Final thesis paper Digital Optical fiber link design

“Heaven`s Light is Our Guide”

DEPARTMENT OF

COMPUTER SCIENCE & ENGINEERING

Rajshahi University of Engineering & Technology

Thesis Title

DIGITAL FIBER OPTIC LINK DESIGN

Author

Md. Nadimul Islam Roll: 093048

Department of Computer Science & Engineering


Supervised by

Dr. Md. Al Mamun

Assistant Professor



ACKNOWLEDGEMENT

At first I want to remember almighty Allah to give me the knowledge and ability to

think and successfully finish my thesis. I would like to express our sincere,

appreciation and deep gratitude to our respective teacher, supervisor, Dr. Md, Al

Mamun Assistant Professor,Department of Computer Science & Engineering

, Department of Computer Science & Engineering , Rajshahi University of

Engineering & Technology (RUET), Rajshahi for his affectionate and incentive

guidance and valuable suggestions throughout the period of experimental work. I also

express our gratitude to the respected teachers of Department of Computer Science &

Engineering , Rajshahi University of Engineering & Technology (RUET), Rajshahi,

for their encouragement and guidance.

Date 15 February, 2013 Author

RUET,Rajshahi. Md. Nadimul Islam

׀

“Heaven`s Light is Our Guide”



CERTIFICATE

This is to certify that this thesis work on “Digital fiber optic link design” by

Md.Nadimul Islam, Roll No: 093048 has been carried out under my supervision.

This thesis work has been submitted in partial fulfillment of the requirement for the

Degree of Bachelor of Science in Computer Science & Engineering, Rajshahi

University of Engineering & Technology (RUET), Rajshahi.

--------------------------------------------

Dr. Md

Assistant Professor


Rajshahi University of Engineering & Technology (RUET)

Date: 15 February, 2013

Rajshahi, Bangladesh.

׀׀

ABSTRACT

At present, all over the world, optical communication system have been seen as one of

the attractive solution to the increasing high data rate in telecommunication systems.

There also has been a tremendous increase in usage of transferring information like

Internet and multimedia applications. As this applications require high gigabyte

bandwidths over distances of hundred of kilometers, larger bandwidth, shorter

interconnection delays, lower levels of power consumption and smaller channel cross

talk compared to conventional electrical counterparts. For this reason, I want to design

and build an inexpensive low bit-rate digital link megabyte range as a gigabyte range

digital fiber-optic link on this thesis.The digital fiber optic link design has been done

for long distance about 100km from transmitter to receiver. The data rate is 400Mbps

at the bit rate error of 10-12

. Four combinations of source, fiber and detector have been

compared. From the rise time budget and power budget the decision of the system

validity has been given. The algorithm and the flow chart of the design has been

included. The software implementation has been done by using MATLAB 7. Also the

reflection of this thesis in included.

׀׀׀

TABLE OF CONTENTS

Page no

ACKNOWLEDGEMENT……………………………………………………………... ׀

CERTIFICATE……………………………………………………………………...… ׀׀

ABSTRACT…………………………………………………………………………..׀׀׀

LIST OF

FIGURES……………………………………………………………….….׀v

LIST OF TABLES………………………………………………………….……........v

CHAPTER 1

INTRODUCTION

1.1 Description of a digital fibre-optic link system......................................................1

1.2 Objectives..................................................................................................................1

1.3 Proposed approach and method to be employed…………………………….......3

1.4 Overview of the thesis...............................................................................................5

CHAPTER 2

LITERATURE REVIEW 2.1 Introduction……………………………………………………………………….6

2.1.1 History of fiber optics………………………………………………….......7

2.1.2 Transmitters………………………………………………………………..8

2.1.3 Recivers……………………………………………………………………9

2.1.4 Fibers……………………………………………………………..……......9

2.1.5 Amplifiers………………………………………………………………...10

2.1.6 Wave-length division multiplexing……………………………………....10

2.1.7 Dispersion………………………………………………………………...10

2.1.8 Inter-modal dispersion…………………………………………………....11

2.1.9 Chromatic dispersion…………………………………………………..…11

2.1.10 Polarization mode dispersion………………………………...................11

2.1.11 Attenuation………………………………………………………………11

2.2.12 Bending Loss……………………………………………………………12

2.1.13 Mechanical Misalignment……………………………………………....14

2.1.14 Coupling loss…………………………………………………………...15

2.2 Overview of the optical link design…………………………………………15

2.2.1 Optical source…………………………………………………………….17

2.2.2 Comparison between (LD) and (LED)…………………………………..17

2.2.3 Fiber splicing…………………………………………………………......18

2.2.4 Optical fiber connectors………………………………………………….19

2.2.5 SC fiber optic connector basic structure………………………………...20

2.2.6 Elements in a sc connector……………………………………………….21

2.2.7 How fiber optic connectors mate…………………………………………22

2.2.8 Fiber optic connectors types and there application………………………22

2.2.9 Small form factor fiber optic connectors…………………………………24

2.2.10 Advantages of fiber-optic communications……………………………..25

2.3 Link power budget……………………………………………………………….27

2.4 Rise time budget………………………………………………………………….28

CHAPTER 3

HARDWARE DESIGN AND IMPLEMENTATION

3.1 Rise time budget…………………………………………………………………30

3.2 Power budget…………………………………………………………………….31

3.3 Quantum noise limited receiver sensitivity……………………………………...31

3.4 Fiber RMS Dispersion Analysis…………………………………………………32

3.5 Maximum Link Length Calculation……………………………………………..33

3.6 Design Example…………………………………………………………………..34

3.3.1 Wavelength region………………………………………………………...34

3.3.2 Source Slection……………………………………………………………34

3.3.3 Fiber selection……………………………………………………………..34

3.3.4 Detector Selection…………………………………………………………35

CHAPTER 4

SOFTWARE DESIGN AND IMPLEMENTATION

4.1 Introduction……………………………………………………………………...41

4.2 Algorithm of the program……………………………………………………….41

4.3 Flow chart of a repeater less link design……………………………………….42

4.4 Program input……………………………………………………………………43

4.5 Program output…………………………………………………………………..43

4.6 Figure of the output……………………………………………………………...45

CHAPTER 4

CONCLUSION AND FUTURE DEVELOPMENT

4.1 Conclution………………………………………………………………………46

4.2 Future development……………………………………………………………..47

CHAPTER 5

REFLECTIONS

5.1 Reflections………………………………………………………………………48

CHAPTER 6

REFERENCE

6.1 Books…………………………………………………………………………….49

6.2 Websites………………………………………………………………………….50

CHAPTER-1

INTRODUCTION

1.1 Description of a digital fiber - optic link system

The fiber optic link system is similar in concept to any type of communication

system. Information source provides an electrical signal to a transmitter comprising an

electrical stage which drives an optical source to give modulation of the light wave

carrier. The optical source provides the electrical-optical conversion may be either a

semiconductor laser (LD) or light emitting diode (LED). The transmission medium

consists of an optical fiber cable and the receiver consists of an optical detector which

drives a further electrical stage and hence provides demodulation of the optical

carrier. A transmission link is a point to point line that has a transmitter on one end

and a receiver on the other, as shown in figure 1.

Communication

Channel

Figure 1: Generic optic communication system

1.2 Objectives

The aim of this thesis is to design and build an inexpensive low bit-rate digital link

Megabyte range as a scaled down model of a gigabyte range digital fiber-optic link. This

link will be used as a demonstrator for principles of digital optical communications for

Optical

Transmitter Input Output

Optical

Receiver

students to perform a number of measurement techniques commonly used in digital

optical communication system.

The first step in designing any fiber optic system requires making careful decisions

based on the operating parameters that apply for each component of a fiber optic

transmission system.

1

The main factors, as shown in the table below, involves bit error rates and data rates

in a digital systems, bandwidth, linearity, and signal-to-noise ( SNR ) ratios in analog

systems, and transmission distances.

All of these considerations are inter-related, and transmission distance is the

predominant consideration.

- Transmission distance will affect the strength of the transmitter output, which

dictates the type of light source used.

