OPTICAL AMPLIFIERS

TABLE OF CONTENTS

INTRODUCTION……………………………………………………………………………………………………………………………….2

TYPES OF AMPLIFIERS………………………………………………………………………………………………………………………3

DOPED FIBER AMPLIFIERS……………………………………………………………………………………………………………….3

ERBIUM-DOPED FIBER AMPLIFIERS....................................................................................................4

Noise.......................................................................................................................................11

Gain saturation.......................................................................................................................12

Inhomogeneous broadening effects........................................................................................13

Polarization effects..................................................................................................................13

SEMICONDUCTOR OPTICAL AMPLIFIERS……………………………………………………………………………………….14

VERTICAL-CAVITY SOA…………………………………………………………………………………………………………………..16

CONCLUSION…………………………………………………………………………………………………………………………………18

REFERENCES………………………………………………………………………………………………………………………………….19

1 DEPT. OF ELECTRONICS & COMMUNICATION

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OPTICAL AMPLIFIERS

INTRODUCTION

An optical amplifier is a device that amplifies an optical signal directly, without

the need to first convert it to an electrical signal. An optical amplifier may be

thought of as a laser without an optical cavity, or one in which feedback from the

cavity is suppressed. Optical amplifiers are important in optical

communication and laser physics.

With the demand for longer transmission lengths, optical amplifiers have become

an essential component in long-haul fiber optic systems. Semiconductor optical

amplifiers (SOAs), erbium doped fiber amplifiers (EDFAs), and Raman optical

amplifiers lessen the effects of dispersion and attenuation allowing improved

performance of long-haul optical systems. 

There are several different physical mechanisms that can be used to amplify a light

signal, which correspond to the major types of optical amplifiers. In doped fiber

amplifiers and bulk lasers, stimulated emission in the amplifier's gain

medium causes amplification of incoming light. In semiconductor optical

amplfiers (SOAs), electron-hole recombination occurs. In Raman

amplifiers, Raman scattering of incoming light with phonons in the lattice of the

gain medium producesphotons coherent with the incoming photons. Parametric

amplifiers use parametric amplfication.

.

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TYPES OF OPTICAL AMPLIFIERS

• Semiconductor Optical Amplifiers (SOAs)

• Erbium Doped Fiber Amplifiers (EDFAs)

• Raman Amplifiers

• Hybrid Amplifiers

LASER AMPLIFIERS

Almost any laser active gain medium can be pumped to produce gain for light at

the wavelength of a laser made with the same material as its gain medium. Such

amplifiers are commonly used to produce high power laser systems. Special types

such as regenerative amplifiers and chirped-pulse amplifiers are used to

amplify ultra short pulses.

DOPED FIBER AMPLIFIERS

Doped fiber amplifiers (DFAs) are optical amplifiers that use a doped optical

fiber as a gain medium to amplify an optical signal. They are related to fiber

lasers. The signal to be amplified and a pump laser are multiplexed into the doped

fiber, and the signal is amplified through interaction with the doping ions. The

most common example is the Erbium Doped Fiber Amplifier (EDFA), where the

core of a silica fiber is doped with trivalent Erbium ions and can be efficiently

pumped with a laser at a wavelength of 980 nm or 1,480 nm, and exhibits gain in

the 1,550 nm region.

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ERBIUM-DOPED FIBER AMPLIFIERS

The erbium-doped fiber amplifier (EDFA) is the most deployed fiber amplifier as

its amplification window coincides with the third transmission window of silica-

based optical fiber.

Two bands have developed in the third transmission window – the Conventional,

or C-band, from approximately 1525 nm – 1565 nm, and the Long, or L-band,

from approximately 1570 nm to 1610 nm. Both of these bands can be amplified by

EDFAs, but it is normal to use two different amplifiers, each optimized for one of

the bands.

The principal difference between C- and L-band amplifiers is that a longer length

of doped fiber is used in L-band amplifiers. The longer length of fiber allows a

lower inversion level to be used, thereby giving at longer wavelengths (due to the

band-structure of Erbium in silica) while still providing a useful amount of gain.

EDFAs have two commonly-used pumping bands – 980 nm and 1480 nm. The

980 nm band has a higher absorption cross-section and is generally used where

low-noise performance is required. The absorption band is relatively narrow and so

wavelength stabilized laser sources are typically needed. The 1480 nm band has a

lower, but broader, absorption cross-section and is generally used for higher power

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amplifiers. A combination of 980 nm and 1480 nm pumping is generally utilised in

amplifiers.

