Optical Amplifiers - Linköping University
Optical AmplifiersMartin HanssonElectronic Devices, Dept. of ElectricalEngineering, Linköping University, SwedenMonday, March 14, 2005ElectronicDevices
OutlinezzBackgroundSemiconductor Optical Amplifiers–––––zFiber Optical Amplifiers–––––zPrinciples of silicon optical amplifier - SOAGainNoiseDynamic behaviorGain clampingErbium doped fiber amplifiers – EDFAGainNoiseOther propertiesMore on pumpingSummaryMonday, March 14, 20052(30)
Background - Optical AmplifierszAmplification in optical transmission systems needed tomaintain SNR and BER, despite low-loss in fibers.zEarly optical regeneration for optic transmission relied onoptical to electron transformation.zAll-optical amplifiers provide optical gain without any signalconversion to the electron domain.zHigher bandwidth demands further emphasize the need forall-optical amplifiers.zTwo types of all-optical amplifiers:– Semiconductor optical amplifiers– Fiber-optical amplifiers.Monday, March 14, 20053(30)
Semiconductor Optical AmplifierszThe semiconductor optical amplifier (SOA) provide opticalgain without optical-to-electronic conversions.zSOA’s are typically used in the following ways: Used as power boosters followingthe source (optical PA). Provide optical amplification forlong-distance communications (inline amplification, repeaters). Pre-amplifiers before the photodetector. All-optical signal processing.Monday, March 14, 20054(30)
Semiconductor Optical AmplifierszSOA’s are based on semiconductor lasers.zOptical feedback of the laser is reduced.zDivided into two sub categories:– Fabry-Perot Amplifiers (FPA)– Traveling Wave Amplifiers (TWA)zThe distinctions depends on the amount of light reflectedback into the cavity.– FPA’s usually has considerable amount of reflections back to thecavity – reflectivity around 0.3, narrow bandwidth ( 0.1 nm with acarrier at 1550 nm).– TWA’s are designed to get as close to a single pass amplificationas possible – reflectivity below 10-3, large bandwidth ( 30 nm).Monday, March 14, 20055(30)
Principle of SOA Efficient semiconductor lasers are usually fabricated as a heterojunction. A doped (n/p) active region is sandwiched between two heavily doped regions. Large concentration of holes and electron in valence band and conduction band,respectively for the cladding materials.p claddingn claddingn activeEg,claddingelectronsholesEFEg,active Eg,claddingEg,activeEg,claddingncladding nactiven-doped region between two degeneratedregions (n and p doped) at equilibriumExample of a SOA (Laser)Monday, March 14, 20056(30)
Principle of SOA A high forward bias applied to thejunction bends the energy bands. Holes in the p cladding injected inthe active region and the larger bandgap of the n cladding confines theholes in the active region. The higher refractive index in theactive region acts as a wave guide forthe emitted light. A laser uses highly reflective facets ofthe cavity thus applying a positivefeedback to the system. In an optical amplifier we are onlyinterested of the gain in a single passthought the amplifier.p claddingn activen claddingElectronflowVbiasHole flowConfinementfactor ΓRefractiveindex profileOptical modep cladding n activen claddingMonday, March 14, 20057(30)
Gain of a SOAzAmplification in a SOA - excited electrons in the active region arestimulated to recombine with the holes and releasing the excessenergy as identical photons.The rate of excited electrons (N) and theG(N) is a material gain coefficient:number of photons (Nph) is given by:dNJ R (N ) v g g (N )N phdtedg (N ) (1)Γσ gV(N N 0 )(2)Γ – optical confinements factorJ – injection current densityσg – differential gainvg – group velocity of light traveling inV Ldw – volume of active regionthe amplifierN0 – carrier density needed for transparency.R(N) – recombination rateMonday, March 14, 20058(30)
Gain of a SOASteady state solution of (1) gives:g (N ) g0g0 IN ph1 1 IsatN ph,sat(3)Where g0 is the small-signal gain given by (4)and Isat by (5).Γσ g J g0 τ s N0 V ed Isat hνLdwΓ 2σ gτ sThe single-pass gain through the amplifier isgiven by integrating over the whole length.Single-pass gain Gs is given by (7). g0Gs exp (Γ α )L (1 I Isat ) (7)(4)(5)Net gain per unit length is given by (6).g Γg (N ) α(6)α is the total loss coefficient per unit length.Monday, March 14, 20059(30)
Gain saturation in a SOAzThe gain of a SOA will saturate if the optic input power is toolarge.zThe high input power will consume many of the EHP’s in theactive region.zThe electrons and holes in the cladding regions of the SOAneeds some finite time to re-occupy the active region.zSaturation of the gain is referred to as the output power forwhich the gain has compressed 3 dB.Monday, March 14, 200510(30)
Gain compression:P3dB hνAηo ln 2τΓdG / dN(8)A - active strip, cross section areaηo – output coupling efficiencyFiber-to-fiber Gain (dB)Gain saturation in a SOA3dBOutput Power (dBm)τ – carrier lifetimeΓ– confinement factordG/dN – differential gainhν – photon energyMonday, March 14, 200511(30)