- Fiber type, as single-mode fiber is better suited to long distance transmission.

- Receiver type and sensitivity.

- Transmission distance dictates the modulation scheme used as some are better

for longer distances than others.

While designing a system can be very complex, there are several techniques to

simplify this process.

One technique is to determine the link's optical loss budget, which evaluates the

transmitter output power, operating wavelength, attenuation of fiber, bandwidth of

fiber and receiver optical sensitivity. Another technique is used to determine the link's

rise time budget. Link’s rise time budget describes the speed of the transmission

device's ability to turn on and off.

Environmental considerations such as temperature will affect the performance of

LEDs and lasers as well as the optical fiber itself. Some environments can cause more

hazards for the fiber optic systems than others. These factor must be consider when

designing a good system.

The cost in setting up of a fiber optic transmission system can also be a critical

consideration. Component considerations such as light emitter type, emitter

wavelength, connector type, fiber type, and detector type will have an impact on both

the cost and performance of a system. A properly engineered system is one that meets

the required performance limits and margins with little extra.

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Table 1: Operating Parameters

1.3 Proposed approach and method to be employed

Table 1 - System Design Considerations

System Factor Considerations/Choices

Transmission Distance System Complexity Increases with

Transmission Distance

Types of Optical Fiber Single-mode or Multimode

Dispersion Incorporate Signal Regenerators or

Dispersion Compensation

Fiber Non-linearities Fiber Characteristics, Wavelengths, and

Transmitter Power

Operating Wavelength 780, 850, 1310, 1550, and 1625 nm Typical

Transmitter Power Typically Expressed in dBm

Source Type LED or LD

Receiver Sensitivity/Overload Characteristics Typically Expressed in dBm

Detector Type PIN Diode, APD

Modulation Code AM, FM, PCM, or Digital

Bit Error Rate (Digital Systems Only) 10-9

, 10-12

Typical

Signal-to-Noise Ratio Specified in Decibels (dB)

Number of Connectors or Splices in the

System

Signal Loss Increases with the Number of

Connectors or Splices

Environmental Requirements & Limitations Humidity, Temperature, Exposure for

Sunlight

Mechanical Requirements Flammability, Indoor/Outdoor Application

In order to do this thesis, a lot of reading and research on Fiber Optic Communication

Link Design will be required. Going through reference books, journals, internet

resource and data sheets also helps in doing this project.

This Fiber Optic Communication Link Design would consist of the following

modules:

1) Photodetector

2) Optical Source

3) Optical Fiber

3

Why do we want to used these modules? The reason will be discussed below:

1) Photodetector – The photodetector is typically a avalanche photodetector

(APD) or a PIN device. Thermal detectors do not achieve the bandwidth

useful for communications. For the specific photodetector we must examine

the responsivity R at the operating wavelength. Responsivity of a

photodetector is usually expressed in amperes per watt, or volts per watt, of

incident radiant power. It is a function of a wavelength of the incident

radiation and of the sensor properties, such as the bandgap of the material of

which the photodetector is made.

2) Optical Source – The basic requirement for the light source depends on the

usage of the optical communication systems ( Long-haul communication or

local area network ). There are 2 types of light sources that can be used, Light

Emitting Diodes ( LEDs ) and Laser Diodes (LDs). Laser diodes have the

advantage of high speeds, narrow spectral width and high power. LEDs have

the advantages of reliability, lower cost, long lifetime and simplicity of design

but they have a lower bandwidth compare to Laser.

3) Optical Fiber – Light waves travels in the optic fiber in the form of modes,

each with a distinct spatial distribution, propagation, velocity, attenuation

coefficient and polarization. Hence the fiber chosen must meet the design

goals after the consideration of :

- Single mode fiber , multimode step index or multimode graded index

- Fiber core size

- Core refractive index

- Bandwidth

- Attenuation dB/km

- Numerical Aperture

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1.4 Overview of the thesis

Chapter 1 as stated by description of a digital fiber - optic link system,

Objectives of the thesis,Proposed approach and method to be

employed of the thesis.

Chapter 2 as stated above serves as a general introduction to digital fibre-optic

communication.

Chapter 3 discribe the hardware design and implementation of this thesis.

Chapter 4 discribe the software design and implementation of this thesis.

Chapter 5 discribe the conclusion and future development of this thesis.

Chapter 6 discribe the reflections of this thesis.

Chapter 7 discribe the reference of this thesis.

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CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

Fiber optic communication is a method of transmitting information and data from one

place to another by sending pulses of light through optical fiber cable. The light

transmitted forms an electromagnetic carrier wave that is modulated to carry

information. The first fiber optic communications is developed in 1970’s and have

revolutionized the telecom industry because of its advantages over electrical

transmission. Optical fiber has largely replaced copper wire communication in core

networks.

The process of using fiber optics in communication involves in the following steps :

Using the transmitter to create the optical signal, relaying the signal in the fiber

network, ensure that the signal will not be distorted or weak and receiving the optical

signal and converting the optical signal into an electrical signal.

Optical fiber is mainly used by telecommunications companies to transmit telephone

signals, Internet communication, and cable television signals. Due to lower

attenuation and interference, advantages of optical fiber over the existing copper wire

are optical signal can travel long-distance and has a large bandwidth. However,

infrastructure development within cities was relatively difficult and time-consuming,

and fiber-optic systems were complex and expensive to install and operate. Due to

these difficulties, fiber-optic communication systems have been installed in long-

distance applications, where they can maximize their network bandwidth, therefore

offsetting the increased cost. The prices for fiber-optic communications have dropped

considerably since 2000 and the price for rolling out fiber to the home has currently

become more affordable and cost-effective than that of rolling out a copper based

network.

6

Figure 2: In fiber-optic communications, information is transmitted by sending light

through optical fibers.

2.1.1 History of fiber optics

Optical fiber was first developed by Corning Glass Work in 1970’s. After a period of

research, the first commercial fiber optic communications system was developed. The

system operates at a wavelength around 0.8µm and uses GaAs semiconductor lasers.

The first generation system operates at a bit rate of 45 Mbps with repeater spacing of

10km. On 22 April 1977, General Telephone and Electronics sent the first live

telephone traffic through fiber optics at 6 Mbps in California.

The second generation fiber optic system was developed for commercial used in the

early 1980’s . The system operates at a wavelength around 1.3µm and used GaAsP

semiconductor laser. In 1981 the single mode fiber was introduced and this improve

the system performance. By 1987, these systems were operating at bit rates up to

1.7Gb/s with repeater spacing up to 50km.

The third generation fiber optic system was developed and operates at a wavelength

of 1.55 µm and uses InGaAsP semiconductor laser and had losses about 0.2 dB/km. In

order to overcome this problem dispersion shifted fibers were used and was designed

to have minimal dispersion of 1.55 µm or limiting the laser spectrum to a single

longitudinal mode. These new development eventually allowed the third generation

systems to operate at 2.5 Gbit/s with repeater spacing in excess of 100km.

7

http://en.wikipedia.org/wiki/File:Laser_in_fibre.jpg

The fourth generation fiber optic communication system is design with the use of

optical amplification that would reduce the need for repeaters and wavelength

division multiplexing to increase the data capacity. These improvements allow the

doubling of the data capacity every 6 months starting in 1992 until a bit rate of 10

Tb/s was achieved in 2001. Recently the bit rates have gone up to 14 Tb/s over a

single 160km line using optical amplifiers. The focus of development for the fifth

generation fiber optic communication system is on extending the wavelength range

over which a wavelength division multiplexing system can operate. Due to the

increased use of internet, demand in upgrading the bandwidth is needed in the next

generation fiber optics system.

2.1.2 Transmitters

Light-emitting diodes (LEDs) and laser diodes (LDs) are the most commonly-used

optical transmitters. The difference between laser diodes (LDs) and light emitting

diodes (LEDs) is that LEDs produce incoherent light, while laser diodes produce

coherent light. For semiconductor optical transmitters they are to be designed to be

compact, efficient, and reliable, while operating in an optimal wavelength range, and

directly modulated at high frequencies.

