medium. As free space optical

communication is a line of sight technology the main issues creating major drawbacks of an FSO

link are fog, absorption, scattering, physical obstructions, and scintillation. All of these

obstructions are created by the environment, which cannot be avoided.

Building a fiber optic network (cable-based network) takes many months. Also it is infrastructure

dependent, which is lost when the customer leaves the building or decides to cancel the service.

In contrast, FSO is a mobile platform, which can be reused in many circumstances. Another

important aspect to note is the environmental benefits of free-space optics. FSO is an

environment friendly option as compared to fiber that requires digging channels in ground for

cabling. So in the end, the benefits are more than the the disadvantages in case of FSO.

3

Figure 1 Range and Data rate of networking technologies [Ref 4]

Figure 1 above shows different type of data networks available and gives a clear idea of their

range and data rate.

1.3. Project Overview

The purpose of this project is to design and implement a free space optical communications link

between a transmitter and a receiver. In the past Wi-Fi, VCELs and LASERs have been used as

the light source for optical communication, but not the LED, except for low speed digital

communication such as remote controls. LASERs and VCELs (Vertical Cavity Surface Emitting

Laser) are capable of switching fast in between low-high intensities. However, they use more

power than LEDs and their set-up cost is high. The LED assembly is effective, compact and less

expensive as it uses less power as compared to other means of communication used such as

LASER or VCEL's. So using a LED in a free space optical communications is a better option for

current wireless communication since it will reduce the overall cost of the assembly of the

system.

Presently existing technologies face risks as far as their security is concerned. FSO can be a

substitute for Wi-Fi as it is wireless, secured and uses line of sight as a communication medium.

LEDs used in free space optical communication, support high speed and secured data transfer

and more importantly, less expensive light sources as compared to LASERs

4

The link designed in this project consists of a transmitter and a receiver both of which is powered

by an external power supply and are portable. The transmitter circuitry takes in a data stream of

"1s (+3.3V)" and "0s (0V)" and then modulates the current through an LED. This light is

captured by the photodiode at the receiver and is converted back to the data stream by the

receiver circuitry. Cost effective parts and low power consumption will reduce the overall cost of

the system. The link is designed for various applications with a range of 40 feet, such as sending

data from computer to computer. The link is designed to have a data transfer rate of 100 Mb/s

over this range. It could also be used in applications such as providing data in a closed room

having a line of sight to PDAs (personal digital assistants), laptops etc. The same amount of data

rate can be achieved, in a link between buildings.

1.4 Applications of free space optics system

Free space optical systems can be a perfect, reliable, cheap option over both a radio

communication link and a fiber optic cable operated link. Although consistency of the system is

based on local atmospheric conditions, a free space optical communication link can be widely

used in urban applications.

The transmitted light in free space is reflected, refracted or absorbed by the objects such as rain,

fog, wind, or sun. For these reasons, in long range communication, larger wavelengths (1550

nm) are used for transmission. In this project the range is short that is why optical

communication is used at a shorter wavelength.

In practical applications various network structures are formed to provide the connectivity over

longer distance

These include:

1) Ring Topology

2) Mesh Topology

3) Star Topology

4) Backbone

Several tens of meters of range can be achieved using these network structures. Examples of

point to point applications can be:

5

1) Broadband link for buildings of organizations or companies.

2) Point to point or point to multipoint systems in urban area.

3) Broadband links over highways or river crossings.

6

CHAPTER 2

Link Equations and Transmitter Circuit

2.1 Link Equation

In order to setup a free space optical link, channel analysis of the link is very important. As the

LED used in this project is an infrared LED, the study of various parameters is required where

the key parameters are:

1. Wavelength of LED.

2. Atmospheric Loss.

3. Output Power.

4. Alignment of transmitter and receiver.

If we neglect the optical efficiency of the free space optic link and detector noise, the link

equation can be written as follows:

Pr = Pt *

* exp (-α * d) (1)

Where,

Pr = Power Received (Watts

Pt = Power transmitted (Watts)

α = Atmospheric Loss

Div = Divergence Angle of beam (degrees)

d = Distance between transmitter and receiver (Meters)

Ar = Receiver aperture area (Meters2)

The link equation shows that the power received is directly proportional to the power transmitted

and the receiver aperture area, while it is inversely proportional to the square of the divergence

angle, the square of the link range and the exponential of the product of the atmospheric

attenuation coefficient and the link range. The only factor that cannot be controlled in this

equation is the atmospheric attenuation coefficient, especially when link operates outdoors. The

rest of the factors such as transmitted power, link range and beam divergence can be controlled

and changed.