The explosion of dense wavelength-division multiplexing (DWDM) applications

makes these optical amplifiers an essential fiber optic system building block.

EDFAs allow information to be transmitted over longer distances without the need

for conventional repeaters. The fiber is doped with erbium, a rare earth element,

that has the appropriate energy levels in their atomic structures for amplifying

light. EDFAs are designed to amplify light at 1550 nm. The device utilizes a 980

nm or 1480nm pump laser to inject energy into the doped fiber. When a weak

signal at 1310 nm or 1550 nm enters the fiber, the light stimulates the rare earth

atoms to release their stored energy as additional 1550 nm or 1310 nm light. This

process continues as the signal passes down the fiber, growing stronger and

stronger as it goes. Figure 2 shows a fully featured, dual pump EDFA that includes

all of the common components of a modern EDFA. 

The input coupler, Coupler #1, allows the microcontroller to monitor the input

light via detector #1. The input isolator, isolator #1 is almost always present.

WDM #1 is always present, and provides a means of injecting the 980 nm pump

wavelength into the length of erbium-doped fiber. WDM #1 also allows the optical

input signal to be coupled into the erbium-doped fiber with minimal optical loss.

The erbium-doped optical fiber is usually tens of meters long. The 980 nm energy

pumps the erbium atom into a slowly decaying, excited state. When energy in the

1550 nm band travels through the fiber it causes stimulated emission of radiation,

much like in a laser, allowing the 1550 nm signal to gain strength. The erbium

fiber has relatively high optical loss, so its length is optimized to provide

maximum power output in the desired 1550 nm band. WDM #2 is present only in

dual pumped EDFAs. It couples additional 980 nm energy from Pump Laser #2

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into the other end of the erbium-doped fiber, increasing gain and output power.

Isolator #3 is almost always present. Coupler #2 is optional and may have only one

of the two ports shown or may be omitted altogether. The tap that goes to Detector

#3 is used to monitor the optical output power. The tap that goes to Detector #2 is

used to monitor reflections back into the EDFA.

Block Diagram of an EDFA

This feature can be used to detect if the connector on the optical output has been

disconnected. This increases the backreflected signal, and the microcontrolled can

set to disable the pump lasers in this event, providing a measure of safety for

technicians working with EDFAs. Figure 3 shows a two-stage EDFA with mid-

stage access. In this case, two single-stage EDFAs are packaged together. The

output of the first stage EDFA and the input of the second stage EDFA are brought

out the user. Mid-stage access is important in high performance fiber optic

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systems. To reduce the overall dispersion of the system, dispersion compensating

fiber (DCF) can be used periodically. However, problems can arise from using the

DCF, mostly the insertion loss reaching 10 dB. Placing the DCF at the mid-stage

access point of the two-stage EDFA reduces detrimental effects on the system, and

allows the users noticeable gain.

Two-stage EDFA with Mid-stage Access

The optical input first passes through optical Isolator #1. Next the light passes

through WDM #1, which provides a means of injecting the 980 nm pump

wavelength into the first length of erbium-doped fiber. WDM #1 also allows the

optical input signal to be coupled into the erbium-doped fiber with minimal optical

loss. The erbium-doped optical fiber is usually tens of meters long. Like the fully

feature, dual pumped EDFA, the 980 nm energy pumps the erbium atoms into an

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excited state that decays slowly. When light in the 1550 nm band travels through

the erbium-doped fiber it causes stimulated emission of radiation. As the optical

signal gains strength, output of the erbium-doped fiber then goes into the optical

isolator #2, the output of which is available to the user. Typically, a dispersion

compensating device will be connected at the mid-stage access point. The light

then travels through isolator #3 and WDM #2, which couples additional 980 nm

energy from a second pump laser into the other end of a second length of erbium-

doped fiber, increasing gain and output power. Finally, the light travels through

isolator #4. Photons amplify the signal avoiding almost all active components, a

benefit of EDFAs. Since the output power of an EDFA can be large, any given

system design can require fewer amplifiers. Yet another benefit of EDFAs is the

data rate independence means that system upgrades only require changing the

launch/receive terminals. The most basic EDFA design amplifies light over a

narrow, 12 nm, band. Adding gain equalization filters can increase the band to

more than 25 nm. Other exotic doped fibers increase the amplification band to 40

nm. Because EDFAs greatly enhance system performance, they find use in long-

haul, high data rate fiber optic communication systems and CATV delivery

systems. Long-haul systems need amplifiers because of the lengths of fiber used.