Light emitting diode (LED) is a forward-biased p-n junction that emitting light

through spontaneous emission and creates a phenomenon referred to as

electroluminescence. The emitted light is incoherent with a relatively wide spectral

width of 30-60 nm. LED light transmission is also inefficient, with only about 1 % of

input power, or about 100 microwatts, eventually converted into launched power

which has been coupled into the optical fiber. However, LEDs are very useful for

low-cost applications due to their relatively simple design.

Communications LEDs are made from gallium arsenide phosphide (GaAsP) or

gallium arsenide (GaAs).that is because GaAsP LEDs can operate at a longer

wavelength than GaAs LEDs, they have a wider output spectrum.

8

The large spectrum width of LEDs causes higher fiber dispersion, considerably

limiting their bit rate-distance product. LEDs are normally used in the local-area-

network applications with bit rates of 10-100 Mbit/s and transmission distances of a

few kilometers.

Figure 3: Red, green and blue LEDs of the 5mm type

2.1.3 Recivers

Photodetector is the main component in an optical receiver. A photodetector converts

light into electricity by using the photoelectric effect. The photodetector is typically a

semiconductor-based photodiode. Several types of photodiodes include avalanche

photodiodes, a p-i-n photodiodes, and p-n photodiodes.

2.1.4 Fibers

The optical fiber consists of a cladding, core, and a UV coated jacket (a protective

outer coating). The cladding guides the light along the core by using the method of

total internal reflection ( TIR ). The core and the cladding (which has a lower-

refractive-index) are made of high-quality silica glass, some are made of plastic as

well. Fusion splicing or mechanical splicing is the method to connect the optical

fibers together and requires special skills and interconnection technology due to the

microscopic precision required to align the fiber cores.

There are two types of optical fiber used in optic communications that is multi-mode

optical fibers and single-mode optical fibers.

9

http://en.wikipedia.org/wiki/File:RBG-LED.jpg

A multi-mode optical fiber has larger core (≥ 50 micrometres), allowing less precise

and cheaper transmitters and receivers to connect.

However, a multi-mode fiber introduces multimode distortion, which often limits the

bandwidth and length of the link. A single-mode fiber has a smaller core (<10

micrometres) and allows much longer, higher-performance links but requires more

expensive components and interconnection methods.

2.1.5 Amplifiers

There are 2 limitations in the transmission distance of a fiber-optic communication

that is the limitation by fiber attenuation and by fiber distortion. These problems can

be eliminated by using opto-electronic repeaters. These repeaters convert the signal

into an electrical signal, and then by using a transmitter to send the signal again at a

higher intensity. The cost of these repeaters is very high when it is used in the modern

wavelength-division multiplexed (WDM) signals.

Optical amplifier is an alternative approach, which amplifies the optical signal

directly without having to convert the signal into the electrical domain. It is made by

doping a length of fiber with the erbium, and pumping it with light from a laser with a

shorter wavelength than the communications signal (typically 980 nm). Amplifiers

have been used widely in new installations.

2.1.6 Wave-length division multiplexing

Wavelength-division multiplexing (WDM) is multiplying the available capacity of an

optical fiber by adding new channels. Using WDM technology that is commercially

available now, the bandwidth of a optic fiber can be divided into as many as 160

channels to support a combined bit rate into the range of terabits per second.

2.1.7 Dispersion

In optical fiber, the maximum transmission distance is limited not only by material

absorption but by severel types of dispersion. Dispersion is caused by a variety of

factors.

10

2.1.8 Inter-modal dispersion

Caused by different axial speeds of different transverse modes that limits the

performance of multi-mode fiber and single-mode fiber supports only one transverse

mode, therefore intermodal dispersion is eliminated.

2.1.9 Chromatic dispersion

Performance in single mode fiber is primarily limited because the index of the glass

varies slightly depending on the wavelength of the light.

2.1.10 Polarization mode dispersion

Single-mode fiber can sustain only one transverse mode, it can carry this mode with

two different polarizations. This phenomenon is called fiber birefringence and can be

counteracted by polarization-maintaining optical fiber.

Figure 4: Cross sections of three types of Polarization Maintaining Fiber

2.1.11 Attenuation

Fiber attenuation is caused by a combination of material absorption, Mie scattering,

Rayleigh scattering, and connection loss. Other forms of attenuation are caused by

physical stresses to the fiber, imperfect splicing techniques, fiber management and

microscopic fluctuations in density.

11

http://en.wikipedia.org/wiki/File:PM_optical_fibres.svg

Figure 5: Light attenuation by ZBLAN and silica fibers

2.2.12 Bending Loss

Radiative Losses occur when optical fiber undergoes a bend of finite radius of

curvature. Fibers can be subject to two types of bends: (a) macroscopic bends having

radii that are large compared with the fiber diameter, for example, such as those that

occur when a fiber cable turns a corner, and (b) random microscopic bends of the

fiber axis that can arise when the fibers are incorporated into cables.

For macroscopic bends, the excess loss is extremely small and is unobservable. As

the radius of curvature decreases, the loss increases exponentially until at a certain

critical radius the curvature loss becomes noticeable. If the bend radius is made a bit

smaller once this threshold point has been reached, the losses suddenly become

extremely large.

When a fiber is bent, the field tail on the far side of the center of curvature must move

faster to keep up with the field in the core, seen in Figure 6, for the lowest order fibre

mode. At a certain critical distance x from the center of the fiber, the field tail would

have to move faster than the speed of light to keep up with the core field. Since this is

not possible the optical energy in the field tail beyond x radiates away.

12

http://en.wikipedia.org/wiki/File:Zblan_transmit.jpg

Figure 6: Bending of fiber

The amount of optical radiation from a bent fiber depends on the filed strength x and

on the radius of curvature R. Since higher order modes are bound less tightly to the

fiber core than lower-order modes, the higher-order modes will radiate out of the fiber

first. Thus, the total number of modes that can be supported by a curved fiber is less

than in a straight fiber. Gloge has derived the following expression for the effective

number of modes Neff that are guided by a curved multimode fiber of radius a :

Neff = N∞ { 1 -

+ (

) ⁄ ]}

Where ∞ defines the graded-index profile, Δ is the core-cladding index difference, n2

is the cladding refractive index, k=2π/λ is the wave propagation constant, and

N∞ =

(n1ka)

2∆

Δ is the total number of modes in a straight fiber.

Another form of radiation loss in optical waveguide results from mode coupling

caused by random micro bends of the optical fiber.

Microbends are repetitive small-scale fluctuations in the radius of curvature of the

fiber axis, as is illustrated. They are caused either by non uniformities in the

manufacturing of the fibre or by non uniform lateral pressures created during the

cabling of the fiber.

13

The latter effect is often referred to as cabling or packaging losses. An increase in

attenuation results from micro bending because the fiber curvature causes repetitive

coupling of energy between the guided modes and the leaky or non guided modes in

the fiber.

2.1.13 Mechanical Misalignment

Radiation losses result from mechanical misalignments because of the radiation cone

of emitting fiber does not match the acceptance cone of the receiving fibre. The

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

fundamental types of misalignment between fibers are shown below

Longitudinal separation occurs when the fibers have the same axis but have a gaps

between their end faces. Angular misalignment results when the two axes form an angle

so that the fiber end faces are no longer parallel. Lateral displacement takes place when

the axes of the two fibers are separated by a distance.

The most common misalignment occurring in practice which also causes the greatest

power loss is lateral displacement. We would be performing this test in this thesis.

This axial offset reduces the overlap area of the two fiber-core end faces, as illustrated in

figure 9 and consequently, reduces the amount of optical power that can be coupled from

one fiber into the other.

(a) (b)

(c)

(d)

Figure 7: (a) Latreal misalignment (b) Gap between ends

(c) Angular misalignment (d) Non flat ends

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2.1.14 Coupling loss

Coupling loss occurs when tow components joint togather. Optical fiber is greatly

subject to coupling loss. Here are some of coupling losses.

