Chapter 9 Semiconductor Optical Amplifiers

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Semiconductor Optoelectronics (Farhan Rana, Cornell University)Chapter 9Semiconductor Optical Amplifiers9.1 Basic Structure of Semiconductor Optical Amplifiers (SOAs)9.1.1 Introduction:Semiconductor optical amplifiers (SOAs), as the name suggests, are used to amplify optical signals.A typical structure of a InGaAsP/InP SOA is shown in the Figure below. The basic structure consistsof a heterostructure pin junction.Metalz 0p InPz LMetalInGaAsPhWn InPThe smaller bandgap intrinsic region has smaller refractive index than the wider bandgap p-doped andn-doped quasineutral regions. The intrinsic region forms the core of the optical waveguide and thequasineutral regions form the claddings. Current injection into the intrinsic region (also called theactive region) can create a large population of electrons and holes. If the carrier density exceeds thetransparency carrier density then the material can have optical gain and the device can be used toamplify optical signals via stimulated emission. During operation as an optical amplifier, light iscoupled into the waveguide at z 0 . As the light propagates inside the waveguide it gets amplified.Finally, when light comes out at z L , its power is much higher compared to what it was at z 0 .9.2 Basic Equations of Semiconductor Optical Amplifiers (SOAs)9.2.1 Equation for the Optical Power:The material gain of the active region can be described by a complex refractive index. Suppose thereal part of the refractive index of the active region is n a , the material group index of the activeMregion n ag, the group index of the waveguide optical mode is n g , the material gain of the activeregion is g , and the mode confinement factor of the active region is a . Then the change in thepropagation vector of the waveguide optical mode due to gain in the active region is given by thewaveguide perturbation theory,

Semiconductor Optoelectronics (Farhan Rana, Cornell University) ng a Mc n ag n i n g g i ga M a a22 n ag where, ngg M n ag g In the presence of gain, the light field amplitude will increase with distance as e a g 2 z and the optical power will increase as e a g z . The factor g is called the modal gain. If P z represents athe optical power (units: energy per sec) then one can write a simple equation for the increase in theoptical power with distance,dP z a g P z dzA time dependent form of the above equation for power propagating in the z-direction will be, 1 P z, t a g P z, t z v g t As the optical signal gets stronger with distance inside the waveguide, and the rate of stimulatedemission also gets proportionally faster, the carrier density inside the active region also changes andcannot be assumed to be the same as in the absence of any optical signal inside the waveguide. In thenext Section, we develop rate equations for the carrier density in the active region.9.2.2 Modeling Waveguide Losses:Material losses (such as those due to free carrier absorption) lead to losses in the waveguide mode.Suppose the material loss is represented by the function (x , y ) . We can represent loss by theimaginary part of the refractive index. The change in the propagation vector due to loss is, o n n E.E * dxdyic o n E.E * dxdy Re Et Ht * . zˆ dxdy 2 Re Et Ht * . zˆ dxdy n g k i k i k k M 222 nkgkk where the sum in the last line represents the sum over all the regions in the cross-section of thewaveguide. The modal loss is equal to the loss of each region weighted by its mode confinementfactor. In the presence of loss, the equation for the optical power becomes,dP z a g P z dzThe time dependent form will be, 1 P z, t a g P z, t z v g t 9.2.3 Rate Equation for the Carrier Density:Recall from the discussion on LEDs that the rate equation for the carrier density in the active regionof a pin heterostructure can be written as,dn i I Rnr n Gnr n Rr n Gr n dt qVaIn the present case, the volume Va of the active region is WhL and the cross-sectional area Aa of theactive region is Wh . The radiative recombination-generation terms in the above equation includespontaneous emission into all (guided and unguided) radiation modes as well as stimulated emission