CATV applications often need to split a signal to several fibers, and EDFAs boost

the signal before and after the fiber splits. There are four major applications that

generally require optical fiber amplifiers: power amplifier/booster, in-line

amplifier, preamplifier or loss compensation for optical networks. Below are

detailed description of each application. Power Amplifier/Booster Figure 4

illustrates the first three application for optical amplifiers. Power amplifiers (also

referred to as booster amplifiers) are placed directly after the optical transmitter.

This application requires the EDFA to take a large signal input and provide the

maximum output level. Small signal response is not as important because the direct

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transmitter output is usually -10 dBm or higher. The noise added by the amplifier

at this point is also not as critical because the incoming signal has a large signal-to-

noise ratio (SNR). 

Three Applications for an EDFA

In-Line Amplifiers In-line amplifiers or in-line repeaters, modify a small input

signal and boost it for retransmission down the fiber. Controlling the small signal

performance and noise added by the EDFA reduces the risk of limiting a system's

length due to the noise produced by the amplifying components. Preamplifiers Past

receiver sensitivity of -30 dBm at 622 Mb/s was acceptable; however, presently,

the demands require sensitivity of -40 dBm or -45 dBm. This performance can be

achieved by placing an optical amplifier prior to the receiver. Boosting the signal

at this point presents a much larger signal into the receiver, thus easing the

demands of the receiver design. This application requires careful attention to the

noise added by the EDFA; the noise added by the amplifier must be minimal to

maximize the received SNR. Compensating for Loss in Optical Networks Inserting

an EDFA before an 8 x 1 optical splitter increases the power to almost +19 dBm

allowing each of the eight output legs to provide +9 dBm, making the output

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almost equal to the original transmitter power. The optical splitter alone has a

nominal optical insertion loss of 10 dB. The transmitter has an optical output of

+10 dBm, meaning that the optical splitter outputs without an EDFA would be 0

dBm. This output power would be acceptable for most digital applications;

however, in analog CATV applications this is the minimal acceptable received

power. Therefore, inserting the EDFA before the optical splitter greatly increases

the output power.

Loss Compensation in Optical Networks

Wideband EDFAs Optical communication systems carrying 100 or more optical

wavelengths require and increase in the bandwidth of the optical amplifier to

nearly 80 nm. Normally employing a hybrid optical amplifier, consisting of two

separate optical amplifiers, allows for separate amplification, one for the lower 40

nm band and the second for the upper 40 nm band. Figure 6 exemplifies the optical

gain spectrum of a hybrid optical amplifier. The solid lines illustrate the response

of two individual amplifier sections. The dotted line, which has been increased by

1 dB for clarity, shows the response of the combined hybrid amplifier.

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Optical Gain Spectrum of a Hybrid Optical Amplifier

The optical fiber amplifier was invented by H. J. Shaw and Michel Digonnet at

Stanford University, California, in the early 1980s. The EDFA was first

demonstrated several years later  by a group including David N. Payne, R. Mears,

and L. Reekie, from the University of Southampton and a group from AT&T Bell

Laboratories, E. Desurvire, P. Becker, and J. Simpson.

Noise

The principal source of noise in DFAs is Amplified Spontaneous

Emission (ASE), which has a spectrum approximately the same as the gain

spectrum of the amplifier. Noise figure in an ideal DFA is 3 dB, while practical

amplifiers can have noise figure as large as 6–8 dB.

As well as decaying via stimulated emission, electrons in the upper energy level

can also decay by spontaneous emission, which occurs at random, depending upon

the glass structure and inversion level. Photons are emitted spontaneously in all

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directions, but a proportion of those will be emitted in a direction that falls within

the numerical aperture of the fiber and are thus captured and guided by the fiber.

Those photons captured may then interact with other dopant ions, and are thus

amplified by stimulated emission. The initial spontaneous emission is therefore

amplified in the same manner as the signals, hence the term Amplified

Spontaneous Emission. ASE is emitted by the amplifier in both the forward and

reverse directions, but only the forward ASE is a direct concern to system

performance since that noise will co-propagate with the signal to the receiver

where it degrades system performance. Counter-propagating ASE can, however,

lead to degradation of the amplifier's performance since the ASE can deplete the

inversion level and thereby reduce the gain of the amplifier.

Gain saturation

Gain is achieved in a DFA due to population inversion of the dopant ions. The

inversion level of a DFA is set, primarily, by the power of the pump wavelength

and the power at the amplified wavelengths. As the signal power increases, or the

pump power decreases, the inversion level will reduce and thereby the gain of the

amplifier will be reduced. This effect is known as gain saturation – as the signal

level increases, the amplifier saturates and cannot produce any more output power,

and therefore the gain reduces. Saturation is also commonly known as gain

compression.

