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OPTICAL AMPLIFIERSOPTICAL AMPLIFIERS

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ContentsContents• IntroductionIntroduction• Doped Fiber amplifiersDoped Fiber amplifiers• Semiconductor Optical AmplifiersSemiconductor Optical Amplifiers• Difference Between DFAs & SOAsDifference Between DFAs & SOAs• Features of Optical AmplifiersFeatures of Optical Amplifiers• ConclusionConclusion• ReferenceReference

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IntroductionIntroduction

• Conventional repeaterConventional repeater

photon-to-electron conversionphoton-to-electron conversion electrical amplificationelectrical amplification retimingretiming pulse shapingpulse shaping electron-to-photon conversionelectron-to-photon conversion

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Works well for moderate-speed single-Works well for moderate-speed single-wavelength operation.wavelength operation.

Becomes complex and expensive for Becomes complex and expensive for high-speed multi-wavelength systems.high-speed multi-wavelength systems.

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• Optical Amplifiers (OAs)Optical Amplifiers (OAs)

Operates completely in the optical domain.Operates completely in the optical domain.

Boost the power levels of lightwave signals for Boost the power levels of lightwave signals for the two long-wavelength transmission the two long-wavelength transmission windows of optical fibers.windows of optical fibers.

Increases power level of the incident light Increases power level of the incident light through a stimulated emission process.through a stimulated emission process.

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OA structure is similar to that of a laser.OA structure is similar to that of a laser.

Does not have the optical feed-back mechanism Does not have the optical feed-back mechanism that is necessary for lasing to take place.that is necessary for lasing to take place.

An OA can boost incoming signal levels, but An OA can boost incoming signal levels, but cannot generate a coherent optical output by cannot generate a coherent optical output by itselfitself..

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Such device can be bidirectional and if it is Such device can be bidirectional and if it is sufficiently linear it may allow multiplex sufficiently linear it may allow multiplex operation of several signals at different operation of several signals at different optical wavelengths.optical wavelengths.

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• The two main OAs types can be classified The two main OAs types can be classified as:as:

Optical Amplifiers

Semiconductor OpticalAmplifiers (SOAs)

Doped FiberAmplifiers (DFAs)

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• SOAs or SOAs or semiconductors laser amplifierssemiconductors laser amplifiers (SLAs) utilize stimulated emission from (SLAs) utilize stimulated emission from injected carrier, andinjected carrier, and

• In DFAs or In DFAs or optical fiber amplifiersoptical fiber amplifiers (OFAs) or (OFAs) or simply simply fiber amplifiersfiber amplifiers, gain is provided by , gain is provided by either stimulated Raman or Brillouin either stimulated Raman or Brillouin scattering or by rare earth dopants.scattering or by rare earth dopants.

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Doped Fiber AmplifiersDoped Fiber Amplifiers• In DFAs, optical fiber core is doped by rare In DFAs, optical fiber core is doped by rare

earth elements e.g. erbium, praseodymium earth elements e.g. erbium, praseodymium etc.etc.

• 1550-nm window1550-nm window

The active medium for operation is created by The active medium for operation is created by lightly doping a silica fiber core with rare-lightly doping a silica fiber core with rare-earth elements such as Erbium (Er) or earth elements such as Erbium (Er) or Ytterbium (Yb). Ytterbium (Yb).

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• 1300-nm window1300-nm window

For this window the DFAs is achieved For this window the DFAs is achieved through doping fluoride-based fiber (rather through doping fluoride-based fiber (rather than silica fibers) with element such as than silica fibers) with element such as Neodymium (Nd) and Praseodymium (Pr).Neodymium (Nd) and Praseodymium (Pr).

• In DFAs population inversion is achieved by In DFAs population inversion is achieved by optical pumping.optical pumping.

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• The basic scheme for optical pumping is as The basic scheme for optical pumping is as shown.shown.

Forward pumped doped fiber amplifierForward pumped doped fiber amplifier..

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• A coupler is used for combining the A coupler is used for combining the information signal and the pump signal.information signal and the pump signal.

• The pump power is generated by The pump power is generated by semiconductor laser diodes at suitable semiconductor laser diodes at suitable wavelengths.wavelengths.

• For higher amplification, more population For higher amplification, more population inversion is desired over the larger length.inversion is desired over the larger length.

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• Types of PumpingTypes of Pumping

Forward pumpingForward pumping

Backward pumpingBackward pumping

Bidirectional pumpingBidirectional pumping

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• The pump power reduces along the The pump power reduces along the propagation direction, the population propagation direction, the population inversion also reduces.inversion also reduces.

• Hence the signal amplification depends Hence the signal amplification depends upon the population inversion profile upon the population inversion profile along the length of doped fiber. along the length of doped fiber.

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Features of DFAsFeatures of DFAs

• Signal for both bit 1 and 0 experiences same Signal for both bit 1 and 0 experiences same amplifier gain when very high bit rate signals amplifier gain when very high bit rate signals are amplified, this results in comparatively are amplified, this results in comparatively less cross-saturation effect in DFAs.less cross-saturation effect in DFAs.

• Cross-saturation effect.Cross-saturation effect.

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• The DFAs can be used as in-line component The DFAs can be used as in-line component in fiber optic link.in fiber optic link.

• The signal polarization does not affect the The signal polarization does not affect the gain of these amplifiers.gain of these amplifiers.

• Major types of DFAs:Major types of DFAs:

Rare earth doped fiber amplifiers,Rare earth doped fiber amplifiers,

Raman fiber amplifiers & Brillouin fiber Raman fiber amplifiers & Brillouin fiber amplifiers.amplifiers.

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Semiconductor Optical Semiconductor Optical AmplifiersAmplifiers

• SOAs areSOAs are

Basically semiconductor lasers which Basically semiconductor lasers which operate below lasing threshold.operate below lasing threshold.