Semiconductor Optoelectronics (Farhan Rana, Cornell University)and absorption by thermal photons in all (guided and unguided) radiation modes. Note that in thebandwidth of interest there will generally be many more unguided modes than guided modes. Weassume that the density of radiation modes in the active region is not modified significantly from theexpression valid for a bulk material and is given by,2 M na nagg p c cThe above approximation turns out to be fairly good even though the optical waveguide does modifythe density of radiation modes from the expression given above.We must now add stimulated emission and absorption from the guided optical mode to the right handside of the above rate equation for the carrier density. Assuming the photon density in the activeregion is n p , the net stimulated emission rate is,cR R g n n pMnagThe material gain g n is carrier density dependent and may be approximated as, n g n g o ln n tr The values of the transparency carrier density n tr range from 1.5x1018 1/cm3 to 3.0x1018 1/cm3 andthe values of g o range from 1000 to 4000 /1cm for most III-V materials. The carrier density rateequation becomes,dn i Ic Rnr n Gnr n Rr n Gr n g n npMdt qVanagIt is better to write the last term on the right hand side in terms of g where, ngg nM agand we get, g dn i I Rnr n Gnr n Rr n Gr n v g g n npdt qVaNote that now the group velocity of the optical mode appears in the last term on the right hand side. Inthe above equation, both the carrier density and the photon density are functions of position inside thewaveguide. More explicitly,dn z, t i I Rnr n z, t Gnr n z, t Rr n z, t Gr n z, t v g g n z, t np z, t dtqVaWe need to relate the photon density n p inside the active region to the optical power P . Since themode confinement factor a is the ratio of the average mode energy density (units: energy per unitlength) inside the active region to the average mode energy density W (units: energy per unit length)in the entire waveguide,Wn p Aa a But, P v g W , therefore,n p Aa aP v gThe effective area Aeff of the optical mode is defined by the relation,

Semiconductor Optoelectronics (Farhan Rana, Cornell University)Aa aThe above definition implies that the photon density in the active region can also be written as,Pnp v g AeffAeff We can now write the carrier density rate equation as,dn z, t i IP z, t Rnr n z, t Gnr n z, t Rr n z, t Gr n z, t g n z, t dtqVa AeffThe above equation together with, 1 P z, t a g n z, t P z, t z v g t are the two basic equations used to analyze semiconductor optical amplifiers.9.3 Operation of Semiconductor Optical Amplifiers (SOAs)9.3.1 Case I – No Gain Saturation:We assume that the SOA is operating in steady state with an extremely small light signal input to theSOA at z 0 . We assume that P (z 0) is so small that P (z) for all z , even after amplification,remains small and, consequently, n p (z) is also small. By small I mean small enough such that onemay ignore the stimulated emission term in the carrier density rate equation compared to the otherrecombination-generation terms. In this case, the steady state carrier density is independent of positionand can be obtained from the equation, I0 i Rnr n Gnr n Rr n Gr n qVaOnce the carrier density is determined, the material gain can be obtained using, n g n g o ln ntr In steady state, the equation for the optical power becomes, P z a g n P z, t z P z P 0 e a g n zThe dimensionless gain G of the amplifier is defined as the ratio of the output power to the inputpower, P (L )G e a g LP (0 )The amplifiers gain is usually specified in dB scale,Gain in dB 10 log10 G 9.3.2 Case II – Gain Saturation:In the more general case, stimulated emission term in the carrier density rate equation cannot beignored. If either the input optical power is large or if the modal gain a g is large, the photon densityn p (z) can also be very large, especially near the output end of the amplifier ( z L ). A large photondensity increases the rate of carrier recombination by stimulated emission. Since photon densityn p (z) is z-dependent, the carrier density n(z) in steady state will also be z-dependent. The situationwill look as follows,

