comparisonamongfiber amplifiers-

7/28/2019 comparisonamongfiber amplifiers-

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EDFA Amplifier

Raman Amplifier

Semiconductor AmplifierA COMPARISON BASED STUDY


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Prepared and Presented By:

Saimunur Rahman

Metric No: C093003

Dept. of Computer Science & Engineering

International Islamic University Chittagong


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Introduction to Optical Amplifiers

In order to transmit signals over long distances (>100 km) it necessary to compensate forattenuation losses within the fiber.

Initially this was accomplished with an optoelectronic moduconsisting of an optical receiver, a regeneration and equalizatiosystem, and an optical transmitter to send the data.

Although functional this arrangement is limited by the optical t

electrical and electrical to optical conversions.

Several types of optical amplifiers have since been demonstrated treplace the OE electronic regeneration systems.


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Introduction to Optical Amplifiers(Cont.)

These systems eliminate the need for E-O and O-E conversions.

This is one of the main reasons for the success of todays opticcommunications systems.


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Optical Amplifiers

The general form of an optical amplifier:


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Optical Amplifier Types

Some types of Optical Amplifiers are:

Semiconductor optical amplifiers(SOAs)

Fiber Raman and Brillouin amplifiers

Rare earth doped fiber amplifiers

The most practical optical amplifiers to date include the SOA and

EDFA types. New pumping methods and materials are also improving the

performance of Raman amplifiers.


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Today we shall talk about these

Optical Amplifiers

andsome comparisons among

them.


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EDFA AmplifierAn Overview


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A figure of EDFA Device


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Inside an EDFA


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History of EDFA Amplifier

Before the invention of EDFAs regenerators we're used to amplifysignal which was very costly and inefficient to use.

Idea for EDFA invented in 1960s

First commercial viable EDFA invented in 1987 by researchers fromSouthampton University and AT&T Bell Laboratories.


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What is EDFA Amplifier?

EDFA stands for Erbium Doped Fiber Amplifier.

Where, Erbium is a chemical element of lanthanide series in periodtable.

Erbium symbol is Er and atomic number is 68.

Erbium looks like a silvery-white solid metal when artificially isolated

Erbium's principal uses involve its pink-colored Er3+ ions, which havoptical fluorescent properties particularly useful in certain lasapplications.

Erbium-doped glasses or crystals can be used as opticamplification media, where erbium (III) ions are optically pumped aaround 980 nm or 1480 nm and then radiate light at 1530 nm stimulated emission.


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What is EDFA Amplifier? (Cont.)

Fig: Erbium-colored glass

This process results in an unusually mechanically simple laser opticaamplifier for signals transmitted by fiber optics.

This is known as Erbium Doped Fiber Amplifier or simply EDFA.


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Erbium doped fiber: Profile


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Basic EDFA overview

EDFA convert optical signal to another amplified optical signalwithout using any electrical domain.

Fig: Basic Block diagram of EDFA


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Working Principle of EDFA

Fig: Energy level transferring block


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Working Principle of EDFA (Cont.)

The 980 nm pump laser excites erbium ions from lower energy level 1into a higher energy level 3.

From level 3 the erbium ions goes to level 2.

From level 2 the erbium ion into x which 1550nm signal which jumpsback to lower level 1.

In this there is emission of 1550nm photon.

This process is known a stimulated emission. EDFA has an amplification window for optical wave analysis for which

the optical fiber has useable gain.

This wavelength range is gain able by a properties of dopained ion, thglass structure of optical fiber and the wavelength and power of pumplaser.


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Schematic diagram of EDFA


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Gain Spectrum for EDFA

Since the gain spectrum of erbium resemblesa 3-level atom it is possible to model the gainproperties using this approach.

Several different wavelength bands havebeen designated for wavelength divisionmultiplexing and EDFAs have been designedto operate in these bands.

The divisions have been designated as:

S-Band1480-1520nm

C-Band1521-1560 nm

L-Band 1561-1620nm


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Optical Gain of EDFA

Rare earth doped optical amplifiers work much like a laser.

The primary difference is that they do not have a resonator.

Amplification occurs primarily through the stimulated missioprocess.

The medium is pumped until a population inversion state achieved. Pump powers are typically several 20-250 mW. An isolatis used to reduce reflections at the input to the amplifier. A narroband optical filter is used to reduce transmission of amplifiespontaneous emission frequency components.

The resultant optical gain depends both on the optical frequencand the local beam intensity within the amplifier section.


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Optical Gain of EDFA (Cont.)