As mentioned above, receiver power is inversely proportional to the product of the atmospheric

attenuation coefficient and the link range. Thus, even if the designer chooses a high transmitter

7

power, a large aperture area and very small beam divergence, the received power might not

change significantly. The exponential factor in the link equation is very important. One way to

control the dominance of the exponential term on the equation is to limit the link range. The link

range is dependent on the application. In this project, we are limited to 40 feet equivalent to a

12m link range with a 100 Mb/s data rate.

The figure 3 below shows the atmospheric attenuation coefficient as a function of wavelength.

There are many transmission windows which are characterized by low attenuation. For example,

the windows located near 850 nm, 1060 nm, 1250 nm, 1560 nm have very low attenuation. Thus,

the 850nm frequency window was selected for this project.

Figure 2 Transmission properties of the atmosphere in the near infrared wavelength range

under clear weather conditions (visibility > 10 miles) [Ref 8]

The above figure 2 shows transmission properties of the atmosphere in the near infrared

wavelength range under clear weather conditions (visibility > 10 miles). The calculation was

done by MODTRAN, a software program developed by the Air Force Research Laboratory. [Ref

8]

8

Based on the absorption properties of the atmosphere for both visible and near infrared range

wavelengths, the transmission properties and the physics of the radiation as it penetrates the

atmosphere are similar but not exactly the same.

Two types of absorption take place in the near infrared range:

1) Absorption due to water particles (moisture) in the air which is unavoidable.

2) Absorption due to the gas particles which will not be considered in this case as this project

uses a LED source of 850nm and this condition only applies to the wavelength > 2000nm.

According to the above figure, if we choose a wavelength between 800 nm to 900 nm or 1000

nm to 1100 nm, the atmospheric loss can be considered less than 0.2 db/km. Therefore α<0.2 can

be used in the above link equation. The detailed calculation of the link equation is given in

section 4.

2.2 Transmitter Circuit

The objective of this project is to design a transmitter which can transmit with a data rate of 100

Mbps at a distance of approximately 40 feet.

Light Output

Figure 3 Block Diagram FSO Transmitter

The block diagram of a transmitter is shown in figure 3. A Modulator is inside the LED driver

and the LED driver is the core component of the transmitter circuit. The LED driver changes the

current through the LED. The LED converts the input electrical signal into an optical signal

suitable for transmission. The input from the signal generator is given to the LED driver. The

driver circuit is used for switching the LED at the given input frequency.

Data Input (Signal

generator 100 MHz)

LED Driver

Transmit Optics

(Lens)

Light source: LED

9

The process of modulation takes place inside the LED driver itself according to the input

frequency received.

The block ‘Transmit Optics’ in figure 3 is the lens used to avoid the divergence of the light

beam. Finally the parallel beam of a light out of the lens is transmitted in free space. The

complete block diagram of the link including the transmitter and the receiver is as shown in

figure 5 below. The details of the receiver are discussed in the project "Free Space Optic

Receiver". This work was performed as separate project but in cooperation with this project. [Ref

12]

LED Driver Free Space

Transmitter Receiver

Lens

Light Source Light detector TIA

Signal generator (LED) (Photodiode)

Limiting Amp

Figure 4 Block diagram of free space optic system [Ref 5]

As shown in the block diagram, the data is provided in the form of square wave using a signal

generator at the frequency desired. The LED driver changes the current through the LED and

accordingly the light intensity. This light enters free space at a certain divergence angle

controlled by the lens. Due to this divergence in the beam, the light will scatter in free space. So

in order to achieve the distance mentioned in the objective, we use lenses on both the transmitter

and receiver, both lenses are the same as shown in figure 5 below.

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Transmitter Side Receiver side

Figure 5 Relationship of light rays to right and left focal points in lens [Ref 11]

The F and F' shown in the figure 5 above are the focal points on the left and the right side

respectively. The focal length adjustment of the Lens and the transmitter on the left is such that

the LED covers the whole lens. A similar adjustment is made on the receiver side. Focal length

adjustment of lens and the photodiode is such that the photodiode covers the whole lens. This

way, the light is converged and focused on the photodiode.