Table:2 Different coupling losses

2.2 Overview of the optical link design

The design of an optical link involves many interrelated variables among the fibre,

source, and photo detector operating characteristics, so that the actual link design and

analysis may require several iterations before they are completed satisfactorily. Since

performance and cost constraints are very important factors, components are chosen

wisely to ensure the desired performance level can be met over the expected system

life time without over specifying the component characteristics.

The following key system requirements are needed in analysing a link:

1. Transmission distance

2. The data rate or channel bandwidth

3. The bit-error rate (BER)

The typical optical communication system consists of a transmitter, optical source,

transmission media, a detector and a receiver. In such a system, transmitter is one of

the key components, when it performs as the interface between electronics and the

emitters.

15

Losses Couses

Source to fiber From source to fiber connection

Fiber splicing Parmanent joint between two fibers

Optical fiber connectors Fiber to connector loss

Figure 8: Typical interface circuit for fibre-optic link

In order to pass electrical information through a fiber, an optical transmitter, is used to

convert electrical impulses into modulated light, which is then launched, or focused,

by the transmitter into the fiber.

The transmitter generally consists of a silicon integrated circuit that converts input

voltage levels from a computer, into current pulses. These current pulses, in turn,

drive a light-emitting diode (LED). The output light from the diode is then focused by

one or more lenses into the fiber. The link budget makes it possible to calculate how

far the link will carry signals without a repeater in attenuation limited systems or how

many connectors and splices can be used at a given distance in dispersion-limited

links .Another aspect of the link will be the consideration of choosing the type of

fiber.

In order to maintain an optimum performance, the connection must also protect the

fibre ends from damage that may occur due to handling (connection and

disconnection), must be insensitive to environmental factors (e.g. Moisture and dust)

must cope with tensile load on the cable. Additionally the connector should ideally be

a lost cost component that can be fitted with relative ease. Hence optical fibre

connectors may be considered in three major areas:

• The fibre termination, which protects and locates the fibre ends;

• The fibre end alignment to provide optimum optical coupling;

• The outer shell, which maintains the connection and the fibre alignment,

protects the fibre ends from the environment and provides adequate

strength at the joint.

16

Use of index matching material in the connector between two jointed fibres can

increase light transmission while keeping dust and dirt away from the fibres.

2.2.1 Optical source

In any fiber optic system the light emitters are the key element. This component

converts the electrical signal into a corresponding light signal that can be injected into

the fiber. The light emitter is an important element because it is often the most costly

element in the system, and its characteristics often strongly influence the final

performance limits of a given link. There are two types of light emitters in wide spread

use: laser diodes (LDs) and light-emitting diodes (LEDs).

Figure 9: LED/LD converting electrical signal to light signal

2.2.2 Comparison between (LD) and (LED)

Parameter Light-Emitting Diode

(LED)

Laser diode (LD)

Output power Linearly proportional to

drive current

Proportional to current

above the threshold

Current Drive current 50 to 100 mA

peak

Threshold current 5 to 40

mA

Coupled power Moderate High

Bandwidth Moderate High

Wavelengths Available 0.66 to 1.65 µm 0.78 to 1.65 µm

Emission spectrum 40nm to 190nm FWHM 0.00001nm to 10nm

FWHM

Cost $ 5 to $ 300 $ 5 to $ 3000

Table 3: Comparison of LED and LD

17

Figure 10: Current vs. Light Output

2.2.3 Fiber splicing

A fiber splice is a permanent joint between two fibers. These are typically used to

create long optical links or in situations where frequent connection and disconnection

are not needed. In making and evaluating such splices, one must take into account the

geometrical differences in the two fibers, fiber misalignments at the joint and the

mechanical strength of the splice. Fusion splice is one of the fiber splicing techniques

that yields a permanent joint.

Fusion splices are made by thermally bonding together prepared fiber ends. Fusion

splicing involves a series of steps. First, the fiber must be exposed by cutting open the

cable. Then the protective plastic jacket must be stripped from a few millimetres to a

few centimetres of the fiber ends to be spliced. The fiber ends must be cleaved to

produce cleave angle that are within 1 to 3 degrees of being perpendicular to the fiber

axis. The ends msut be kept clean until they are fused.

The next step is alignment of the fibers, which may be done manually or automatically

by different splicer models. After preliminary alignment, the ends may be prefused for

about a second with a moderate arc that cleans their ends and rounds their edges. These

ends are then pushed together, allowing power transmission to be tested to see how

accurately they are aligned. After results are satisfactory, the arc is fired to weld the

two fiber ends together. Care must be taken care to ensure proper timing of the arc so

the fiber ends are heated to the right temperature.

18

After the joint cools, it can be recoated with a plastic material to protect against

environmental degradation. The spliced area can also be enclosed in a plastic jacket.

The entire splice assembly is then enclosed mechanically for protection, which in turn

is mounted in a splice enclosure. The case around the individual splice provides strain

relief.

This technique can produce very low splice losses (typically averaging less than 0.01

dB). However care must be exercised in this technique, since surface damage due to

handling, surface defect growth created during heating, and residual stresses induced

near the joint as a result of changes in the chemical composition arising from the

material melting can produce a weak splice.

2.2.4 Optical fiber connectors

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

applications. Their uses range from simple single channel fiber to fiber connectors in a

benign location to multichannel connectors used in harsh military filed environments.

Some of the principal requirements of a good connector design are as follows:

• Low coupling losses: The connector assembly must maintain stringent alignment

tolerances to assure low mating losses. These low losses must not change significantly

during operation of after numerous connect and disconnects.

• Interchangeability: Connectors of the same type must be compatible from one

manufacturer to another.

• Ease of assembly: A service technician should readily be able to install the connector

in a field environment. The connector loss should also be fairly insensitive to the

assembly skill of the technician.

• Low environmental sensitivity: Conditions such as temperature, dust and moisture

should have a small effect on connector loss variations.

19

• Low cost and reliable construction: The connector must have a precision suitable to

the application, but its cost must not be a major factor in the fiber system.

• Ease of connection: Generally, one should be able to mate and demate the connector

simply by hand.

2.2.5 SC fiber optic connector basic structure

More than a dozen types of fiber optic connectors have been developed by various

manufacturers since 1980s. Although the mechanical design varies a lot among

different connector types, the most common elements in a fiber connector can be

summarized in the following picture. The example shown is a SC connector which

was developed by NTT (Nippon Telegraph and Telephone) of Japan.

Figure 11: A SC Connector Sample

Figure 12: SC Connector Structure

20

2.2.6 Elements in a sc connector

• The fiber ferrule.

Figure 13: SC Connector Fiber Ferrule

SC connector is built around a long cylindrical 2.5mm diameter ferrule, made of

ceramic (zirconia) or metal (stainless alloy). A 124~127µm diameter high precision

hole is drilled in the center of the ferrule, where stripped bare fiber is inserted through

and usually bonded by epoxy or adhesive. The end of the fiber is at the end of the

ferrule, where it typically is polished smooth.

• The connector sub-assembly body:

The ferrule is then assembled in the SC sub-assembly body which has mechanisms to

hold the cable and fiber in place. The end of the ferrule protrudes out of the sub-

assembly body to mate with another SC connector inside a mating sleeve (also called

adapter or coupler).

• The connector housing:

Connector sub-assembly body is then assembled together with the connector housing.

Connector housing provides the mechanism for snapping into a mating sleeve

(adapter) and hold the connector in place.

• The fiber cable:

Fiber cable and strength member (aramid yarn or Kevlar) are crimped onto the

connector sub-assembly body with a crimp eyelet. This provides the strength for

mechanical handing of the connector without putting stress on the fiber itself.

21

• The stress relief boot:

Stress relief boot covers the joint between connector body and fiber cable and protects

fiber cable from mechanical damage. Stress relief boot designs are different for

900µm tight buffered fiber and 1.6mm~3mm fiber cable.

