Chapter-1

OPTICAL FIBER COMMUNICATION

1.1 INTRODUCTION

Today’s communication has very important role in life. For better communication we

need transmission medium offering large capacity with low communication losses.

The optical fiber is the only transmission medium offering such large bandwidth upto

GHz. To amplify an optical signal with a conventional repeater, one performs optical

to electrical conversion, electrical amplification, pulse shaping, and then electrical to

again optical conversion. Another way is that to amplify optical signal by using the

optical amplifiers which operates completely in the optical domain to boost(amplify)

the power levels of multiple light wave signals over spectral bands of 30 nm. Optical

amplifiers can amplify signals at different wavelength simultaneously. Optical fiber

communications typically operate in a wavelength region corresponding to one of the

following “telecom windows. The first window at 800–900 nm was originally used

GaAs/AlGaAs-based laser diodes and light emitting diodes (LEDs) served as

transmitters, and silicon photodiodes were suitable for the receivers . However, the

fiber losses are relatively high in this fiber amplifiers are not well developed for this

spectral region.

1.2 EVOLUTION OF OPTICAL FIBER

Therefore, the first telecom window is suitable only for short distance transmission.

The second telecom window utilizes wavelengths around 1.3 μm, where the loss of

silica fibers is much lower and the fibers chromatic dispersion is very weak, so that

dispersive broadening is minimized. This window was originally used for long-haul

transmission. However, fiber amplifiers for 1.3μm (based on, e.g. on praseodymium-

doped glass) are not as good as their 1.5-μm counterparts based on erbium. Also, low

dispersion is not necessarily ideal for long-haul transmission, as it can increase the

effect of optical non linearity. The third telecom window, which is now very widely

used, utilizes wavelengths around 1.5μm. The losses of silica fibers are lowest in this

region, and erbium-doped fiber amplifiers are available which offer very high

performance. Fiber dispersion is usually anomalous but can be tailored with great

flexibility (→dispersion-shifted fibers).

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1.3 Advantages of optical fiber communications over electrical cables

The capacity of fibers for data transmission is huge: a single silica fiber can carry

hundreds of thousands of telephone channels, utilizing only a small part of the

theoretical capacity . In the last 30 years, the progress concerning transmission

capacities of fiber links has been significantly faster than e.g. the progress in the

speed or storage capacity of computers.

The losses for light propagating in fibers are amazingly small: ∼ 0.2 dB/km for

modern singlemode silica fibers, so that many tens of kilometres can be bridged

without amplifying the signals.

A large number of channels can be re-amplified in a single fiber amplifier, if

required for very large transmission distances .

Due to the huge transmission rate achievable, the cost per transported bit can be

extremely low.

Compared with electrical cables, fiber-optic cables are very lightweight, so that

the cost of laying a fiber-optic cable can be lower.

Fiber-optic cables are immune to problems that arise with electrical cables, such

as ground loops or electromagnetic interference (EMI).

1.4 OPTICAL AMPLIFIERS

Optical amplifiers have really revolutionized the field of fiber optics communication.

Optical amplifiers are in general bit rate transparent and can amplify signals at

different wavelength simultaneously. Optical amplifiers are mainly of two types i.e.

Semiconductor optical amplifiers and Fiber amplifiers . These are further classified

into travelling wave semiconductor optical amplifier, Fabry-perot semiconductor

optical amplifier, Erbium doped fiber amplifier, Raman & Brillouin fiber amplifiers.

All optical amplifiers increase the power level of incident light through a stimulated

emission to occur or an optical power transfer process. In SOAs and EDFAs (erbium

doped fiber amplifiers),the mechanism for creating the population inversion that is

needed for stimulated emission to occur is same as is used in laser diodes. Although

the structure of such an optical amplifier is similar to that of laser, it does not have the

optical feedback mechanism that is necessary for lasing to take place. Thus, an optical

amplifier can boost incoming signal levels, but it cannot generate a coherent output by

itself. The basic operation is shown in Figure, here the device absorbs energy supplied

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from an external source called the pump. The pump supplies the energy to electrons in

an active medium, which raises them to higher energy levels to produce a population

inversion. An incoming signal photon will trigger these excited electrons to drop to

lower levels through a stimulated emission process. Since one in coming trigger

photon stimulates many excited electrons to emit photons of equal energy as they

drop to the ground state, the result is an amplified optical signal.