Semiconductor Optoelectronics (Farhan Rana, Cornell University)P(z)n(z)z 0z Lz 0z LThe carrier density, and consequently the gain g , are both reduced near z L . This is called “gainsaturation”; light which is amplified by a gain medium ends up reducing the gain of that medium. Inother words light starts “eating” the hand that feeds it. Gain saturation makes the amplifier nonlinear.9.3.3 Input-Output Characteristics of SOAs – A Simple Solvable Model:The complete non-linear equations of an SOA are difficult to solve analytically. However, withcertain approximations, an analytic solution can be obtained. We assume that the material gain can beapproximated by a linear model, ng n g o ln ntr dg o n ntr a o n ntr dn n ntrThe linear model holds well at least for carrier densities near the transparency carrier density. Thequantity a o is called the differential gain (units: cm2). We also assume that the recombinationgeneration rates can also be approximated with a linear model, Rnr n Gnr n Rr n Gr n n ni r n rHere, r is a phenomenological recombination time. With these approximations we can write thefollowing set of equations for operation in the steady state,dP z n z n P z a aotrdz(1) i I n z P z ao n z ntr qVa r AeffThe second equation gives us, i I rn z ntr qVa 1 ao r ntrP z AeffThe above equation shows that the reduction of the carrier density and the saturation of the gain isgoverned by the denominator. We write the above expression as,

Semiconductor Optoelectronics (Farhan Rana, Cornell University) i I rn z ntr qVa1 ntrP z Psat(3)where, APsat effao rThe quantity Psat defines the optical power at which gain saturation cannot be ignored. WhenP z Psat gain saturation can be ignored and carrier density can be determined assuming theoptical power is zero. The unsaturated value of the modal gain is, n z n i I r n a g * a a a aotr P z Po tr sat qVa and the unsaturated value of the amplifier gain is, g * LG* e aPlugging the result in (3) into (1) gives, dP z a g * P z P z dz 1 Psat It is clear from the above equation that if P z Psat then the amplifier gain is just the unsaturated g * L . Solution of the above equation via direct integration gives,gain G* e a G * ag * ag * L ln G 1 ag * P 0 a g * e 1 a g * G 1 1 ag * L ln G Psat G * ag * ag * 1 G e G 1 G In the above equation G is the amplifier gain defined as P L P 0 . The above equation can be usedto obtain G as a function of the unsaturated modal gain and the input optical power. Since theamplifier gain depends on the input power, the amplifier is nonlinear. The nonlinearity is due to gainsaturation. When P 0 Psat the amplifier gain G equals the unsaturated value G * . As the inputpower P(0) increases, the optical power P(L) at the output becomes large enough to cause asignificant reduction in the carrier density n(z) close to z L , and when the carrier densitydecreases, the gain G , which can also be written as,L aao n( z ) dzG also decreases. This is gain saturation. Two important figures of merit of SOAs are the inputsaturation power and the output saturation power. The input saturation power is the input opticalpower at which the amplifier gain G decreases by a factor of two (or by 3 dB) from the unsaturatedvalue G * . The output saturation power is the output optical power at which the amplifier gaindecreases by a factor of two (or by 3 dB). The input saturation power is given by the expression,e0

Semiconductor Optoelectronics (Farhan Rana, Cornell University) 2 ag * 1P 0 a g * 1 g * Psat a G * 2 2 1 The output saturation power is, P L a g * G * 2 ag * 1 1 Psat 2 G * 2 2 ag * 1 The maximum output saturation power the amplifier can produce is obtained by taking the limitG* assuming that the ratio a g * remains constant. For example, G * can be increased byincreasing the length of the amplifier. The maximum output saturation power is, P L a g * 1 1 2 ag * Psat The above equation shows that the maximum value of the output saturation power is of the order ofPsat . More insight can be obtained by plotting P(L) vs P(0) and the gain G vs the output powerP(L) and vs the input power P(0) . These graphs are shown below for G * equal to 28 dB and theratio g * equal to 2. All the quantities are plotted in decibels (dB).aThe plots show:i)the decrease in the amplifier gain with the input optical power when the input optical powerexceeds Psat G * .ii) the saturation of the output optical power at large input powers to values close to Psat .