For basic discussion consider a two-level homogeneouslybroadened medium.

The gain coefficient can be expressed as:

Here, 0is the peak gain, is the optical frequency of the incidentsignal, 0is the transition frequency, P is the optical power of theincident signal, T2 is the dipole relaxation time, and Ps is thesaturation power.

Typically T2 is small < 1ps, and the saturation power Ps depends ongain medium parameters such as the fluorescence time and thetransition cross section.


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Gain and noise figure of EDFA(Sample)

Fig: A Characteristic plot of gain and noise figure for an erbium doped fiber

amplifier pumped ~30 mW at 980 nm.


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EDFA Gain Equalization

Gain equalization can beaccomplished in several ways:

Thin film filters

Long period fiber gratings

Chirped fiber Bragg gratings


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Gain Flattering


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Characteristics of EDFAs:(Advantages)

High power transfer efficiency from pump to signal power (> 50%).

Wide spectral band amplification with relative flat gain (>20dB)useful for WDM applications.

Saturation output> 1mW (10 to 25 dBm).

Gain-time constant long (>100 msec) to overcome patternineffects and inter-modulation distortions( low noise).

Large dynamic range.

Low noise figure.

Polarization independent.

Suitable for long-haul applications.


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Disadvantages of EDFAs:

Relatively large devices (km lengths of fiber) not easily integratedwith other devices.

ASE amplified spontaneous emission. There is always some outputeven with no signal input due to some excitation of ions in the fiberspontaneous noise.

Cross-talk effects.

Gain saturation effects.


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Applications of EDFA

EDFA can be used as: Power amplifiers

Inline amplifiers,

As well as preamplifiers.


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Raman AmplifierAn Overview


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A figure of Raman Amplifier Device


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What is Raman Amplifier?

A Raman amplifier is a device which takes input and amplified the same direction or opposite direction with pump laser.

Here is a very important rule/formula we must consider for thamplification.

Wavelength < Wavelength

Usually a few tens of nm


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The mechanism behind RamanAmplifier

The mechanism behind the Raman amplification is StimulateRaman Scattering (SRS).

SRS is a non linear effect of optical fiber.

For the SRS the optical power must b greater than the threshold thappen at least minimum 500mW. This is a codition.

Now we will look at how it actually happens.


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The mechanism behind RamanAmplifier (Cont.)

The photon of pump beam p is scattered bymolecules in the fiber medium and becomethe lower energy photon s .

The valance of the energy becomes vibrationand dissipated in the fiber medium.

For instance optical power this non linear

effect can transfer most of the pump powerp into signal powers.


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Raman Gain Coefficient

The frequency difference between pand s has to match a relationship inorder to fully use of this non lineareffect.

This is sown here by using Raman gaincoefficient graph.


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Raman Gain Coefficient (Cont.)

First picture is showing the pump laser at1535nm which has more higher signal todata signal.

In second picture, the pump laserpower transferred to the signal power asshown here.


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Raman Amplifier Types

There are basically two types of Ramanamplifiers as given here:

Distributed Raman Amplifier (DRA) uses thetransmission fiber itself as the medium, intowhich a backward pump is injected.

Discrete (Lumped) Raman Amplifier (RA) The

amplifier consists of a coil of dedicated fibertogether with pumps.


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Real World Raman AmplifierApplication

For getting the full benefits ofamplification EDFA and Ramanamplifiers as used together.

Distributed amplifier amplifies thesignal in a backward direction.

EDFA amplifier amplifies the signal

in a forward direction. Here we have shown the figure of

signal levels and how it changed.

i f ifi


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Properties of Raman Amplifiers:

The peak resonance in silica fibers occurs about13THz from the pump wavelength. At 1550nmthis corresponds to a shift of about 100 nm.

As indicated power is transferred from shorterwavelengths to longer wavelengths.

Coupling with the pump wavelength can be

accomplished either in the forward or counterpropagating direction.

Power is coupled from the pump only if thesignal channel is sending a 1 bit.

P A t t E t d th


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Pump Arrangement to Extend theRange for St. Raman Amplification:

An array of laser diodes can be used toprovide the Raman pump.

The beams are combined and thencoupled to the transmission fiber.

The pump beams can counterpropagate to the direction of the signal

beams.

Diffi lti ith R A lifi


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Difficulties with Raman Amplifiers

The Pump and amplified signals are at different wavelengths. Therefothe signal and the pump pulses will separate due to dispersio(waveguide dispersion) after a certain propagation distance.