Figure1.1: Generic Optical Fibers

1.4.1 Principle & Theory

To achieve optical amplification, the population of upper energy level has to be

greater than that of lower energy level, i.e. N2> N1, where N1, N2 is population

density of lower and upper state . This condition is known as population inversion.

This can be achieved by exciting electron into higher energy level by external source

called pumping. Stimulated emission occur, when incident photon having energy E=

hc/λ interact with electron in upper energy state causing it return to lower state with

creation of second photon, where h is Plank constant, c is velocity of light and λ is the

wavelength of light . So light amplification occurs, when incident photon & emitted

photon are in phase and release two more photon, continuation of this process

effectively creates avalanche multiplication. Therefore amplified coherent emission is

obtained.

1.5 Types of Optical Amplifiers

Optical amplifiers were classified on the basis of device characteristics i.e. whether it

is based on:

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Figure1.2: Types Of Optical Amplifiers

In-line Optical Amplifiers

In a single-mode link, the effects of fiber dispersion may be small so that the main

limitation to repeater spacing is fibre attenuation. Since such a link does not

necessarily require a complete regeneration of the signal, simple amplification of the

optical signal is sufficient. Thus, an in-line optical amplifier can be used to

compensate for transmission loss and increase the distance between regenerative

repeaters.

Figure1.3: Inline Optical Amplifier

Preamplifier

Figure shows an optical amplifier being used as a front-end preamplifier for an optical

receiver. Thereby weak optical signal is amplified before photo detection so that the

Signal-to-noise ratio degradation caused by thermal noise in the receiver electronics

can be suppressed. Compared with other frontend devices such as avalanche

photodiodes or optical heterodyne detectors, an optical preamplifier provides a larger

gain factor and a broader bandwidth.

Figure1.4:- Preamplifier

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Power Amplifier

Power or booster amplifier applications include placing the device immediately after

an optical transmitter to boost the transmitted power, as figure shows [11, 12]. This

serves to increase the transmission distance by 10-100 km depending on the amplifier

gain and fiber loss. As an example, using the boosting technique together with an

optical pre-amplifier at the receiving end can enable repeater less undersea

transmission distances of 200-250 km. One can also employ an optical amplifier in a

local area network as a booster amplifier to compensate for coupler-insertion loss and

power-splitting loss.

Figure1.5:-booster amplifier

1.5.1 Semiconductor Optical Amplifiers

Semiconductor Optical Amplifiers (SOAs) uses the principle of stimulated emission

to amplify an optical information signal. Optical input signal carrying original data

enters to semiconductor’s active region through coupling. The coupling is required

because the mode field diameter of single mode beam is 9.3Mm, while size of active

region is less. Injection current delivers the external energy to pump elements at

conduction band. The input signal stimulated the transition of electrons down to

valence band & emission of photon with same energy & same wavelength as the input

signal, so amplified optical signal is obtained. SOA is of two types - Fabry –Perot

Amplifier (FPA) & Travelling Wave Amplifier (TWA). Fabry-Perot Amplifier (FPA)

is same as SOA. In this, light entering the active region is reflected several times from

cleaved face & amplified as it leaves the cavity. Travelling Wave Amplifier (TWA) is

the SOA form.

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Figure1.6: Device Structure Of SOA

Simple SOA are almost the same as regular index-guided FP lasers. The back facet is

pigtailed to allow the input of signal light. The main problem is that it has been

difficult to make SOAs longer than about 450 μm. In this short distance there is not

sufficient gain available on a single pass through the device for useful amplification to

be obtained. One solution to this is to retain the reflective facets (mirrors)

characteristic of laser operation. Typical SOAs have a mirror reflectivity of around

30%. Thus the signal has a chance to reflect a few times within the cavity and obtain

useful amplification. In TWA, there is an active medium without reflective facets, so

that input signal is amplified by a single passage through active region. Practical

active region without reflective facets was made by covering the facets of

semiconductor material by antireflection coating, tilting the active region with respect

to facet and using buffer material between active region & facet to also reduce

reflectance R as small as 10-4.

1.5.2 EDFA AMPLIFIERS:

Definition: (Erbium-Doped Fiber Amplifier) A device that boosts the signal in an

optical fiber. Introduced in the late 1980s, the EDFA was the first successful optical

amplifier. It was a major factor in the rapid development of fiber-optic networks in

the 1990s, because it extended the distance between costly regenerators. This EDFA

is designed for the Dense Wavelength Division Multiplexing (DWDM) applications.