Require population inversion so that Require population inversion so that stimulated emission and hence stimulated emission and hence amplification can take place.amplification can take place.

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• SOAs have a similar structure to Fabry-SOAs have a similar structure to Fabry-Perot laser diode but with anti-reflection Perot laser diode but with anti-reflection design elements at the endfaces.design elements at the endfaces.

• The population inversion is achieved by The population inversion is achieved by means of electrical energy.means of electrical energy.

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• Typically made from group III-V Typically made from group III-V compound semiconductors, though any compound semiconductors, though any direct band gap semiconductors be used.direct band gap semiconductors be used.

• A SOA has two facets and reflectivities of A SOA has two facets and reflectivities of these facets decide whether the device will these facets decide whether the device will operate as a SOA or a semiconductor laser.operate as a SOA or a semiconductor laser.

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• Classified into two main groupsClassified into two main groups

Fabry-Perot amplifiers (FPAs)Fabry-Perot amplifiers (FPAs)

Traveling wave amplifiers (TWAs)Traveling wave amplifiers (TWAs)

• When reflectivities are non-zero but quite When reflectivities are non-zero but quite small, the signal passes through the cavity small, the signal passes through the cavity several times.several times.

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• Fabry-Perot amplifierFabry-Perot amplifier (FPA) (FPA)

When the signal within the cavity reduces When the signal within the cavity reduces with the successive passes and dies out with the successive passes and dies out ultimately.ultimately.

• Traveling wave amplifierTraveling wave amplifier (TWA) (TWA)

When the facet reflectivities are zero, input When the facet reflectivities are zero, input signal passes through the device only once. signal passes through the device only once.

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Schematic diagram of an SOASchematic diagram of an SOA

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Features of SOAsFeatures of SOAs

• The SOA is of small size and electrically The SOA is of small size and electrically pumped.pumped.

• Potentially less expensive than the EDFA Potentially less expensive than the EDFA (Erbium-doped fibre amplifiers)(Erbium-doped fibre amplifiers)

• Integrated with semiconductor lasers, Integrated with semiconductor lasers, modulators, etc.modulators, etc.

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• The SOA has higher noise, lower gain, The SOA has higher noise, lower gain, moderate polarization dependence & high moderate polarization dependence & high nonlinearity with fast transient time.nonlinearity with fast transient time.

• High optical nonlinearity makes SOAs High optical nonlinearity makes SOAs attractive for all optical signal processing like attractive for all optical signal processing like all-optical switching and wavelength all-optical switching and wavelength conversion.conversion.

• When √When √R1R2 < 0.17R1R2 < 0.17, the FPA can be , the FPA can be approximated as TWA.approximated as TWA.

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Difference Between DFAs Difference Between DFAs & SOAs& SOAs

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Features of Optical Features of Optical AmplifiersAmplifiers

• In similar manner to electronic repeaters, OAs In similar manner to electronic repeaters, OAs may be employed in a simplex mode where each may be employed in a simplex mode where each transmitted optical signal is carried on a separate transmitted optical signal is carried on a separate fiber link.fiber link.

Simplex modeSimplex mode

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Duplex modeDuplex mode

• OAs have the ability to operate OAs have the ability to operate simultaneously in both directions at the simultaneously in both directions at the same carrier wave length.same carrier wave length.

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ConclusionConclusion

• OAs operate solely in the optical domain OAs operate solely in the optical domain with no interconversion of photons to with no interconversion of photons to electrons.electrons.

• The OA can be used for any kind of The OA can be used for any kind of modulation at virtually any transmission modulation at virtually any transmission rate.rate.

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• Such systems do not require full Such systems do not require full regeneration of the transmitted digital regeneration of the transmitted digital signal at each repeater, and optical signal at each repeater, and optical amplification of the signal proves amplification of the signal proves sufficient.sufficient.

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• Semiconductors laser amplifiers (SLAs), can Semiconductors laser amplifiers (SLAs), can be classified intobe classified into

Fabry-Perot amplifiers (FPAs)Fabry-Perot amplifiers (FPAs) Traveling wave amplifiers (TWAs)Traveling wave amplifiers (TWAs)

• Fiber amplifiers, can be classified intoFiber amplifiers, can be classified into

Rare earth doped fiber amplifiersRare earth doped fiber amplifiers Raman fiber amplifiers and Brillouin fiber Raman fiber amplifiers and Brillouin fiber

amplifiersamplifiers

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ReferenceReference• Semiconductor Optical AmplifiersSemiconductor Optical Amplifiers, Michael J. , Michael J.

Connelly, 2002, Kluwer Academic Press.Connelly, 2002, Kluwer Academic Press.

• Optical Fiber CommunicationsOptical Fiber Communications, John M. , John M. Senior, 2003, Prentice Hall of India.Senior, 2003, Prentice Hall of India.

• Optical Fiber CommunicationsOptical Fiber Communications, John Gover, , John Gover, 1993, Prentice Hall of India.1993, Prentice Hall of India.

• Optical Fiber CommunicationsOptical Fiber Communications, G. Kieser, G. Kieser,, 1991, McGraw Hill Inc.1991, McGraw Hill Inc.

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• Yatindra Nath Singh ,Yatindra Nath Singh ,Studies on Placement of Studies on Placement of Semiconductor Optical Amplifiers in Semiconductor Optical Amplifiers in Wavelength Division Multiplexed Star and Wavelength Division Multiplexed Star and Tree Topology NetworksTree Topology Networks, , Thesis, September Thesis, September 1996.1996.

• ““Encyclopedia of Semiconductor Optical Encyclopedia of Semiconductor Optical Amplifier”; Amplifier”; http://www.wikipedia.com

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