2.2.7 How fiber optic connectors mate

Figure 14: FC Connectors Mating

Unlike electronic connectors, most fiber optic connectors don’t have jack and plug

design. Instead a fiber mating sleeve (adapter, or coupler) sits between two

connectors. At the center of the adapter there is a cylindrical sleeve made of ceramic

(zirconia) or phosphor bronze. Ferrules slide into the sleeve and mate to each other.

The adapter body provides mechanism to hold the connector bodies such as snap-in,

push-and-latch, twist-on or screwed-on. The example shown above are FC connectors

with a screwed-on mechanism.

2.2.8 Fiber optic connectors types and there application

Both examples shown above are for single fiber cable (simplex) which is easy to

install. However there are also duplex and multi-fiber connector designs. Below are

loosely divided family types of fiber connectors which sometimes overlap.

ST connector – simplex only, twist-on mechanism. Available in single mode and

multimode.

It is the most popular connector for multimode fiber optic LAN applications . It has a

long 2.5mm diameter ferrule made of ceramic (zirconia), stainless alloy or plastic. It

mates with a interconnection adapter and is latched into place by twisting to engage a

spring-loaded bayonet socket.

22

Figure 15: ST Connector and ST Adapter (mating sleeve)

FC connector – simplex only, screw-on mechanism. Available in single mode and

multimode.

FC connector also has a 2.5mm ferrule (made of ceramic (zirconia) or stainless alloy)

.It is specifically designed for telecommunication applications and provides non-

optical disconnect performance. Designed with a threaded coupling for durable

connections. It has been the most popular single mode connectors for many years.

However it is now gradually being replaced by SC and LC connectors.

Figure 16: FC Connector

SC connector – simplex and duplex, snap-in mechanism. Available in single mode

and multimode.

SC was developed by NTT of Japan. It is widely used in single mode applications for

its excellent performance. SC connector is a non-optical disconnect connector with a

2.5mm pre-radiused zirconia or stainless alloy ferrule. It features a snap-in (push-pull)

connection design for quick patching of cables into rack or wall mounts. Two simplex

SC connectors can be clipped together by a reusable duplex holding clip to create a

duplex SC connector.

Figure 17: Simplex SC Connector

23

2.2.9 Small form factor fiber optic connectors

A number of small form factor fiber optic connectors have been developed since the

90s’ to fill the demand for devices that can fit into tight spaces and allow denser

packing of connections. Some are miniaturized versions of older connectors, built

around a 1.25mm ferrule rather than the 2.5mm ferrule used in ST, SC and FC

connectors. Others are based on smaller versions of MT-type ferrule for multi fiber

connections, or other brand new designs. Most have a push-and-latch design that

adapts easily to duplex connectors.

LC connector – simplex and duplex – push and latch – 1.25mm ferrule. Available in

single mode and multimode.

Externally LC connectors resemble a standard RJ45 telephone jack. Internally they

resemble a miniature version of the SC connector. LC connectors use a 1.25mm

ceramic (zirconia) ferrule instead of the 2.5mm ferrule. LC connectors are licensed by

Lucent and incorporate a push-and-latch design providing pull-proof stability in

system rack mounts. Highly favored for single mode applications.

Figure 18: LC Connector – Simplex and Duplex

MU connector – simplex, duplex – snap in, 1.25mm ferrule.

MU connectors and adapters were developed by NTT, and have push-pull

mechanism. They are called “mini SC” and are more popular in Japan. Applications

include high-speed data communications, voice networks, telecommunications, and

dense wavelength division multiplexing (DWDM). MU connectors are also used in

multiple optical connections and as a self-retentive mechanism in backplane

applications.

24

Figure 19: MU Connector – Simplex and Duplex

E2000 connector. 1.25mm ferrule, snap-in mechanism. Also called LX.5 connector.

Available in single mode and multimode.

Externally a E2000 connector looks like a miniature SC connector. The connector is

easy to install, with a push-pull latching mechanism which clicks when fully inserted.

It features a spring-loaded shutter which fully protects the ferrule from dust and

scratches. The shutter closes automatically when the connector is disengaged, locking

out impurities which could later lead to network failure, and locking in potentially

harmful laser beams. When it is plugged into the adapter the shutter opens

automatically.

Figure 20: E2000 Connector

2.2.10 Advantages of fiber-optic communications

• Low transmission loss

Development of optical fibers over the past 40 years has resulted in the production of

optical fibers which exhibit very low attenuation or transmission loss. Fibers have been

fabricated with losses as low as 0.1 dBkm-1 and this feature facilitates the

implementation of communication links with extremely wide repeater spacing.

25

• Small size and weight

Optical fibers have small diameters which are often no greater than the diameter of a

human hair. Even with protective coatings they are far smaller and much lighter than

corresponding copper cables.

• Electrical Isolation

Optical fibres which are fabricated from glass, or sometimes a plastic polymer, are

electrical insulators and therefore unlike their counterparts, they do not exhibit earth

loop and interface problems.

• Large bandwidth

The optical carrier frequency yields a far greater potential transmission bandwidth

than metallic cable systems. Information-carrying capacity of optical fiber system has

proved far superior to the best copper cable systems. By comparison the losses in

wideband coaxial cable systems restrict the transmission distance to only a few

kilometres at bandwidths over one hundred megahertz.

• System reliability

These features primarily stem from the low loss property of optical fiber cables which

reduces the requirement for intermediate repeaters or line amplifiers to boost the

transmitted signal strength. Furthermore, the reliability of the optical components is no

longer a problem with predicted lifetimes of 20 years and above, henceforth reducing

the maintenance time and costs.

• Low cost

Optical fibres offer potential for low cost line communication compared to those with

copper conductors. Overall system cost when utilizing optical fibre communication on

long-haul links is substantially less than those for equivalent electrical line systems.

26

• Immunity to Interference and Crosstalk

Optical fibers form a dielectric waveguide and are therefore free from electromagnetic

interference (EMI), radio-frequency interference (RFI), or switch intransients giving

electromagnetic pulses (EMP). Henceforth, operation of an optical communication

system does not require shielding from EMI.

• Signal Security

The light from optical fibers does not radiate significantly and therefore they provide a

high degree of signal security. Unlike situation with copper cables, a transmitted

optical signal cannot be obtained from a fiber in a non-invasive manner (i.e without

drawing optical power from the fiber.)

2.3 Link power budget

An optical power loss model for a point to point link is shown in Fig (21 ). The optical

power received at the photodetector depends on the amount of light coupled into the

fiber and the losses occurring in the fiber and at the connectors and splices. The link

loss budget is derived from the sequential loss contributions of each element in the

link. Each of these loss is expressed in decibels (dB) as

Loss = 10 log Pout / Pin

Figure 21: Link Diagram

27

where Pin and Pout are the optical powers emanating into and out of the loss element,

respectively. A link margin of 6 – 8 dB is generally used for the systems that are not

expected to have additional components incorporated into the link in the future.

The link loss budget simply considers the total optical power loss that is allowed

between the light source and the photodetector, and allocates this loss to cable

attenuation, connector loss, splicer loss, and system margin. Table (4) shows an

example of the link power budget calculation.

Optical Transmitter Output Power -13dBm

Receiver Sensitivity -42dBm

Required margin 29dBm

System loss:

Table 4: Link Power Budget

In this example, we have an excess power margin of 29dB – 28dB = 1 dB. Thus we

have a total headroom of 7dB when we include the system margin of 6dB.

2.4 Rise time budget

The rise time budget determines the dispersion limitation of a link. Dispersion is the

broadening of a pulse as it travels through an optical fiber. We will define the rise time

due to the material (chromatic) ( t mat ) and modal dispersion ( t mod ). Thus the total rise

time of the system is the rms of all the rise times.

Total rise time : t sys = ( t2

tx + t2mat +t

2mod + t

2rx )

1/2

28

Fiber Loss 3.5dBm/km X 5 km 17.5dB

Connector loss 1dB x 2 connectors 2dB

Total Splicing Loss (0.5dB X 5) 2.5dB

System margin (Headroom) 6dB

Total Attenuation 28dB

For digital data, two main coding schemes are used. They are the return-to-zero (RZ)

and non-return-to-zero (NRZ). The NRZ method occupies the entire bit width (bit

period), whereas the RZ method occupies only a portion of the bit period.