The device features excellent gain flatness, low noise figure and wide operating

wavelength range. It also has good network control interface.

1.5.2.1 Setup and Operation Principle

A typical setup of a simple erbium-doped fiber amplifier (EDFA) is shown in Figure.

Its core is the erbium-doped optical fiber, which is typically a single-mode fiber. In

the shown case, the active fiber is “pumped” with light from two laser diodes

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(bidirectional pumping), although unidirectional pumping in the forward or backward

direction (co-directional and counter-directional pumping) is also very common.

Figure1.7: Setup of an EDFA Amplifier

The pump light, which most often has a wavelength around 980 nm and sometimes

around 1450 nm, excites the erbium ions (Er3+) into the 4I13/2 state (in the case of 980-

nm pumping via 4I11/2), from where they can amplify light in the 1.5-μm wavelength

region via stimulated emission back to the ground-state manifold 4I15/2. The setup

shown also contains two “pig-tailed” (fiber-coupled) optical isolators. The isolator at

the input prevents light originating from amplified spontaneous emission from

disturbing any previous stages, whereas that at the output suppresses lasing (or

possibly even destruction) if output light is reflected back to the amplifier. Without

isolators, fiber amplifiers can be sensitive to back reflections. Very high signal gains,

as used, e.g., for the amplification of ultra short pulses to high energies, are usually

realized with amplifier chains, consisting of several amplifier stages with additional

optical elements (e.g. isolators, filters, or modulators) in between.

1.5.2.2 AMPLIFICATION MECHANISM

Whereas semiconductor optical amplifiers use external current injection to excite

electrons to higher energy levels, optical fiber amplifiers use optical pumping. In this

process, one use photons it directly raises electrons into excited states. The optical

pumping process requires the use of three energy levels. The top energy level to

which the electron is elevated must lie energetically above the desired lasing level.

After reaching its excited state, the electron must release some of its energy and drop

to the desired lasing level. From this level, a single photon can than trigger the excited

electron into stimulated emission, whereby the electron releases its remaining energy

in the form of a new photon with a wavelength identical to that of single photon .since

the pump photon must have a higher energy than the signal photon, the pump

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wavelength is shorter than the signal wavelength. To get a phenomenological

understanding of how an EDFA works, we need to look at the energy level structure

of erbium. The erbium atoms in silica are Er3+ ions, which are erbium atoms that have

lost three of their outer electrons.fig shows a simplified energy-level diagram and

various energy-level transition process of these Er3+ ions in silica glass. The two

principal levels for telecommunication applications are a metastable level (the so

called 4I13/2 level) and the 4I11/2 pump level .the term “metastable” means that the

lifetimes for transitions from this state to the ground state are very long compared

with the lifetimes of the states that led to this level. The metastable, the pump and the

ground state levels are actually bands of closely spaced energy levels that form a

manifold due to the effect known as Starks splitting. Furthermost, each stark level is

broadened by thermal effects into an almost continuous band.

1.5.2.3 EDFA Architecture

Optical fiber amplifier consists of a doped fiber ,one or more pump lasers, a passive

wavelength coupler ,optical isolators , and tap couplers as shown in fig. the

dichroic(two-wavelength) coupler handles either 980/1550 nm wavelength

combinations to couple both the pump and signal optical powers efficiently into the

fiber amplifier. The tap couplers are wavelength – insensitive with typical splitting

ratios ranging from 99:1 to 95:5.they is generally used on both sides of the amplifier

to compare the incoming signal with the amplified output. The optical isolators

prevent the amplified signal from reflecting back into the device, where it could

increase the amplifier noise and decrease the amplifier efficiency.

Three possible configurations of an EDFA:

(a) Co-directional pumping

(b) Counter-directional pumping

(c) Dual-pump scheme

The pump light is usually injected from the same direction as the signal flow. This is

known as codirectional pumping. It is also possible to inject the pump power in the

opposite direction to the signal flow, which is known as counter-directional pumping.

As shown in Fig can employ either a single pump source or use dual pump schemes,

the resultant gains typically being +17 db and +35 dB, respectively.

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Figure1.8:- (a) Co-directional pumping (b) Counter-directional pumping

(c) Dual pumping

.

Counter-directional pumping allows higher gains but co-directional pumping gives

better noise performance. In addition, pumping at 980 nm is preferred, since it

produces less noise and achieves larger population inversion than pumping at 1480

nm.