The photodiode converts this light into an electrical signal. This electrical signal is fed into a

trans-impedance amplifier, which is typically used as a preamplifier in a circuit. This can convert

a very low level signal into a significantly amplified signal with an adequate bandwidth. The

next stage in the circuit is a limiting amplifier. This amplifier boosts the output signal of the

trans-impedance amplifier, which typically converts a low voltage differential signal into a

constant amplitude binary signal. All the details of the receiver will be discussed in a separate

project, Free Space Optic Receiver. [Ref 12]

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CHAPTER 3

Transmitter Design

In this section, all of the core components of the transmitter are discussed in detail including the

component specifications.

3.1 LED

A free space optical system based on a LED has been designed. Choosing the correct LED is a

tough challenge. There are a few parameters which play an important role in choosing the LED:

1. Switching Speed

2. Wavelength

3. Divergence angle

4. Optical output power

There will always be a trade-off between these components as the market availability of the

products is limited. The original proposed LED for this project was the OPF 345. Although this

LED had a fast switching speed with a fall time (tf) and rise time (tr) both equal to 3.5 ns, it was

rejected due to low output power. The LED used in this project was the SIR-568ST3F. It has

greater rise and fall times than OPF345, but significantly better output power.

The two important time periods to discuss in the switching speed of the LED are fall time (Tf)

and rise time (Tr). See figure 6.

th 1

tf tr tf 0

T T T

Figure 6(a) Figure 6(b)

Fig 6 Switching speed calculation

Figure 6(a) shows two full cycles of an ideal waveform. To determine the maximum switching

speed, fmax, the hold time, th, is assumed to be zero. The actual waveform with rise and fall times,

tr and th, is shown in figure 6(b), where, T = tr + tf

12

Thus,

Switching Speed (fmax) =

=

=

= 71.42 MHz (2)

One cycle would correspond to two bits of data ("0" and "1").

So,

Data Rate = 2 x (fmax) = 2 x 71.42 MHz = 142.84 Mbps.

Therefore, a data rate of 142.84 Mbps can be achieved. According to the specification sheet 50

MHz is the cut off frequency of the LED. If the frequency is increased further, attenuation will

be seen in the output. But with the use of a limiting amplifier on the receiver side, a proper

square waveform can be seen at the output. This output waveform is shown in the section 4

performance and testing.

The next parameter is wavelength selection, as discussed in section 2.1 of this report; the

wavelength was selected to be 850 nm. The third parameter is the divergence angle. When the

LED emits light in free space, it never emits it in a straight line. The angle at which the IR LED

emits light is known as the divergence angle. It is shown in the figure 7 below.

Free Space

Divergence angle

IR LED

Figure 7 Explanation of divergence

The LED SIR-568ST3F used in this project, has a half viewing angle of 13 deg. The figure 8

below shows the concept of the half viewing angle.

13

Ө Ө1/2

LED Ө1/2 = half viewing angle

Figure 8 Half and full viewing angle [Ref 9]

The luminous intensity alone does not decide received power. Both luminous intensity and

viewing angle need to be taken into account. According to the figure, half viewing angle θ1/2, is

the off-axis angle where the LED's luminous intensity is half the intensity, of the on-axis

intensity. Two times θ1/2 is the full viewing angle. The LED used in this project has 26 degree

full viewing angle which is shown in the figure 8.

The output power of LED is the fourth parameter to be considered. The structure and function of

a LED is shown below in figure 9.

Figure 9 Generation of photon (light) [Ref 7]

The source of light LED is a semiconductor device, which can emit visible as well as infrared

light. As shown in the figure, when the light emitting diode is forward biased, i.e. the anode is

connected to a positive voltage and the cathode is connected to a negative voltage, electrons and

protons come together to release the energy in the form of photons. This whole process is known

14

as electroluminescence. The output is in the form of light and the optical power of this light can

be measured. According to the specification sheet of the LED, it is 13 mW.

3.2 LED Driver

The second component in the transmitter design is the LED driver. The basic function of a driver

circuit is to switch the LED on and off according to the input signal provided to the driver. The

application plays a very important role in selecting the LED driver circuit.

Often, in many applications, the switching speed of the LED is not important. But when it comes

to building a system for data transmission, the switching speed has to be fast to provide the

required data rate. So, the LED driver chosen should be fast enough to support the specified data

rate. The MAX3967A driver circuit was proposed in this project. It is a LED driver with an

operating data rate of up to 270 Mbps. The typical implementation of the circuit as per the data

sheet of LED driver IC is as shown in the figure 10 below.