Gain RipplezUse a anti-reflective layer on the facets of the laser cavity toreduce the positive feedback .zIdeal anti-reflective layers hard to obtain.zResults in ripple in the gain due to the different modes ofthe laser cavity.zAmount of ripple depends on gain and reflectivity.(1 GR )2Ripple (1 GR )2(9)G – Gain of amplifierR – Facet reflectivityMonday, March 14, 200512(30)
Methods reducing the ripplezPlace the wave guide at an angle to the facet.zEnd the wave guide before the facet (window region).zUsing a combination of all three methods can result in R 10-5Residual reflections are not directlyreflected back to the cavityLight starts to diverge in the windowregion. Reflections are not reflectedback to the cavity.claddingp claddingn claddingn activenp claddingwindow regionn activeMonday, March 14, 200513(30)
Polarization Dependent GainzzzActive region without symmetry causes light with differentpolarization to be amplified differently.A difference in gain between TE and TM mode of thetransmitted light can be as high as tens of dB withoutcountermeasures.Some tactics to reduce the PDG:– Restore symmetry of the active region. Hard to control in industrial processes because active region needs tobe small for single mode.– Introduce a tensile strain of a laser cavity that emits TE polarizedlight.Applied The cavity starts emitting TM polarized light. Strain can be carefully controlled.ForceEmittedlightMonday, March 14, 200514(30)
Noise in a SOAzStimulated emission is not solely responsible for the lightamplification in the SOA.zSpontaneous recombination of EHP’s will also be amplified(amplified spontaneous emission).Noise figure of an semiconductor opticalamplifier (NF)NF where2nspηinsp (10)N2N2 N1(11)N1 and N2 is the number of carriers inground and excited states,respectively.ηi is the input coupling loss.Typical ASE spectrum of a SOA.Monday, March 14, 200515(30)
Effects at dynamic operationzLarge input signal to amplified in a SOA compress the gain.zFor WDM systems the gain compression will cause interchannel crosstalk.– A large input will compress the gain, limiting the available EHP’sused for amplification of the other channelszThe gain compression can be used in all-optical signalprocessing applications.– Wavelength conversion.– Cross-gain modulation.– Cross-phase modulation.Fast dynamic response of a SOA.Monday, March 14, 200516(30)
Gain clampingzGain clamping is used to reduce the inter-channel crosstalkfor WDM systems.zUse distributed Bragg reflectors (DBR) on the facets of thecavity of the amplifier.zWavelength selective feedback in the cavity.zLaser mode created at a wavelength outside of theinteresting amplification band.Monday, March 14, 200517(30)
Gain clampingzzzzA gain clamped SOA has a gain-vs.-output power that isconstant over a large power range.The laser power is used as a reservoir of optical energywhich removes the gain compression.When the laser energy is consumed laser action turns off.Amplifier saturates very fast.Monday, March 14, 200518(30)
Fiber Optical AmplifierszFiber optical amplifiers are based on rare-earth-dopedfibers.zAmplification is obtained at different wavelength dependingon which rare-earth-ions that is used.zMost commonly used is Erbium (Er) with atomic number 68,placed among a Lanthanides in the periodic system.zSilica-fibers doped with Er ions can obtain high gain at awavelength of 1550 nm.zFiber optic amplifiers can be used as:– Power amplifiers– Repeaters, in-line amplifiers– Pre-amplifiersMonday, March 14, 200519(30)
Erbium Doped Fiber AmplifierzAn Erbium-ion doped fiber pumped with light of certainwavelengths.zErbium ions are excited to any of their excited states.zMost common pump wavelengths used are 980 nm and 1480 nm.zExcites the Erbium-ions to the second and first excited energylevel, respectively.zElectrons in the 4I11/2 energy level leavesthat energy level for 4I13/2 with a spontaneouslife time of τ32.zzThe transition between 4I11/2 and 4I13/2 are anon-radiative transition that emits a quantumvibration to the crystal lattice (phonon).Light with wavelength between 1520 nm and1570 nm induce stimulated emission in theEr-ions.τ32 1 µsτsp 10 ms980nm1480nmEr3 4I11/24I13/21520 –1570 nm4I15/2Monday, March 14, 200520(30)
Erbium Doped Fiber AmplifierzA basic EDFA setup includes optical isolators, wavelengthselective couplers, pump lasers, and the fiber itself.zFiber can be pumped with light that either co- or counterpropagates with the amplified light, or both.zThe optical isolators are used to limit the ASE and any lasingmodes in the fiber.General Erbium-doped fiber configurationMonday, March 14, 200521(30)