1.6 APPLICATIONS

Erbium-doped Amplifiers in Telecom Systems

The power of a data transmitter may be boosted with a high-power EDFA before

entering a long fiber span, or a device with large losses, such as a fiber-optic

splitter. Such splitters are widely used e.g. in cable-TV systems, where a single

transmitter is used to deliver signals into many fibers.

A fiber amplifier may also be used in front of a data receiver, if the arriving signal

is weak. Despite the introduction of amplifier noise, this can improve the signal-

to-noise ratio and thus the possible data transmission rate, since the amplifier

noise may be weaker than the input noise of the receiver. It is more common,

however, to use avalanche photodiodes, which have some built-in signal

amplification.

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In-line EDFAs are used between long spans of passive transmission fiber. Using

multiple amplifiers in a long fiber-optic link has the advantage that large

transmission losses can be compensated without (a) letting the optical power drop

to too low levels, which would spoil the signal-to-noise ratio, and (b) without

transmitting excessive optical powers at other locations, which would cause

detrimental nonlinear effects due to the unavoidable fiber nonlinearities. Many of

these in-line EDFAs are operated even under difficult conditions, e.g. on the

ocean floor, where maintenance would be hardly possible.

Although data transmitters are normally not based on erbium-doped devices,

EDFAs are often part of equipment for testing transmission hardware. They are

also used in the context of optical signal processing.

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CHAPTER-2

SIMULATION SOFTWARE:- OPTISYSTEM

2.1 OVERVIEW

In an industry where cost effectiveness and productivity are imperative for success,

the award winning OptiSystem can minimize time requirements and decrease cost

related to the design of optical systems, links, and components. OptiSystem is an

innovative, rapidly evolving, and powerful software design tool that enables users to

plan, test, and simulate almost every type of optical link in the transmission layer of a

broad spectrum of optical networks from LAN, SAN, MAN to ultra-long-haul. It

offers transmission layer optical communication system design and planning from

component to system level, and visually presents analysis and scenarios. Its

integration with other Optiwave products and design tools of industry leading

electronic design automation software all contribute to OptiSystem speeding your

product to market and reducing the payback period.

2.2 Specific Benefits

• Provides global insight into system performance

• Assesses parameter sensitivities aiding design tolerance specifications

• Visually presents design options and scenarios to prospective customers

• Delivers straightforward access to extensive sets of system characterization data

• Provides automatic parameter sweep and optimization

• Integrates with the family of Optiwave products

2.3 Applications

Created to address the needs of research scientists, optical telecom engineers, system

integrators, students and a wide variety of other users, OptiSystem satisfies the

demand of the evolving photonics market for a powerful yet easy to use optical

system design tool.

OptiSystem enables users to plan, test, and simulate:

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• WDM/TDM or CATV network design

• SONET/SDH ring design

• Transmitter, channel, amplifier, and receiver design

• Dispersion map design

• Estimation of BER and system penalties with different receiver models

• Amplified System BER and link budget calculations

2.4 Key Features and Functionality

2.4.1 Component Library

The OptiSystem Component Library includes hundreds of components that enable

you to enter parameters that can be measured from real devices. It integrates with test

& measurement equipment from different vendors. Users can incorporate new

components based on subsystems and user-defined libraries, or utilize co-simulation

with a third party tool such as MATLAB or SPICE.

2.4.2 Integration with Optiwave Software Tools

OptiSystem allows you to employ specific Optiwave software tools for integrated and

fiber optics at the component and circuit level: OptiSPICE, OptiBPM, OptiGrating,

and OptiFiber.

2.4.3 Mixed signal representation

OptiSystem handles mixed signal formats for optical and electrical signals in the

Component Library. OptiSystem calculates the signals using the appropriate

algorithms related to the required simulation accuracy and efficiency.

2.4.4 Quality and performance algorithms

In order to predict the system performance, OptiSystem calculates parameters such as

BER and Q-Factor using numerical analysis or semi-analytical techniques for systems

limited by inter symbol interference and noise.

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2.4.5 Advanced visualization tools

Advanced visualization tools produce OSA Spectra, signal chirp, eye diagrams,

polarization state, constellation diagrams and much more. Also included are WDM

analysis tools listing signal power, gain, noise figure, and OSNR per channel.

2.4.6 Data monitors

You can select component ports to save the data and attach monitors after the

simulation ends. This allows you to process data after the simulation without

recalculating. You can attach an arbitrary number of visualizers to the monitor at the

same port.