A 1 psec pump pulse at 600nm separates from a 1 psec Stokes pulsin~30 cm.

A second problem is that the pump power decreases along the fib

length due to linear absorption and scattering Raman gain is great

at the input end.

A final problem results from amplifying spontaneous Raman photonThis occurs when the pump power is increased to offset attenuatiolosses and spontaneous Raman photons are coupled into the guidemode all along the length of the fiber. This increases noise.

A di i i ti b t EDFA


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A discrimination between EDFAand RA after a long brief

C bi d EDFA d RA


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Combined EDFA and RA

With only an EDFA at transmit end theoptical power level decreases over thefiber length.

With an EDFA and Raman the minimumoptical power level occurs toward themiddle, not the end of the end of thefiber.

A li ti f R A lifi


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Application of Raman Amplifier

Raman Amplifiers can be used as: Preamplifiers

Power amplifiers

Distributed amplifiers in a number of digital and analogical transmissionexperiments.


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SemiconductorAmplifierAn Overview

A figure of Semiconductor Amplifie


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A figure of Semiconductor AmplifieDevice

Semiconductor Amplifier


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Semiconductor Amplifier

An electrical current passed through thedevice that excites the electrons in theactive region.

When photon(light) travel through the activeregion it can cause these electron to losesome of their extra energy in the form ofmore photons that match the wavelength of

the initial ones.

Therefore, an optical signal passing throughthe active region is amplified and is said tohave experienced gain.

Semiconductor Amplifier (Cont )


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Semiconductor Amplifier (Cont.)

Both edges of the SOA are designed to have very low reflectivity sthat there are no unwanted reflections of the signal within thsemiconductor itself.

This is the main difference from regular laser that have reflectivfacets in order to build up the intensity of light within thsemiconductor material.

SOA: Amplification Process


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SOA: Amplification Process

Semiconductor have valance andconduction band.

At thermal equilibrium valance band hashigher population.

Under population inversion conditionconduction band will have higherpopulation.

Population inversion is achieved by forwardbiasing the p-n junction.

SOA Design


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SOA Design

Characteristics of SOA:


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Characteristics of SOA:

Polarization dependent require polarization maintaining fiber.

Relatively high gain ~20 dB.

Output saturation power 5-10 dBm.

Large BW.

Can operate at 800,1300,and 1500nm wavelength regions.

Compact and easily integrated with other devices

Characteristics of SOA (Cont )


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Characteristics of SOA (Cont.)

Can be integrated into arrays

High noise figure and cross-talk levels due to nonlinear phenomenosuch as 4-wave mixing. This feature restricts the use of SOAs.

Limited in operation below 10Gb/s. (Higher rates are possible withlower gain.)

SOA Vs Semiconductor Laser


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SOA Vs. Semiconductor Laser

Both are similar and in principle and construction.

Essentially Fabry-Perot cavities, with amplification achieved byexternal pumping.

The key of SOA is to preventing self-oscillations gathering laseroutput.

SOAs is electrically pumped by injected current.

SOA Applications


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SOA Applications

Power booster.

In-line amplifier.

Detector preamplifier.

Optical switching element.

Wavelength converter.

Optical Amplifiers (in short)


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Optical Amplifiers (in short)

Advantages

And

Disadvantages

ErDoped Fiber Amplifier EDFA


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ErDoped Fiber Amplifier EDFA

Advantages:

High gain (4050 dB),

Low noise (35 dB),

Low polarization sensitivity,

EDFAs are fully compatible with the rest of the fiber optic transmission li

Limitations:

Large size,

High pump power consumption (efficiency 10dB/1mW).

Raman Amplifier (RA)


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Raman Amplifier (RA)

Advantages:

Low noise (35 dB).

Wide gain bandwidth (up to 10 nm).

Distributed amplification within the transmission fiber.

Limitations:

Low gain (10 dB).

Requirement of high pump power.

Semiconductor Optical Amplifier


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Semiconductor Optical Amplifier

Advantages:

Small size. Transmission bidirectional.

Smaller output power then EDFA.

Less expensive then EDFA.

Limitations:

Lower gain (2030 dB) then EDFA.

Higher noise (712 dB) then EDFA.

Polarization dependence.

High nonlinearity.


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A short comparison

Between

discussed amplifiers

Optical Amplifier: Comparison


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Optical Amplifier: Comparison


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Thank You GuysFor

Tolerating me

AndPlease make dua for me

Question/ Answer


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Ques o / s e

Any Questions in your mind regarding this topic?

Feel free to ask

And

I would love to answer them

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