1 0 1 0 1 0 1 0 1 0 1 0 Bit Stream

Clock

RZ Data

NRZ data

Figure 22: RZ and NRZ Coding Schemes

A general rule of thumb is the system rise time should be less than a percentage of one

of these methods. The bit period is seconds is the reciprocal of the bit rate in Hertz.

Tsys(RZ) ≤ 35% (

) = 35% (Bit Period)

Tsys(NRZ) ≤ 70% (

) = 70% (Bit Period).

29

CHAPTER 3

HARDWARE DESIGN AND IMPLEMENTATION

3.1 Rise time budget

For a return-to-zero signal, the pulse duration is half the repetation period T, which

lead to

ts = (.7T) / RRZ………………………………………………………………………..3.1

A similar argument for the NRZ code the pulse duration(τ) and the repetition period

(T) are both equal to 1/R, where R is the data rate. A reasonable estimate of the

required rise time ts is that it be no more than 70% of the pulse duration. The rise time

must be limited to

ts = .7τ = .7/RNRZ……………………………………………………………………...3.2

The total rise time must be apportioned between the light source, the fiber and the

photo detector in the manner indicated by

ts2

= tLS2+tF

2+tPD

2……………………………………………………………………..3.3

There is a relationship between the fiber’s Rise time and it’s pulse spread. The

equation is given by

tF = .35/f3-dB(electrical) = ∆τ…………………………………………………..................3.4

The fiber’s rise time is given by

∆τ = L Mt ∆λ…………………………………………………………………………3.5

Here, Mt = Net dispersion factor and it is found from matrial dispersion factor (Mt) and

waveguide dispersion factor (Mg)

The matrial dispersion factor and the waveguide dispersion factor opposes one another.

According to condition ts of equation (3.3) must be equal or less than the system rise

time of (3.1) or (3.2)

30

3.2 Power budget

The purpose of the power budget is to ensure that enough power will reach the receiver

to maintain relable performance during the entire system life time. In the preparation of

link power budget, certain parameters like required optical power level pr at the

receiver to meet the system requirements, coupling losses etc are required.

After computing various losses and fixing safety margin,power budget of the link is

calculated by the following equations.

Power margin in dB, Pm = Pt-Pr(min)-Lsf -NLff –αL- Lfd- Lc........................................3.6

Where, pt = source output power …………..dBm

Pr (min) = Minimum receiver power ……….dB

Lsf = source to fiber coupling loss…………..dB

Lfd = Fiber to detector coupling loss………..dB

Lff = Fiber to fiber coupling loss……………dB

N = Number of splice

L = fiber link length…………………………km

α = Attenuation coefficient of fiber ………..dB/km

Lc = Connector loss(1dB each for the transmitter and receiver)

A power margin pm ≥ 6dBm is acceptable otherwise some components need to be

upgraded. With Pm ≤ 6dBm, system will become less reliable.

3.3 Quantum noise limited receiver sensitivity

A quantum noise limited receiver sensitivity provides the ultimate in detection. We

will compute it’s sensitivity to provide a benchmark against which other receiver’s can

be measured. The error rate for quantum limited system with negligible dark current is

given by

Pe =exp(-ns )

Or,BER=exp(-ns)……………………………………………………………………3.7

31

Where ns is the average number of photoelectrons generated when a binary one is

received. The number of incident photon’s necessary to produce ns electrons is ns/ɳ

where ɳ is the quantum efficiency .

The peak optic power in a rectangular pulse is related to ns by

P=(hfns)/( ɳτ)

=(hcns)/( ɳλτ)……………………………………………………………………3.8

Where,

τ= the pulse duration

h= the plank constant

λ=the operating wavelength

For NRZ signals, τ=1/R, the power is given by

P = (hcns)/( ɳλ)………………………………………………………………………3.9

For RZ signals, τ=1/2R, the power is given by

P=(2hcns)/( ɳλ)………………………………………………………………………3.10

The sensitivity of PIN receiver’s is 28 dB poorer and the APD is 20 dB than the

quantum noise limited receiver sensitivity.

3.4 Fiber RMS Dispersion Analysis

There are generally two sources of dispersion: material dispersion,which comes from a

frequency dependent response of a material to waves; and a modern

dispersion.considering both the modal and material dispersion in fiber optic the rms

pulse width of the fiber is determined as follows

σf =√ (

)…………………………………………………………………3.11

where σmat and σmod are the rms pulse widths due to modal and material dispersion

(and usually expressed in ps/km-nm) of the fiber respectively.

32

For SM fiber modal dispersion does not contribute and therefore σf becomes same as

σmat .The optical fiber link may consist of several concatenated sections and the fiber in

each section may have different dispersion characterstics.Further there may be mode

mixing at the splices and the connectors. As a consequence, propagation delay

associated with different modes tensed to average out. In the absence of mode mixing

σmod for SI fiber is

σmod = (n1∆L)/(2√3c)………………………………………………………………3.12

where c is rhe velocity of light and the parameter ∆ depends on the core and cladding

refraction indics.For Graded Index (GI) fibers, delay time is a fraction of the refractive

index profile (g). The minimum intermodal rms pulse broadening with an optimum g is

σmod = (n1∆L)/(20√3c)………………………………………………………………3.13

The rms pulse width due to material dispersion is determined as follows,

σ mat=׀Dmat׀σλL………………………………………………………………………3.14

Where, σλ is the rms spectral width of the source and Dmat is the dispersion parameter.

3.5 Maximum Link Length Calculation

The maximum link length is determined presuming that link is attenuation limited and

there is no dispersion effect and again when the link is dispersion limited and there is

no attenuation effect. And the maximum practicable link length is taken as optimums

the two are equal at the desired data rate.For dispersion limited link ,maximum data

rate that can be transmitted over an optical fiber system is given by

R=(1/4 σ sys)………………………………………………………………………3.15

Where R is data rate in Mbps

33

The maximum allowable fiber dispersion will be

σal = √((1/4R)2-σtx

2-σrx

2 ) ns

σal = √(250/R)2-σtx

2-σrx

2……………………………………………………………..3.16

Therefore the maximum link length under dispersion limited condition is determined as

Ld (max) = (L*σal)/σf (km)…………………………………………………………3.17

The formula for maximum link length under attenuation limited condition is

Ls (max) = [ Pt(dBm)-Pr(dBm)]/α km……………………………………………….3.18

3.6 Design Example

Design a digital fiber optic link that must transmit a 600 Mbps NRZ pulse train over a

100 km path with an bit error rate of 10-12

without using any repeater .

3.6.1 Wavelength region

We know for long distance communication attenuation should be minimum as a result

high wavelength region that is third optic region has been considered in this thesis.

3.6.2 Source Slection

Two types of source have been considered in this thesis (1 ) LED Source (2) LASER

Source. Between this two sources LASER diode source is more preferable for

following reasons

1) Spectrsl width is less for LASER diode source.

2) Less rise time than LED.

3) More coherent

4) Coupling efficiency is better than LED.

5) Attenuation loss is less.

3.6.3 Fiber selection

In this thesis the fiber has been fixed.Multimode graded index fibers produce less

modal pulse distortion than multimode step index fiber.Because of this advantage most

glass multimode fiber is graded index .

34

On the other hand, single- mode graded –index fibers provide no advantage over

single- mode step-index fibers with respect to pulse spreading . Again the attenuation

loss of single- mode step-index fiber is less than other fibers. So, in this thesis single-

mode-step-index fiber (SI-SM) fiber has been fixed.

3.6.4 Detector Selection

Two types of detector have been chosen in the link design (1)PIN (2)APD.

This two will be compared and better one will be selected in the design .The PIN is

cheaper , less sensitive to temperature , requires lower reverse bias voltage than the

APD. The APD gain is needed when the system is loss limited, as occurs for long

distance.

