optical amplifiers

Optical Amplifiers

There are three type of optical amplifiers:

Semiconductor optical amplifier

Erbium Doped Fiber Amplifiers (EDFAs)

Raman Amplification

Types of Optical Amplifiers

Semiconductor Optical Amplifier Semiconductor Optical Amplifier

As bandwidth demand rises,the construction of optical packet switching nodes targeting optical routers would benefit from fast optical switches

Semi conductor optical amplifier(SOA)provides this high speed switching capability,as well as high gain, high extinction ratio and high integration potential

SOA is a key technology for several other functions such as all optical wavelength conversion,optical selection,regeneration,booster and in-line amplification


• An SOA is based on the same technology as a Fabry Perot Laser diode

• Such a LASER consists of an amplifying device inside a cavity(Fabry Cavity)

• The amplification is achieved by externally pumping the energy levels of the material

• In order to achieve only amplification functions it is necessary to protect the device against self oscillations generating the LASER effect

• This is accomplished by blocking cavity reflections using an anti reflection(AR)coating and the technique of angle cleaving the chip facets

• Unlike EDFAs which are optically pumped SOAs are electrically pumped by injected current


Structure of SOA Structure of SOA

• The basic SOA consists of a central active section about 600m long and two passive sections at the input and output sides of the chip around 100m

• The central active layer is based on separate confinement heterostructure(SCH)and consists of 0.2m thick tensile bulk active layer embedded between two 0.1m thick quaternary layers

• The central active layer is tapered over a length of 150m,which allows optical coupling to an underlying passive wave guide

Structure of SOA Structure of SOA

• Residual reflectivity of less than 10-4 to ensure a gain ripple below 0.5dB

• Low optical loss so as to attain a net gain as high as 30dB

• High material gain to allow low drive current operation

• High output saturation power, defined as the output power for which gain is reduced by 3dB

• Chip to coupling loss of less than 3dB per facet

• Polarization sensitivity of less than 0.5dB,because the polarization state of the optical signal coming from a link fiber is usually random

Key Parameters required for SOA Key Parameters required for SOA

Depending upon the efficiency of the AR(anti reflection)coating SOAs can be classified as:

Resonant Devices

Traveling wave devices(TW)

Types of SOA Types of SOA

Types of SOA (Contd) Types of SOA (Contd)

Resonant SOAs are manufactured using an AR coating with a reflectivity of around 10-2 .They typically feature a gain ripple of 10 to 20 dB and a bandwidth of 2 to 10GHz

TW coating incorporate a material with a reflectivity less than 10-4,gain ripple of a few dB and a bandwidth of 5THz

Schematic representation of SOA

Application of SOAs:

Amplifiers

Switching

Wavelength conversion and regeneration

Selection and Inversion

SOA - Amplifier SOA - Amplifier

Discrete stand-alone SOAs can be used as compact booster amplifiers(a standard device for single channel operation,a gain clamped version for WDM operation)

To achieve high sensitivity optically pre-amplified receivers as an interesting alternative solution to replace avalanche photodiodes for data rates of 40Gbps or higher

Noise figure is a key factor for amplification applications:

Noise figure is defined as: nsp/c1

Where nsp is the inversion factor c1 is the overall input loss

SOA - Switching SOA - Switching

Optical cross connects(OXCs)constitute a major application area for SOAs

SOA gate arrays are well suited for fast switching on the 1550nm wavelength range

An SOA gate array is an array of devices monolithically integrated on the same substrate and used as a gate

When the injected current is high it passes light through with some amplification,if injected current falls near zero the device blocks the light and thus an array acts as a switch

SOA – Wavelength converter and Regenerator SOA – Wavelength converter and Regenerator

Wavelength conversion can be achieved through cross phase modulation(XPM)performed in an SOA based Mach-Zehnder interferometer

A wavelength conversion operation incorporating a cascade of two SOA based MZ-interferometer wavelength converters used in a co-propogation scheme can yield 3R regeneration

The first stage performs re-shaping and re-timing,then the second stage matches the chirp of the output data for transmission over a high dispersion link

SOA – Selection and Inversion SOA – Selection and Inversion

A wavelength selector has been created using SOA gates placed between two phased array wavelength de-multiplexer

Spectral inversion is a mirror effect achieved in the signal spectrum between higher and lower frequencies which are inverted

It can be implemented using optical conjugation in a SOA structure in high bit rate transmissions using standard single mode fibers

EDFA-Erbium Doped Fiber Amplifier EDFA-Erbium Doped Fiber Amplifier

Erbium is a rare metallic earth element that is used to amplify light signals sent along fiber optic cable

When Erbium is lined in a fiber optic material such as glass, and light is pumped through it the result is an “Erbium Doped Fiber Amplifier”

These amplifiers provide a large gain,which occurs when the fiber is “pumped”by an additional light input at wavelengths shorter than 1.55m

EDFAs allow optical signals to be transmitted over long distances without the need for signal regeneration particularly in DWDM

EDFA-Erbium Doped Fiber Amplifier EDFA-Erbium Doped Fiber Amplifier

Basic operation of EDFA System Basic operation of EDFA System

A coupler combines the input signal at wavelength of 1.5m with a pump input of =1480nm or 980nm

The coupled signal passes through Erbium Doped Fiber which can go for many miles, until it passes through an isolator

The isolator will basically de-couple the combined signal and output an amplified original input signal

The function of the pump is to use light to lift atoms into specific excited states

Basically optical pumping requires there to be some optical transition between ground state and excited state

Principle of Amplification in EDFA Principle of Amplification in EDFA

EDFA Band

Principle of Amplification in EDFA Principle of Amplification in EDFA

Optical pumping requires some transition from ground state to excited state,semiconductor laser diode is a realistic pump source

Through experimentation and research it was found that 532nm and 980nm were the optimum wavelengths for pumping,which provided 1.35dB/mw and 2.2dB/mw of gain

By absorbing a pumped photon, electrons are elevated to a higher energy state

These electrons decay quickly from this state to a third metastable state,this state is called metastable because electron will remain in this state for 10ms before spontaneously decaying to the ground state and emitting a photon in the process

Principle of Amplification in EDFA(Contd..) Principle of Amplification in EDFA(Contd..)