2.4.7 Hierarchical simulation with subsystems

To make a simulation tool flexible and efficient, it is essential to provide models at

different abstraction levels, including the system, subsystem, and component levels.

OptiSystem features a truly hierarchical definition of components and systems,

enabling you to employ specific software tools for integrated and fiber optics at the

component level, and allowing the simulation to be as detailed as the desired accuracy

dictates.

2.4.8 Powerful Script language

You can enter arithmetical expressions for parameters and create global parameters

that can be shared between components and subsystems using standard VB Script

language. The script language can also manipulate and control OptiSystem, including

calculations, layout creation and post-processing when using the script page.

2.4.9 State-of-the-art calculation data-flow

The Calculation Scheduler controls the simulation by determining the order of

execution of component modules according to the selected data flow model. The main

data flow model that addresses the simulation of the transmission layer is the

Component Iteration Data Flow (CIDF). The CIDF domain uses run-time scheduling,

supporting conditions, data-dependent iteration, and true recursion.

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2.4.10 Report page

A fully customizable report page allows you to display any set of parameters and

results available in the design. The produced reports are organized into resizable and

moveable spreadsheets, text, 2D and 3D graphs. It also includes HTML export and

templates with pre-formatted report layouts.

2.4.11 Bill of materials

OptiSystem provides a cost analysis table of the system being designed, arranged by

system, layout or component. Cost data can be exported to other applications or

spreadsheets.

2.4.12 Multiple layouts

You can create many designs using the same project file, which allows you to create

and modify your designs quickly and efficiently. Each OptiSystem project file can

contain many design versions. Design versions are calculated and modified

independently, but calculation results can be combined across different versions,

allowing for comparison of the designs.

2.5 New Features IN Optisystem

The most comprehensive optical communication design suite for optical system

design engineers is now even better with the release of OptiSystem version 8.0 also

available in 32-bit and TRUE 64-bit editions. The latest version of OptiSystem

features a number of new features and enhancements to address the design of passive

optical network (PON) architectures using orthogonal frequency division multiplexed

(OFDM) signals, optical coherent detection systems and injection- locked Fabry-Perot

laser diodes (F-P LD). The OptiSystem API has been extended to support OptiSPICE,

the first circuit design software for analysis of integrated circuits including

interactions of optical and electronic components. OptiSystem is the default

waveform viewer and signal integrity analyzer of OptiSPICE.

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Fabry-Perot Laser

A new model of a wavelength-locked Fabry-Perot laser diode (F-P LD) based on the

rate equations for the semiconductor laser diode. Fabry-Perot laser is a cost- effective

source for the wavelength-division multiplexed passive optical networks.

DUOBINARY, CSRZ AND DPSK Transmitters

New transmitters encapsulate the complexity of advanced modulation formats such

as duobinary, CSRZ and DPSK facilitating the design of fiber-optic communication

networks.

OFDM Modulator/Demodulator

OFDM can be applied in optical long haul transmission systems and have many

advantages over conventional single-carrier modulation format. The new components

allow for the simulation of OFDM transmitters and receivers, supporting different

types of modulation schemes such as BPSK, QPSK, QAM, etc.

Yb Doped Fiber Dynamic

A new time domain Stimulated Brillouin Scattering (SBS) model for high-power

Ytterbium doped fiber amplifiers. The new model describes the interplay between the

first and second-order Stokes, pump, and signal in double-clad fiber amplifiers.

2.6 MORE Features in OptiSystem

Bi-Directional AWG

New feature empowers the unique bi-directional capabilities of OptiSystem,

facilitating the design of AWG based PONs.

Microwave Components

New sophisticated library of components including 180 and 90 degree hybrid

couplers, DC blockers, power splitters and combiners. An ideal solution for ROF

simulation applications.

MLSE (Maximum Likelihood Sequence Estimate)

An advanced component feature using the Viterbi algorithm to equalize the input

signal through a dispersive channel.

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Optical Fibers and Amplifiers

A new discretization parameter for broadband sampled signals offers improved

performance, accuracy, and convergence for doped amplifier gain and Brillouin

calculations. Four-Wave Mixing, Stimulated Brillouin Scattering, Self-Phase

Modulation, Cross-Phase Modulation, and Stimulated Raman Scattering are all

included with the optical fiber models of OptiSystem.

Free Space Optics (FSO)

New feature enabling the simulation of complex inter-satellite communication links.

Constellation and Polar Diagrams

A new calculation engine in OptiSystem used to estimate symbol error in user defined

regions and targets.

