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Optical amplifiers and their applicationsRef: Optical Fiber Communications by:G. Keiser; 3rd edition

Optical AmplifiersTwo main classes of optical amplifiers include:Semiconductor Optical Amplifiers (SOA) andDoped Fiber Amplifiers (DFA)

Basic operation of optical amplifiers

Semiconductor Optical AmplifiersRef: Optical Fiber Communications by:G. Keiser; 3rd editionThere are two types of SOAs:--- Fabry- Perot amplifiers (FPA)When the light enters FPA it gets amplified as it reflects back and forthbetween the mirrors until emitted at a higher intensity.It is sensitive to temperature and input optical frequency.---Non-resonant traveling-wave amplifiers (TWA)It is the same as FPA except that the end facets are eitherantireflection coated or cleaved at an angle so that internal reflectiondoes not take place and the input signal gets amplified only onceduring a single pass through the device. They widely used becausethey have a large optical bandwidth, and low polarization sensitivity.

Non-resonant traveling-wave amplifiers (TWA)External PumpingExternal pumping injection creates population inversion similar to LASERs.The rate equations can be defined as: n (t )n (t ) R p (t ) R st (t ) τr tJ (t )R p (t ) qdis the external pumping rate, J(t) is the current density, d is theactive layer thickness, and τr is the combined time constantcoming from spontaneous-carrier recombination mechanism.Rst(t) is the stimulated emission and it is equal to:Rst (t ) Γav g ( n nth ) N ph gv g N ph

External Pumping (Cont )where vg is the group velocity of the incident light, Г, optical confinementfactor, a is the gain constant, nth is threshold carrier density, Nph is thephoton density and g is the overall gain per unit of length.NphPs v g ( hv )( wd )where Ps is the power of optical signal, w and d are width and thethickness of active area respectively.Under steady state condition, variation of n vs time is zero, therefore:R p Rst nτr

External Pumping (cont )Substituting for Rp and Rst and solvingfor g yields:J nth g0qd τ rg v g N ph 1 /(Γaτ r ) 1 N ph / N ph;satwhereN ph;sat 1Γavgτ rand Jn g 0 Γ a τ r th qd τ r where go is the zero or small-signal gain per unit of length(in the absence of the signal input)

Amplifier GainAmplifier gain or signal gain G is defined as:G P s , outP s , inOr as we saw in the case of laser: G exp Γ g m α L exp[g ( z )L ] where, gm , α, and L are the material gain coefficient, the effectiveabsorption coefficient of the material and amplifier length respectively. g(z)is the overall gain per unit of length.g(z) can written as:g0g (z) P (z)1 sPamp , satgo, Ps(z), and Pamp,sat are the unsaturated mediumgain per unit of length in the absence of signal input,internal signal power at z, and amplifier saturationpower.

Amplifier GainThe increase in the light power in incremental length of dz canbe expressed as:dP g ( z ) Ps ( z )dz 11g 0 dz P ( z) Pamp , sat sWhich can show: 11 gdz 0 0 P ( z ) P amp , satPs , in sLnowPs , outand finally one can see that: dP dP G 1 Pamp , satPs ,in G ln 0 G where Go exp (goL) is the single-pass gain in the absence oflight.

Amplifier gain versus power

Erbium-Doped Fiber Optic AmplifiersErbium energy-level diagram andamplification mechanism

EDFA configurations

EDFA Power-Conversion Efficiency (PCE) and GainThe input and output power of an EDFA can beexpressed:Ps ,out Ps ,inλp Pp ,inλsThe Power Conversion Efficiency (PCE) is defined as(always less than unity)PCE Ps ,out Ps ,inPp ,in Ps ,outPp ,inλp 1λsWe can also write the amplifier gain as:G Ps , outPs , in 1 λ p P p , inλ s Ps , in

Optical AmplifiersEDFA Power-Conversion Efficiency (PCE) and GainIn order to achieve a specific maximum gain G, the input signalpower can NOT exceed a value given byP s , in λpP p , inλsG 1

Optical AmplifiersEDFA Power-Conversion Efficiency (PCE) and GainThe maximum gain in a 3 level laser medium of length L can also be given asfollow (in addition to pump power, the gain depends on the filter length)Gmax exp(ρσ e L )where ρ is the rare-earth element concentration and σe is the signalemission cross section.Therefore the maximum gain or power will be defined as: λ p P p , inG min exp (ρσ e L ),1 λ s Ps , in Ps ,out λp min Ps ,in exp(ρσ e L ), Ps ,in Pp ,in λs

Gain versus EDFA length

Absorption and Emission Cross-Sections in EDFA The effect of absorption and emissionefficiencies in external pumping in EDFAare realized by defining new parameterscalled Absorption Cross-Section, σa andEmssion Cross-Section, σe respectively.σa determines the pumping rate. If thepumping power is Pp and Er groundstate population is N0, the pumping rateis WpN0 where,σe determines the medium gain, g σeN2. N2 is metastable (inversion layer)population N1Stimulated emission rate, Rs is:Where Ps-in is the incident light power.Therefore the pumping gain will be:L is the length of the pump.σ a PpWp hνARs Vg gN ph Gp Pp outPp inσ e Ps in N 2hνA e ( σ e N 2 σ a N 0 ) L

Components for Optical CommunicationsMaterials for self-studies Passive WDM Active ComponentsModulators,Diodes,Switches,Routers

Some applications of Light polarization

Optical DiodeRutileHalf Wave PlateFaraday RotatorCalcite, RutileHalf waveplateRutileFaradayRotator

Transmission windows

WDM

Optical Interference

Fiber Bragg grating

Fiber Bragg grating fabricationPhase Mask: Direct Imprinting248 nm LaserΛPMPhase MaskGe dopedFiberDiffractionm -10thorder(Suppressed)Diffractionm 1

Reflection grating

Typical WDM Network

Simple demultiplexer function

Extended add/drop multiplexer

Tunable optical filter

Semiconductor Optical Amplifiers There are two types of SOAs: --- Fabry- Perot amplifiers (FPA) When the light enters FPA it gets amplified as it reflects back and forth between the mirrors until emitted at a higher intensity. It is sensitive to temperature and input optical frequency.---Non-resonant traveling-wave amplifiers (TWA)

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