Four types of combination are considered

1)LED-SI-SM-PIN

2) LED-SI-SM-APD

3) LD-SI-SM-PIN

4) LD-SI-SM-APD

LED-SI-SM-PIN combination

Rise time budget

From (3.1) the system rise time budget

ts = .7/RNRZ

=.7/400X106

=1.75ns

The matrial dispersion factor for ts = 1.75ns, is M = -20 ps/(nm-km)

And the waveguide dispersion factor, Mg = 4.5 ps/(nm-km)

The net dispersion factor, Mt = 20-4.5

= 15.5 ps/(nm-km)

Now, L = Length between transmitter and Receiver = 100km

∆λ = spectral width = 50 nm

35

From (3.5) the fibers rise time is given by,

∆τ = L MT ∆λ

= 100x15.5x50

= 77.5ns

From figure (3.4) and (3.5)

tF = ∆τ = 77.5ns

LED’s rise time, tLS = 3ns

PIN photodetector’s rise time tPD = .3ns

From figure (3.3) The total rise time must be

ts=

⁄

= √[(3)2+(77.5)

2+(.3)

2]

= 77.5586ns

Which exceeds the total system rise time budget. So, next higher cost combination will

have to be considered.

LED-SI-SM-APD combination

Rise time budget

The net dispersion factor, Mt = 15.5 ps/(nm-km)


∆λ = spectral width = 50ns


∆τ = L MT ∆λ

= 100x15.5x50

= 77.5ns


tF = ∆τ = 77.5ns

36

LED’s rise time, tLS = 3



ts=

⁄

= √[(3)2+(77.5)

2+(.25)

2]

= 77.5584ns

Which exceeds the total system rise time budget. So, next higher cost combination will

have to be considered.

LD-SI-SM-PIN combination

Rise time budget



∆λ = spectral width = .15ns


∆τ = L MT ∆λ

= 100x15.5x.15

= .2325ns


tF = ∆τ = .2325ns




ts=

⁄

= √[(1)2+(.2325)

2+(.3)

2]

= 1.06961ns

Which less than the total system rise time budget. So, the power budget can be done.

37

Power budget

A quantum-limited receiver sensitivity:

From (3.7) the error rate for quantum limited system with negligible dark current is

given by

BER = exp(-ns)

Or, ns = - In(BER)

= -In(10-12

)

= 27.631

= 28 (approximately)

From(3.9) the peak optic power in a rectangular pulse is related to ns by

P = (hcns)/( ɳλ)

= (6.63x10-34

x28x400x106x 3x10

8)/(1x1.55x10

-6)

= 1.485nw

In dB form P = -58.28 dB

The sensitivity of PIN receiver is 28dB poorer. So, the sensitivity of the PIN receiver

will be Pr (min) = -58.28+28

= -30.28nw

Pt = Source output power =3.1623mw or 5dBm

Pr (min) = Minimum receiver power = -30.28dB

Lsf = Source to fiber coupling loss = 3 dB

Lfd = Fiber to detector coupling loss = .2 dB

Lff = iber to fiber coupling loss = .1dB

N = Number of splice = 50

L = fiber link length = 100km

α = Attenuation coefficient of fiber = .25dB/km

Lc = Connector loss = 2dB (1dB each for the transmitter and receiver)

From (3.5),we get

Power margin in dB

Pm = Pt - Pr (min) - Lsf - NLff – αL - Lfd - Lc…………………………………………3.6

= 5-(-30.28)-3-50x.1-.25x100-.2-2

= 0.08dB

Which is very less than the minimum power budget (6dB) required.

38

LD-SI-SM-APD combination

Rise time budget



∆λ = spectral width = .15ns


∆τ = L MT ∆λ

= 100x15.5x.15

= .2325ns


tF = ∆τ = 77.5ns




ts=

⁄

= √[(1)2+(.2325)

2+(.25)

2]

=1.0567 ns

Which less than the total system rise time budget. So, the power budget can be done.

Power budget

APD’s receiver’s sensitivity:

The sensitivity of APD receiver is 20dB poorer than a quantum limited receiver

sensitivity. So, the sensitivity of the APD receiver will be

Pr (min) = -58.28+20

= -38.28nw

Pt = Source output power = 3.1623mw or 5dBm

Pr (min) = Minimum receiver power = -38.28dB

Lsf = Source to fiber coupling loss = 3 dB

Lfd = Fiber to detector coupling loss = .2 dB

Lff = iber to fiber coupling loss = .1dB

N = Number of splice = 50

L = fiber link length = 100km

α = Attenuation coefficient of fiber = .25dB/km

Lc = Connector loss = 2dB (1dB each for the transmitter and receiver)

From (3.5), we get

Power margin in dB

Pm = Pt - Pr (min) - Lsf - NLff – αL - Lfd - Lc…………………………………………3.6

= 5-(-38.28)-3-50x.1-.25x100-.2-2

= 8.08dB

Which is greater than the minimum power budget (6dB) required.

LD-SI-SM-APD combination is the only one combination that satisfy both rise time

budget and power budget. So, LD-SI-SM-APD is the correct design.

6.6 Maximum link Length Calculation

L = length of the fiber = 100km

σλ= The rms spectral width of the source =.15nm

Dmat = the dispersion parameter = 15.5 ps/(nm-km)

The rms pulse width due to matrial dispersion is determined from(3.14) as follows

σmat= ׀ Dmat׀ σλ L

= 20 x.15x100

= 300ps

= .3ns

For the step index fiber, σmod = the rms pulse width due to modal = 0

σmat= The matrial dispersion of the fiber

From (3.11) The rms Spectral width is given by

σf =√ (

)

= .2325ns

39

From (3.16) we get

σal=√(250/R)2-σtx

2-σtx

2

=√(250/400)2-.5

2-.125

2

= .3536ns

Therefore, the maximum link length under dispersion limited condition is determined

from (3.17) as: Ld (max) = (L* σal)/ σf

= (100)x(.3536)/.3

= 117.8667km

From (3.18) The formula for maximum link length under attenuation limited condition

is:

La(max)= [ Pt(dBm)-Pr(dBm)]/α km

= (5+38.25)/.25

= 173.00km

40

CHAPTER 4

SOFTWARE DESIGN AND IMPLEMENTATION

4.1 Introduction

In this thesis software implementation has been done by using MATHLAB 7

software. The four combinations has been compared in the program and the correct

design has been chosen from those combinations.

4.2 Algorithm of the program

(1) Strat the program;

(2) Input R,BER,L;

(3) Select combination according to lowest cost;

(4) Make rise time budget

(5) If total rise time is less than the system rise time

then go to (6) otherwise go to (3)

(6) Evaluate Pr for given R,BER

(7) Determine Pm

(8) If the Pm is less than 6dB than go to (9) otherwise go to (3)

(9) List out source, fiber and detector

10) Calculate maximum length under attenuation limitted condition and

dispersion limited condition.

11) Stop the program

41

4.3 Flow chart of a repeterless link design

42

Specify R,BER,L

If

Pm>=6dB

Is Total rise time

<=System rise time

Start

Stop

pppp

Select source, fiber,detector

Select

different

combinations

Evaluate Pr for

Given R,BER

Determine Pm

Make rise time budget

Evaluate Maximum

Link Length

Make power

budget

List out

source,fiber,detector

All

combination

tried

NO

NO

YES

YES

4.6 Figure of the output

combination Rise time

budget (ns)

Power

budget

(dB)

Attenuation

limited

Maximum

Distance

(km)

Dispersion

Lmitted

Maximum

Distance (km)

Comment

LED-SI-SM-PIN 77.5586>1.75 Wrong

Design

LED-SI-SM-APD 77.5584>1.75 Wrong

Design

LD-SI-SM-PIN 1.06961 < 1.75 0.08<6 Wrong

Design

LD-SI-SM-APD 1.0567< 1.75 8.08>6 173.00 177.8667 Current

design

Figure: Design Output

45

CHAPTER 4

CONCLUSION AND FUTURE DEVELOPMENT

4.1 Conclution

In conclusion, the outcome of this Capstone thesis has been achieved. It is a good

learning experience for me. The whole designing and development process was

passed through. The designing, developing and conducting of technical investigations

and experiments of the capstone thesis was tough but enjoyable as I have learn a lot

more on fiber optics design.