Figure 10 Typical operating circuit using MAX2967A [Ref 10]

The dimension and the packaging information of the driver MAX3967A IC is as shown in the

figure 11 below.

15

Figure 11 Pin configurations [Ref 10]

As shown here in figure 11, the 4mm × 4mm QFN package is very difficult to mount. Therefore

the evaluation kit has been used in this circuit. The evaluation kit has the IC already mounted on

it by the manufacturer. Although the typical operating circuit is as shown in fig.10, the following

is the typical evaluation kit operating circuit used in this project.

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Figure 12 Evaluation kit operating circuit [Ref 10]

As shown in the figure 12, the LED goes in the position D1 in such a way that the anode is

connected to pin 10, which is out-, and the cathode is connected to pin 9, which is out+.

In this project, instead of using PECL data inputs, we have used a single ended input and

terminated the other input, to avoid the complexity of implementing differential inputs. There are

several jumpers to configure the kit. According to the instructions, JU9 in above figure is left

17

open to enable the output. A jumper is placed on JU8 to attach MODSET to the potentiometer

and the potentiometer is adjusted to 300 ma. R2 resistor shown in the figure 12 is removed from

the LED to enable the maximum output from LED. 3.3v is applied to Vcc and ground to power

up the circuit. [Ref 10]

The block diagram of the driver IC MAX3967 is as shown in figure 13 below.

Figure 13 Block diagram MAX3967A [Ref 10]

The main components in Figure 13 are the reference voltage generator, modulation current

generator, prebias current generator, and the input and output buffer. For the input buffer, the

proposed input in the data sheet is PECL (Positive Emitter Coupled Logic) data input. But, to

avoid the complexity of implementing the differential input, this project uses a single ended

digital square wave to drive the LED. According to the optical evaluation kit information,

Tempco is the modulation current temperature coefficient, which is provided by the reference

voltage generator. In the reference voltage generator, pin Tc is connected to Tcnom to get the

18

medium value of Tempco. The value of Tempco ranges from 2500 ppm/0C to 12000 ppm/

0C.

Thus, according to specification sheet, medium value considered is 3600 ppm/0C.

Amplitude modulation is used in the modulation current generator. This circuit determines the

amplitude of the modulation current based on the voltage at the MODSET pin. In the prebias-

current generator, to improve the switching speed of the LED, the voltage at the terminal prebias

(Vprebias) can be applied. In the output buffer, the modulation current is amplified and the RC

filter is also connected to avoid overshoot.

19

CHAPTER 4

Performance and testing

In this section, the testing and performance of the free space optical link are discussed in detail.

The input chosen in this project was a square wave with a frequency ranging from 50 MHz to

100 MHz with amplitude of 1 Vp-p. This was selected to verify that a data rate of up to 100 Mbps

can be achieved.

Figure 14 Input square wave

Figure 14 shows the output of signal generator. This square wave is given as an input to LED

driver to drive the LED. The LED driver is Maxim's MAX3967A which is specifically designed

for driving the source up to a data rate of 270Mbps. The circuit of the driver was discussed in

detail in the section 3. The actual LED driver board used in this project is as shown in figure 15

below.

20

Figure 15 Actual transmitter PCB

As seen in the above image, two outputs OUT+ and OUT- and one of the inputs, IN- are

terminated with 50 Ω terminators. The output suitable for transmission in free space is taken

from the LED and the input is the single ended square wave input (1V peak to peak) shown in

figure 14. The set up of the whole system is as shown in figure 16.

21

Figure 16 Actual setup of FSO system

Starting from the left side on the upper bench is the Oscilloscope, a Power Supply, and two

signal generators. On the lower bench both the transmitter and the receiver are placed parallel

with each other. Due to the space restrictions in the lab, a line of sight of 40 feet in any one

direction cannot be achieved. Thus, a mirror is placed at a distance of 20 feet from both the

transmitter and the receiver. The light coming out of the LED can be reflected back and focused

on the receiver with the help of the lens. The alignment was the most critical part of the project

as the light transmitted by the LED is at 850nm wavelength. Because this is an infrared range it

cannot be seen with the naked eye. But with the technique described below, the setup is aligned

without having to use an infrared camera to observe the beam.