Semiconductor Optoelectronics (Farhan Rana, Cornell University)SOAs with large output saturation powers are desirable. In order to increase the output saturationpower one must increase the value of Psat and the value of the ratio a g * .9.4 Amplified Spontaneous Emission (ASE) in SemiconductorOptical Amplifiers (SOAs)9.4.1 Introduction:Spontaneously emitted photons into all the unguided radiation modes leave the active region soonafter emission. Spontaneously emitted photons into the guided radiation mode travel along thewaveguide and get amplified via stimulated emission. This amplified spontaneous emission (ASE)exits from the output end of the amplifier along with the amplified input signal. ASE is undesirablebut unavoidable. It is considered a part of the noise added by the optical amplifier.9.4.2 Amplified Spontaneous Emission:Suppose the optical waveguide of the SOA supports only a single guided mode. When we say a“single mode waveguide” we do not mean that only a single radiation mode is guided. What we meanis that the waveguide only supports a single transverse optical mode. For this single transverse mode,the propagation vector is a function of frequency, as shown below, and different values of correspond to different longitudinal modes of the waveguide. If the length of the waveguide is L then periodic boundary conditions give L 2 differentlongitudinal modes in an interval . From previous Chapters we know how to calculate thespontaneous emission rate into a single radiation mode. The expression for the spontaneous emissionrate has the same form as that for the stimulated emission rate except that the photon occupation ofthe mode is taken to be unity. The spontaneous emission rate into a longitudinal mode of frequently per unit volume of the active region per second is,1v g g nsp VpHere, Vp is the modal volume of the mode and equals Aeff L . To proceed further, we will make someassumptions that will simplify things. We assume that:a) There is no input optical signal.b) The photons travelling in the waveguide are entirely due to spontaneous emission andamplified spontaneous emission and not coming from any input signal or amplified inputsignal.

Semiconductor Optoelectronics (Farhan Rana, Cornell University)c) The photon density everywhere in the waveguide is small enough to not cause any significantreduction in the carrier density due to stimulated recombination. Consequently, carrier densitycan be calculated as if there were no photons in the waveguide. and the spontaneous emission factorKnowing the carrier density, we can calculate the gain gnsp which are both functions of the photon frequency . As assumed, the gain will be the * .unsaturated gain gSuppose the ASE optical power at frequency moving in the z-direction is given by P z, .Consider a small waveguide segment of length z located at z . The increase in power from z toz z due to the addition of spontaneously emitted photons is,1P z z, P z, v g g nsp Aa z Vpzz zThis implies, vg P z, a g n sp z L The above equation contains only the spontaneous emission contribution. We also add the stimulatedemission-absorption and loss contributions to get, vg P z, a g P z, a g n sp z L The solution subject to the boundary condition P z 0, 0 is, v g a g e ag L 1 P ( z L, ) n( )sp L a g Note that the ASE power is roughly proportional to the gain G e ag L of the amplifier. The above expression gives the ASE power at the output ( z L ) in only one longitudinal radiationmode. To get the total ASE power coming out at z L we need to sum the power in all thelongitudinal modes, d d v g a g PASE L P ( z L, ) L n sp ( ) e ag L 1 0 2 0 2 L a g We can convert the above integral into a frequency integral by noting that, 1 v g and get, d a g 1P ( z L, ) nsp ( ) e ag L 1 a g 0 2 v g0 2 The integral is non-zero and significant only within a bandwidth roughly equal to the gain bandwidth. 0 , n is infinite, but the product g n ( ) is always finite,For frequencies at which gspsp d PASE L and therefore the integrand is also finite. An equal amount of ASE power comes out from the inputend of the amplifier.

Semiconductor Optoelectronics (Farhan Rana, Cornell University)Usually an optical filter is placed in front of the SOA to cut down the ASE in unused bandwidth.Suppose the filter has a center frequency f and a band width f . Then the ASE power goingthrough the filter is, a g f f e a g f L 1 PASE n( ) fspf a g f 2

Semiconductor Optical Amplifiers 9.1 Basic Structure of Semiconductor Optical Amplifiers (SOAs) 9.1.1 Introduction: Semiconductor optical amplifiers (SOAs), as the name suggests, are used to amplify optical signals. A typical structure of a InGaAsP/InP SOA is shown in the Figure below. The basic structure consists of a heterostructure pin junction.File Size: 1MB

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