Overall I conclude that this digital fiber optic link can serve as a good and useful

educational for students that will be taking up the optic communications course. The

various measurement techniques carried out in this thesis will help them to enhance

their knowledge and gain better understanding in the field of fiber optics link system.

The optical transmission is a major innovation in the filed of telecommunications.

This new technology in the communications field will depend on the economic

viability of fiber systems compared to conventional system such as copper that require

high cost. Its feasibility is being demonstrated in many on going thesis and trials such

as the National Broadband Network that is current being install is wide in Singapore

as well as few other asian countries.

An advance in very high speed digital application lines makes it necessary to develop

long haul information superhighways capable of transmitting data at high bandwidth.

Conventional copper cables are stretched to their limits, and fiber optic transmission

is the technology for the future communication systems. Optical fibres can transmit

more information than copper wire. With the available wavelength spectrum of light

divided into a series of parallel channels, thousand of signal can be transmitted along

a single fibre. Transmission rates of the fiber optics has been increasingly increase

since and a lot of big corporation is doing a lot of research and development work to

even increase the bandwidth and transmission distance of the fibre optics.

46

4.2 Future development

Presently, the work we have completed so far has not maximised the full performance

of the digital link. We can improve the full performance of the link by accumulating

even more data to compare and generate more precise data range.

Live testing of the link performance will also help in improving the performance as

what we have done here is just mathematical calculations. Live testing will identify

any performance issue such as actual optic fiber attenuation, quality of connectors,

quality of splices and quantity of splices and connectors in the link.

Nevertheless, future work can be done by using some design software such as Labview

that can be used to simulate waveforms and analyse waveforms other than using

traditional equipment such as oscilloscope. In this way the students can actually

maximise the performance of the digital link. Finally, if the above changed can be

done, the digital link can be served as an excellent module for students doing lab

experiments.

47

CHAPTER 5

REFLECTIONS

5.1 Reflections

From this thesis, I have learnt on how to design the fiber optic communication link.

This thesis also helps me to understand my job better as I’m in the fiber optic

engineering. Through this capstone project, I have gain new skills and knowledge that

not taught in UniSim. Researching of thesis and developing of project is part self-

learning progress. My analytical skills also improved based on the literature research

and seeking advice from my thesis supervisor.

The success that I have achieved is that I can finish thesis in time, the thesis objective

of calculating the fiber optic link design and to find the best combination of raw

material to maximize the fiber optic link design is met. My discontent is I am unable

to apply my theory to experiment, as UniSIM does not provide the lab equipments.

This capstone thesis has taught me valuable lessons in work organize, discipline and

time management. I have a few setbacks during the progress of achieving the results,

being able to improve my weakness in these areas and work toward my goals has

become my greatest determination.

Despite the fact completing this capstone required a lot of time, the anticipation of the

outcome is a success. This thesis has enriched me as I gained more confident in

completing a given task.

At last, the LD-SI-SM-APD combination is the only one combination that satisfy both

rise time budget and power budget. So, LD-SI-SM-APD is the correct design from

other three combination.

48

CHAPTER 6

REFERENCE

6.1 Books

[1] Palais, Joseph C.,“ Fiber Optic Communications”, Fifth Edition, Prentice Hall.,

2005.

[2] B.Saleh and M.Teich, Fundamental of Photonics, New York, John Wiley & Sons,

2000.

[3] J. Wilson and J.Hawkes, Optoelectronics, An introduction, 3rd

Edition., London,

Prentice Hall Europe, 1998.

[4] John M.Senior, “Optical Fibre Communication”, Prentice-Hall., 1999.

[5] Gerd Keiser, “ Optical Fiber Communication”, 3rd

Edition International Editions,

McGraw Hill 2000.

[6] James C.Daly, Fibre Optics”, CRC Press, 1984.

[7] Dennis Derickson, “Fibre Optic Test and Measurement”, Prentice Hall 2002.

[8] Govind P.Agrawal, “Fiber Optic Communication Systems, 3rd

Edition” John

Wiley & Sons, Inc Publications, 2002.

[9] Jeff Hechf, “Understanding Fiber Optics”, 2nd

Edition, Sams Publishing 1996.

[10] Roy Blake, “Comprehensive Electronic Communication”, West Publishing

Company,1997

[11] William Stallings,“Data and Computer communications”. Macmillan, 1991

[12] D J H, Maclean, “Optical Line Systems”, John Wiley and Sons, 1996

6.2 Websites

[1] Fujikura Single Mode Optical Fiber,

http://www.fujikura.co.jp/eng/products/tele/o_fiber/td101001.html

http://www.fujikura.co.jp/eng/products/tele/o_fiber/td101002.html

[2] Fujikura Arc Fusion Splicer Specifications

http://www.fujikura.co.jp/eng/products/tele/o_f_splicers/td70005.html

49

[3] Draka Cableteq Fiber Optic Cables

www.DrakaMOG.com

[4] Wave Optics, Single Mode Optical Fibers

http://www.waveoptics.com/

[5] Nec InGaAsP Pulsed Laser Diode Module

www.datasheetcatalog.com

www.nec.co.jp

[6] Hamamatsu Pin Photodiode

http://jp.hamamatsu.com/en/product_info/

[7] Anritsu Corporation

www.anritsu.com

[8] JDS Uniphase, Laser Diode, Light Source,

www.jdsu.com.sg

[9] Mitsubishi Laser Diode

http://www.mitsubishichips.com/Global/common/cfm/eLineUp.cfm?FOLDER=/

product/opt/laserdiode

[10] OKI Laser Diode and Led

http://www.okisemiopto.com

[11] Transition Networks

www.transition.com

[12] Honeywell – Application Note

http://content.honeywell.com/sensing/prodinfo/fiberoptic/application/on7eng.pdf

50

LIST OF FIGURES

Figure 1: Generic optic communication system……………………………………….1

Figure 2: In fiber-optic communications, information is transmitted by sending

light through optical fibers………………………………………………….7

Figure 3: Red, green and blue LEDs of the 5mm type………………………………..9

Figure 4: Cross sections of three types of Polarization Maintaining Fiber…………..11

Figure 5: Light attenuation by ZBLAN and silica fibers…………………………….12

Figure 6: Bending of fiber……………………………………………………………13

Figure 7: (a) Latreal misalignment (b) Gap between ends

(c) Angular misalignment (d) Non flat ends………………………………14

Figure 8: Typical interface circuit for fibre-optic link…………………………………..16

Figure 9: LED/LD converting electrical signal to light signal………………………..17

Figure 10: Current vs. Light Output……………………………………………………..18

Figure 11: A SC Connector Sample………………………………………………….20

Figure 12: SC Connector Structure…………………………………………………..20

Figure 13: SC Connector Fiber Ferrule………………………………………………21

Figure 14: FC Connectors Mating……………………………………………………22

Figure 15: ST Connector and ST Adapter (mating sleeve)………………………….23

Figure 16: FC Connector……………………………………………………………..23

Figure 17: Simplex SC Connector……………………………………………………23

Figure 18: LC Connector – Simplex and Duplex…………………………………….24

Figure 19: MU Connector – Simplex and Duplex…………………………………...25

Figure 20: E2000 Connector…………………………………………………………25

Figure 21: Link Diagram…………………………………………………………….27

Figure 22: RZ and NRZ Coding Schemes…………………………………………..29

LIST OF TABLES

Table 1: Operating Parameters………………………………………………………3

Table:2 Different coupling losses…………………………………………………...15

Table 3: Comparison of LED and LD……………………………………………….17

Table 4: Link Power Budget………………………………………………………..28

Table 5: Design Output………………………………………………………………45

thesis submitted partial fulfillment of the requirements for the

Degree of Bachealor of Science

In

Computer Science & Engineering

By

Md. Nadimul Islam

To

Dear Md. Al Mamun Sir

the



Rajshahi-6204,Bangladesh.

February- 2013