To align the system, both the transmitter and the receiver are mounted on a sturdy metal

mounting plate (L-shaped) at a same height. The mirror is adjusted such that both the transmitter

and the receiver are visible in the middle of the mirror. Lenses are placed in front of the

22

transmitter and the receiver. The focal point is adjusted such that the LED and the photodiode

both cover the entire area of the lens. This way the system is adjusted properly. The only thing

that remains is to adjust the mirror to reflect the beam, particularly on the lens, in front of

photodiode. The mirror is such that it is moved in three dimensions to get the best signal

strength. The oscilloscope channel is configured at 100 mv/division to check the output coming

out of the receiver.

Link Analysis

According to the LED specification sheet, a graph of emitting strength versus forward current is

shown in figure 17.

Figure 17 Emitting strength versus Forward current

The current drawn by the LED is calculated manually. That is a small value resistance was

placed in series with the LED and the voltage across the resistance was recorded using a multi-

meter. The value of current calculated was 124 ma. As all the above values shown in the graph

are logarithmic. The slope of the line is calculated from one point (70, 50) on the line along with

the Y-axis intercept. The equation of the line can be written as

Log Y = M * Log X + C

C = Log 10 = y intercept of (0,10) point.

Log 50 = M * [Log 70] + Log 10

1.698 = M * 1.854 + 1

M = 0.3782

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This value is the general slope of the line shown in the figure 17. So, if we substitute all values

with the above calculated slope in equation, the emitting strength can calculated as follows.

Y = 0.3782 * Log (124ma) + Log 10

Y = Emitting Strength (Es) = 61.88 MW/SR (3)

Or approximately Es = 60 MW/SR.

By definition, 1 steradian cuts the area of the sphere equal to 1/R2. As shown in figure 18,

26 deg 5 cm (r = 2.5cm)

5 cm

Figure 18 Divergence of the beam

1 Steradian will cut the area of 25 cm2 at a distance of 5cm. But the area of the lens with a radius

of 2.5 cm is 6.25 cm2. At an angle of 26 degrees and a distance of 5cm most of the light will

enter into lens but some amount of power will be lost. Therefore the exact amount of emitted

power can be calculated as follows:

Emitted Power =

= 15 MW.

Therefore, Emitted Power (Pe) = 15 MW

Emitted Power (Pe ) = 15 MW (4)

24

Figure 19 Power measurement with optical meter

The LED and the Photodiode are 40 feet (13 meters) apart as shown in figure 19. With the help

of an optical meter (which detects the light and calculates the intensity of light in mw), 3db

points of the light beam are measured. This means, at that point, 70% of the power is observed.

The half viewing angle at which 70 % of optical power recorded is approximately 110.

To calculate the exact amount of power entering the receiver, some geometrical calculations are

done. Looking at the figure 19, a cone is formed by the light emitted by the LED. At a distance

of 13 m, if we consider the upper half right angle triangle, all the sides can be calculated and the

power can be determined at the receiver side. The calculations are shown below.

The half viewing angle is 110

Cosine (11) =

=

= 0.9817

Therefore, hypotenuse =

= 13.2423

Now,

Sin (11) =

=

= 0.1908

Opposite Side (b) = 13.2423 * 0.1908 = 2.5267

Thus, the received power is calculated as:

25

PR= PT *

(5)

Where, Ap = Area of Lens via which light enters into Photodiode

At = Area of light cross section at the receiver lens.

The total area of the cross section of the light beam at a distance of 13m is equal to: π (R2).

= π (2.5267)2 = 20.0464 m

2

At = 20.0472 m2 (6)

The area of the lens Ap with radius 2.5 cm can be calculated as follows:

π (2.5)2 = 19.63 * 10

-4 m

2 (7)

Combining equations (5), (6), and (7) generates the received power.

Pr =

= 1.4687 * 10

-6 W

Pr = 1.47 µw (8)

Thus a transmitted power by the LED of 15 mw results in a value of Pr (1.4687µw). At the

receiving end this received power is focused on the photodiode by the lens. The photodiode is

followed by a trans-impedance amplifier and a limiting amplifier for boosting and stabilizing the

received signal. The following equations show the further analysis of the link.

The flux responsivity of the photodiode is a very important factor. It can be defined as the ratio

of the generated photocurrent to the incident optical power of the source. It is measured in A/W.

As per the data sheet of the photodiode OPF430, the flux responsivity specified is 0.55 A/W.