Advanced Analysis Toolsets

The photonic all-parameter analyzer measures polarization mode dispersion (PMD)

and records multiple traces simultaneously. This robust new feature can measure

insertion loss (IL), differential group delay (DGD), polarization chromatic dispersion

(PDC), depolarization rate, dispersion, dispersion slope, and group delay (GD).

S-Parameter Extractor

The signal characteristics from an optical transmitter input and receiver output can be

extracted and exported into an industry standard touchstone format for s-parameters,

benefiting EDA tools that offer integrated S-Parameter support which effectively

reduces the design cycle time.

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CHAPTER-3

IMPLEMENTATION AND SIMULATION

3.1 INTRODUCTION

The main performance characteristics of EDFAs such as high gain, low noise figure,

high output power and gain flatness can be achieved in dual or more stage amplifiers.

The dual stage design provides capabilities to suppress ASE noise by the mid-stage

optical isolator which reduces amplifier saturation and positively contributes to the

gain, power and noise characteristics. In such devices, the first stage can be viewed as

a low-noise preamplifier, and the second stage as a power amplifier. High gain

flatness of the amplifier is achieved by using a mid-stage Gain Equalization Filter

(GEF).

3.2 OBJECTIVE

As the light travels along the fiber, signal intensity reduces due to various factors

such as coupling with isolator and due to properties of fiber itself. Thus, to

overcome this problem we will provide amplifiers in the mid stage Gain flattened

fiber (GFF).

At the 2nd stage when EDFA will be used, high gain, low noise figure, high output

power and gain flatness can be achieved.

3.3 SIMULATION SETUP

The simulation setup is shown in the figure. In this two stage EDFA is used with mid

stage gain equalization filter. In first stage EDFA consist of active fiber, Pump Laser,

coupler and an isolator with a filter. Pumping is done with a laser diode radiating

powerful light at a wavelength other than an information signal’s wavelength. An

information signal is transmitted in the vicinity of 1500nm frequency and input power

is 80mw but pump laser radiate at 980nm frequency and power is 110mw. Both the

optical information and optical pumping beams are put in the same fiber by a coupler

i.e. pump coupler co-propagation. Its feature lower noise but also low output power.

Then isolator is used. Its function to prevent back reflected light from penetrating the

amplifier fiber; otherwise this light will also be amplified. A fiber amplifier i.e.

erbium doped fiber which is bi-directional having length of 40m and here residual

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pump is used as optical null. A filter separates any remnant of light power from the

information signal.

Figure3.1:- Simulation setup for two stage EDFA with mid stage GEF

Similarly in second stage of EDFA, CW laser which operates at 1600nm frequency

and power 120mw, pumping is done at 1480nm frequency by using coupler co-

propagation pump, then an isolator and EDF then again counter co-propagation is

used so that we can get higher output but also higher noise. Then gain flattening filter

is used to flatten the gain. Both stages output is fed into analyzer i.e. dual port WDM

analyzer (used for display gain and noise figure). Optical spectrum analyzer is used

for graphical representation between output power and wavelength.

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3.4 Calculation of the whole project

Figure3.2:- Calculation Of Whole Project

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3.5 Output of Optical spectrum analyzer

Figure3.3:-output of optical spectrum

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3.6 Output gain and noise figure

Figure3.4 output gain and noise figure

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REFERENCES

1. Aggrawal G.P., Fiber optic communication systems, John Wiley & Sons, Newyork,

1997.

2. Mynbev , Fiber-Optic Communication Systems, John Wiley & Sons , Newyork

2001.

3. E. Desurvire, J. L. Zyskind, and C. R. Giles, “Design optimization for efficient

erbium-doped fiber amplifiers,” J. Lightwave Technol., vol. 8, pp. 1730-1741, Nov.

1990.

4. N. A. Olsson, “Lightwave systems with optical amplifiers”, J. Lightwave Technol.

LT- 7, 1071 (1989).

5. D. O. Caplan, “Laser communication transmitter and receiver design”, J. Opt. Fiber

Commun. Rep. 4, 225 (2007).

6.International Telecommunication Union (ITU), http://www.itu.int/home/index.html.

7. C. R. Giles and E .Desurvire, “Modeling erbium-doped fiber amplifiers,” J.

Lightwave Technol. 9 (2),271 (1991).

8. http://www.vpiphotonics.com/

9. http://www.optiwave.com/products/system_overview.html

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