This atom while in the metastable state excited state can be stimulated to emit its energy by another photon

The emitted photon flux in this case is coherent with the stimulating photon flux,as this process continues an incident beam is amplified with propogation through the Erbium doped fiber core

Gain, saturation and noise characteristics contribute to overall gain in EDFA and can be tweaked and tuned for optimum performance using the following parameters:

o Amplifier Fiber Length

o Pump Absorption Band

o Wave guide Characteristics

Parameters for optimum performance Parameters for optimum performance

Amplifier Fiber Length:

Because of pump decay along an Erbium doped fiber,a non-uniform medium inversion occurs

Fiber becomes lossy after a certain length because the transmission medium is absorbing when not inverted

Optimum length is dependent on input pump power since a longer length of inverted medium can be achieved by higher pump power

Pump Absorption Band:

If we assumed a fixed signal wavelength,three pump bands would be associated with three different pumping effects

In the two level method,when the pump wavelength approaches signal wavelength,required power tends to infinity,because of this pump wavelength should be tuned away from the signal wavelength

However pump wavelength should not be very far from signal wavelength because the absorption co-efficient decreases

Hence we require more pump power to get the photons to the excited state,due to absorption co-efficient corresponding to a higher saturation power

Parameters for optimum performance Parameters for optimum performance

(ESA:Excited State Absorption)

2 Level: Excited/Stable

3 Level:Excited/Metastable/Stable

EDFA-Summary EDFA-Summary

Few meters of regular fiber doped with a tiny amount of erbium ions:

Signal passes through this fiber along with light from pump laser

Pump laser excites Erbium ions which give extra energy to signal

Amplification possible at many wavelengths around 1550nm

Pumping with 980nm laser is more effective than 1480nm pumping

Commonly used in submarine systems, and increasingly on land

Raman Amplification Raman Amplification

o A Raman amplifier uses intrinsic properties of silica fiber to obtain signal amplification,this means transmission fiber can be used as a medium for amplification

o An amplifier working on the principle of Raman amplification is called a Distributed Raman Amplifier(DRA)

Raman Amplification(Contd..) Raman Amplification(Contd..)

Raman Amplifier(Principle) Raman Amplifier(Principle)

oThe physical property behind DRA is called SRS.this occurs when a sufficiently large pump wave is co-launched at a lower wavelength than the signal to be amplified

oRaman gain strongly depends on pump power and frequency offset between pump and signal

oAmplification occurs when the pumped photon gives up its energy to create a new photon at the signal wavelength plus some residual energy which is absorbed as phonons(vibrational energy)

Raman Amplifier(Principle) Raman Amplifier(Principle)

o As there is a wide range of vibrational states above ground state,a broad range of possible transitions are providing gain

o Raman gain increases almost linearly with the wavelength offset between signal and pump peaking at about 100nm and then dropping rapidly with increased offset

o The position of the gain bandwidth within the wavelength can be adjusted simply by tuning the wavelength

oThus Raman amplification potentially can be achieved in every region of the transmission window of the optical transmission fiber(It only depends on the availability of powerful pump sources at the required wavelength)

oThe disadvantage of Raman amplification is the need for high pump powers to provide a reasonable gain

Pros and Cons of Raman Amplifier over EDFA Pros and Cons of Raman Amplifier over EDFA

Advantages over EDFA:Low Noise Build upSimple design,as direct amplification is achieved in the optical fiber and no special transmission medium is requiredFlexible assignment of signal frequencies,as Raman gain depends on the pump wavelength and not on a wavelength sensitive material parameter,such as emission cross section of dopant in the Erbium Doped Fiber(EDF)Broad gain bandwidth is achievable by combining Raman amplification effect of several pump waves that are placed carefully in wavelength domain

Disadvantages:Not only specially launched pump waves but also but also some of the WDM channels may provide power to amplify the other channels this would result in power to amplify other channels and thus cross talk leading to de-gradationDegrading effects like Raman scattering and backward Rayleigh scattering also affects the performance

Comparison of Raman Amplifier and EDFA Comparison of Raman Amplifier and EDFA

Characteristic Doped-Fiber Amplifier Raman Amplifier

Amplification Band depends on dopant depends on availability of pump wavelengths

Amplification Bandwidth 20 nm, more for multiple dopants/fibers 48 nm, more for multiple pump waves

Gain 20 dB or more, depending on ion concentration, fiber length, and pump configuration

4–11 dB, proportional to pump intensity and effective fiber length

Saturation Power depends on gain and material constants equals about power of pump waves

Pump Wavelength 980 nm or 1480 nm for EDFAs 100 nm lower then signal wavelength at peak gain

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