Flux Responsivity =

According to the calculations above the incident optical power on the photodiode is 1.47 µw.

substituting this value into above equation, the generated Photocurrent is determined to be:

0.55 =

Photocurrent = Ip = (1.4687 * 10-6

)* 0.55 = 0.807 * 10-6

A

26

Ip = 0.81 µA (9)

This means that as per the function of the photodiode, it converts the incident light of power

1.4687 µw into a signal of photocurrent, 0.807 µA. This signal is fed into the trans-impedance

amplifier, which is referred to as the TIA below. The function of the TIA is basically to convert

the input current into a voltage signal. The key factor to discuss here is the trans-resistance of the

TIA.

The trans-resistance of the TIA according to the data sheet is 24000.

Trans-resistance =

24000 =

Substituting the value of IP from equation (9),

Vout = 19.4 mv (10)

The next component in the circuit is a limiting amplifier. The function of the limiting amplifier is

to accept a wide range of inputs and provide a constant level of output voltage with controlled

edge speeds. According to the specification sheet of the limiting amplifier, MAX3748, the VIN-

MIN (input sensitivity) is 5 mVP-P and VIN-MAX is 1200 mVP-P. The output voltage received at the

output of the TIA (19.4 mv) is fed into the limiting amplifier.

27

Figure 20 Final output waveform image

28

Figure 21 Final output captured on the oscilloscope

Figure 20 and figure 21 shows the final output waveforms collected from the limiting amplifier.

The oscilloscope channels are adjusted to 100 mv/div. As shown in the figure output square

wave of more than 700 mv is received. This is a usable output. These waveforms are captured at

10 Mbps data rate. The figure 22 below shows the wave form at 100 Mbps data rate. The purple

and the green are the two waveforms received from the out+ and out-. The orange waveform is

the difference between both the waveforms which is usable output data.

29

Figure 22 Final output at 100Mbps

30

CHAPTER 5

Summary and Conclusion

As described in this report, a free space optical communication link with a range of 40 feet and a

data rate of 100 Mbps was designed and tested. In the beginning the LED used was the OPF345.

This LED had a fast switching speed with a capability of providing the data rate of 250 Mb/s, but

a low output power of 25 micro watts. Obtaining the proposed link range was not possible

because of the low output power. In the final design the SIR-568ST3F IR LED is used. This

LED was relatively slow with a switching speed of 71 Mb/s and with a capability of providing a

data rate of 142 Mb/s and an output power of 13 mw. The alignment of the transmitter and the

receiver was critical. To achieve the link range of 40 feet, a new setup with two LED's in parallel

was considered. The experiment was not successful as the modulation current got divided into

two LED's and the divergence of the beam increased. Therefore, this idea was rejected and

experiment was performed with the single LED.

With the proper technique of adjusting the lens, transmitter, receiver and the mirror the required

link range and the data rate was achieved. Future work should be focused on studying the pre-

bias voltage, the modulation currents and determine if it is possible to further reduce thermal

noise created by the photodiode. It is also possible to reduce the noise and achieve an accurate

output by avoiding interference caused by the other wireless devices such as cell phones and

routers. This can be achieved by using good shielding techniques. A package should be created

in which an internal power supply (battery), casing and lens are integrated, making it portable

and sturdy. The transmitter and the receiver could be interfaced with a data source and memory

such as USB and a hard drive. The receiver prototype should be developed into a commercial

receiver with more filtering and error correction stages along with interfacing.

31

BIBILIOGRAPHY

1. Free-Space Optics: Enabling Optical Connectivity in Today’s Networks by

Heinz Willebrand, Ph.D., and Baksheesh S. Ghuman. 2002.

2. “Handbook of Optical Through the Air Communications,” by David A

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(Retrieve Date: October 2012)

3. Reference: www.lightpointe.com. (Retrieve Date: March 2012)

4. Introduction to FSO technology from MRV communication.

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Hervé; Boisrobert, Christian; de Fornel, Frédérique; Favennec, Pierre-Noël) Publisher Wiley-

ISTE

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http://www.lightpointe.com/images/LightPointe_How_to_Design_a_Reliable_FSO_System.pdf

(Retrieve Date: April 2012)

9. [http://www.theledlight.com/technical1.html] (Retrieve Date: November 2012)

10. http://www.maximintegrated.com/datasheet/index.mvp/id/4794 (Retrieve Date: July 2012)

11. http://eyesvisions.com/physics/28 (Retrieve Date: June 2012)

12. Free Space Optic Receiver: California State University, Northridge.