To achieve optimum noise performance DFAs are operated under a significant

amount of gain compression (10 dB typically), since that reduces the rate of

spontaneous emission, thereby reducing ASE. Another advantage of operating the

DFA in the gain saturation region is that small fluctuations in the input signal

power are reduced in the output amplified signal: smaller input signal powers

experience larger (less saturated) gain, while larger input powers see less gain.

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The leading edge of the pulse is amplified, until the saturation energy of the gain

medium is reached. In some condition, the width (FWHM) of the pulse is reduced.

Inhomogeneous broadening effects

Due to the inhomogeneous portion of the linewidth broadening of the dopant ions,

the gain spectrum has an inhomogeneous component and gain saturation occurs, to

a small extent, in an inhomogeneous manner. This effect is known as Spectral hole

burning because a high power signal at one wavelength can 'burn' a hole in the gain

for wavelengths close to that signal by saturation of the inhomogeneously

broadened ions. Spectral holes vary in width depending on the characteristics of

the optical fiber in question and the power of the burning signal, but are typically

less than 1 nm at the short wavelength end of the C-band, and a few nm at the long

wavelength end of the C-band. The depth of the holes are very small, though,

making it difficult to observe in practice.

Polarization effects

Although the DFA is essentially a polarization independent amplifier, a small

proportion of the dopant ions interact preferentially with certain polarizations and a

small dependence on the polarization of the input signal may occur (typically < 0.5

dB). This is called Polarization Dependent Gain (PDG). The absorption and

emission crossections of the ions can be modeled as ellipsoids with the major axes

aligned at random in all directions in different glass sites. The random distribution

of the orientation of the ellipsoids in a glass produces a macroscopically isotropic

medium, but a strong pump laser induces an anisotropic distribution by selectively

exciting those ions that are more aligned with the optical field vector of the pump.

Also, those excited ions aligned with the signal field produce more stimulated

emission. The change in gain is thus dependent on the alignment of the

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polarizations of the pump and signal lasers – i.e. whether the two lasers are

interacting with the same sub-set of dopant ions or not. In an ideal doped fiber

without birefringence, the PDG would be inconveniently large. Fortunately, in

optical fibers small amounts of birefringence are always present and, furthermore,

the fast and slow axes vary randomly along the fiber length. A typical DFA has

several tens of meters, long enough to already show this randomness of the

birefringence axes. These two combined effects (which in transmission fibers give

rise to polarization mode dispersion) produce a misalignment of the relative

polarizations of the signal and pump lasers along the fiber, thus tending to average

out the PDG. The result is that PDG is very difficult to observe in a single

amplifier (but is noticeable in links with several cascaded amplifiers).

SEMICONDUCTOR OPTICAL AMPLIFIERS

Semiconductor optical amplifiers (SOAs) are essentially laser diodes, without end

mirrors, which have fiber attached to both ends. They amplify any optical signal

that comes from either fiber and transmit an amplified version of the signal out of

the second fiber. SOAs are typically constructed in a small package, and they work

for 1310 nm and 1550 nm systems. In addition, they transmit bidirectionally,

making the reduced size of the device an advantage over regenerators of EDFAs.

However, the drawbacks to SOAs include high-coupling

loss, polarization dependence, and a higher noise figure. Figure 1 illustrates the

basics of a Semiconductor optical amplifier.

Semiconductor optical amplifiers (SOAs) are amplifiers which use a

semiconductor to provide the gain medium. These amplifiers have a similar

structure toFabry–Pérot laser diodes but with anti-reflection design elements at

the endfaces. Recent designs include anti-reflective coatings and

tilted waveguide and window regions which can reduce endface reflection to less

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than 0.001%. Since this creates a loss of power from the cavity which is greater

than the gain it prevents the amplifier from acting as a laser.

Semiconductor Optical Amplifier

Semiconductor optical amplifiers are typically made from group III-V compound

semiconductors such as GaAs/AlGaAs, InP/InGaAs, InP/InGaAsP

andInP/InAlGaAs, though any direct band gap semiconductors such as II-VI could

conceivably be used. Such amplifiers are often used in telecommunication systems

in the form of fiber-pigtailed components, operating at signal wavelengths between

0.85 µm and 1.6 µm and generating gains of up to 30 dB.

The semiconductor optical amplifier is of small size and electrically pumped. It can

be potentially less expensive than the EDFA and can be integrated with

semiconductor lasers, modulators, etc. However, the performance is still not

comparable with the EDFA. The SOA has higher noise, lower gain, moderate

polarization dependence and high nonlinearity with fast transient time. The main

advantage of SOA is that all four types of nonlinear operations (cross gain

modulation, cross phase modulation, wavelength conversion and four wave

mixing) can be conducted. Furthermore, SOA can be run with a low power laser.

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This originates from the short nanosecond or less upper state lifetime, so that the

gain reacts rapidly to changes of pump or signal power and the changes of gain

also cause phase changes which can distort the signals. This nonlinearity presents

the most severe problem for optical communication applications. However it

provides the possibility for gain in different wavelength regions from the EDFA.

"Linear optical amplifiers" using gain-clamping techniques have been developed.

Modern optical networks utilize SOAs in the follow ways: Power Boosters: Many

tunable laser designs output low optical power levels and must be immediately

followed by an optical amplifier. ( A power booster can use either an SOA or

EDFA.) In-Line Amplifier: Allows signals to be amplified within the signal

path. Wavelength Conversion: Involves changing the wavelength of an optical

signal. Receiver Preamplifier: SOAs can be placed in front of detectors to enhance

sensitivity.

VERTICAL-CAVITY SOA

A recent addition to the SOA family is the vertical-cavity SOA (VCSOA). These

devices are similar in structure to, and share many features with, vertical-cavity

surface-emitting lasers (VCSELs). The major difference when comparing

VCSOAs and VCSELs is the reduced mirror reflectivities used in the amplifier

cavity. With VCSOAs, reduced feedback is necessary to prevent the device from

reaching lasing threshold. Due to the extremely short cavity length, and

correspondingly thin gain medium, these devices exhibit very low single-pass gain

(typically on the order of a few percent) and also a very large free spectral

range (FSR). The small single-pass gain requires relatively high mirror

reflectivities to boost the total signal gain. In addition to boosting the total signal

gain, the use of the resonant cavity structure results in a very narrow gain

bandwidth; coupled with the large FSR of the optical cavity, this effectively limits

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operation of the VCSOA to single-channel amplification. Thus, VCSOAs can be

seen as amplifying filters.

Given their vertical-cavity geometry, VCSOAs are resonant cavity optical

amplifiers that operate with the input/output signal entering/exiting normal to the

wafer surface. In addition to their small size, the surface normal operation of

VCSOAs leads to a number of advantages, including low power consumption, low

noise figure, polarization insensitive gain, and the ability to fabricate high fill

factor two-dimensional arrays on a single semiconductor chip. These devices are

still in the early stages of research, though promising preamplifier results have

been demonstrated.

CONCLUSION

In the past few years research into all-optical amplification has been intensified.

The performance expectations of both semiconductor and fibre amplifiers are

becoming better understood and the number of possible applications is rapidly

increasing. The particular attraction of linear optical amplifiers is that in most cases

they offer very wide bandwidths, up to 5 THz, which is attractive for many of the

wideband signaling schemes proposed for future networks. Thus it is hoped that

optical amplifiers will allow the direct amplification of wavelength- and

frequency-division multiplexing and other wideband schemes such as subcarrier

modulation. There is still considerable work to be done, however, in evaluating the

amplifier performance in certain cases, for example in the presence of a large

number of closely spaced channels.

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REFERENCES

1.  Mears, R.J. and Reekie, L. and Poole, S.B. and Payne, D.N.: "Low-

threshold tunable CW and Q-switched fiber laser operating at 1.55μm",

Electron. Lett., 1986, 22, pp.159-160

2. R.J. Mears, L. Reekie, I.M. Jauncey and D. N. Payne: “Low-noise Erbium-

doped fiber amplifier at 1.54μm”, Electron. Lett., 1987, 23, pp.1026–1028

3.  E. Desurvire, J. Simpson, and P.C. Becker, High-gain erbium-doped

traveling-wave fiber amplifier," Optics Letters, vol. 12, No. 11, 1987, pp.

888–890

4. M. J. Connelly, Semiconductor Optical Amplifiers. Boston, MA: Springer-

Verlag, 2002. ISBN 978-0-7923-7657-6

5. Ghosh, B.; Mukhopadhyay, S. (2011). "All-Optical Wavelength encoded

NAND and NOR Operations exploiting Semiconductor Optical Amplifier

based Mach-Zehnder Interferometer Wavelength Converter and Phase

Conjugation System". 

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