Gain in EDFA’szThe amplification in a EDFA is supplied when incoming lightstimulates the Er-ions to return to the ground state and emittingthe excessive energy as coherent light.Gain in the EDFA is defined as (12) where g(λ,z) is the gain coefficient over the length of the ED fiberaccording to (13).The emission coefficient and absorption coefficient are given by (14) and (15) respectively.Γs is the confinement factor of the fiber, nEr is the concentration of Er-ions in the core, σe and σa arethe signal emission and absorption cross sections as functions of wavelength.LPG(λ ) out g (λ, z ) dzPin0(12)1 dP (λ, z ) P (λ, z ) dz g * (λ )N2 ( z ) α (λ )N1( z )g ( λ, z ) g * (λ ) Γs nEr σ e (λ )α (λ ) Γs nEr σ a (λ )(13)(14)(15)Monday, March 14, 200522(30)
Gain in EDFA’szzThe gain spectrum of a EDFA is not flat over a widewavelength range.Gain coefficient depends highly on the inversion of thefiber. 100 % inversion – all ionsexcited to first excited energystate or higher. -100 % inversion – non ofthe ions excited and incominglight is absorbed.Monday, March 14, 200523(30)
Gain saturationzzzPsat Gain saturation occurs when the stimulated emission isbalanced by the absorption of pump energy.The higher the pump power the more excited Er-ions andthe higher saturation power.Psat defined as the power where the gain coefficient isreduced by half.(σ eshν s Ac σ as )Γsτ sp σ P 1 as p σ P th es p (16)Where σes and σas are the emission andabsorption cross sections, respectively, at thesignal wavelengthAc is the core are area, τsp is the spontaneouslifetime of the first excited state of the Er-ions,and Pp is the pump power.The pump threshold for transparency is given by (17).Below the pump threshold the gain coefficient isnegative, because there are several non-excited ionsin the fiber that absorbs the incoming signal.Ppth σ as hν s Acσ es Γpτ spσ ap(17)Where hνp is the pump photon energy, Γp is theconfinement factor of the pump mode and σap is thepump absorption cross section.Monday, March 14, 200524(30)
Noise in EDFA’szEDFA also experience amplified spontaneous emission(ASE).zLight emitted by spontaneous decay of excited erbium ionscaptured by the waveguide and amplified.zASE acts as background noise to the amplified signal.NF 2nsp(18)nsp is the spontaneous emission factor.nsp σ es N2σ es N2 σ as N1EDFA’s can be efficiently inverted and NFcan therefore be close to 3 dB which is thefundamental quantum limit of opticalamplifiers.(19)The closer nsp is to 1 the lower thenoise.Monday, March 14, 200525(30)
Coupling LosszMismatch between the Er-doped fiber modes andtransmission fiber modes.zEr-doped fibers usually 2-4 µm in diameter – ordinarytransmission fiber have a diameter of 8-10 µm.zDirect Butt-coupling would have coupling loss of severaldB.zFusion splice is used to couple the fibers.zDoping in splice region can be controlled so that aoptimized low-loss tapered region is formed.zTotal input and output coupling noise of a EDFA fiber usingspliced fusion regions is usually less than 1.5 dB.Monday, March 14, 200526(30)
PolarizationzBecause of the symmetric core of the Er-doped fiber, thegain is virtually independent of polarization.zOne of the main advantages of EDFA’s compared to SOA’s.zSmall polarization dependence by different polarization ofions in fiber.Monday, March 14, 200527(30)
More on pumpingzzzWhy is the two pump wavelengths of 980 nm and 1480 nmchosen?The Er3 ions next four excited energy levels correspondsto pumping wavelengths of 514 nm, 532 nm, 667 nm, and800 nm.Why not use any of these wavelengths?Monday, March 14, 200528(30)
More on pumpingzPump light of any of the six specific wavelength will excitethe Er-ions to the corresponding energy level.zThe ions decays nonradiatively down to the first excitedstate.zLaser diodes developed for 665 nm and 800 nm - could beused for pumping Er-doped fibers.zPump efficiency for shorter wavelengths is lowered due toexcited state absorption (ESA)zESA - pump light excites Er-ions at the first excited state tohigher stateszAbsorbs the pump light and thus reduces amplification.zEfficient pumping is achieved at the wavelengths for 980nm and 1480 nm, which is way they are chosen.Monday, March 14, 200529(30)
SummaryzOptical amplifiers provide amplification in fiber optictransmission without opto-electron conversions.zTwo types of optical amplifiers – semiconductor opticalamplifiers and fiber optical amplifiers.zSOA’s based on lasers – can be either wide or narrow bandzMain application besides amplification – all optical signalprocessing.zEDFA’s provide gain in a fiber by pumping it with laser light– Optimal for signal wavelength close to 1.55 µm.zVertically no polarization dependence on the gain.Monday, March 14, 200530(30)
Mar 14, 2005 · Background - Optical Amplifiers zAmplification in optical transmission systems needed to maintain SNR and BER, despite low-loss in fibers. zEarly optical regeneration for optic transmission relied on optical to electron transformation. zAll-optical amplifiers provide optical g
RF/IF Differential Amplifiers 5 Low Noise Amplifiers 7 Low Phase Noise Amplifiers 10 Gain Blocks 11 Driver Amplifiers 13 Wideband Distributed Amplifiers 13 Power Amplifiers 15 GaN Power Amplifiers 18 Variable Gain Amplifiers 19 Analog Controlled VGAs 19 Digitally Controlled VGAs 20 Baseband Programmable VGA Filters 21 Attenuators 22
Semiconductor optical amplifiers (SOAs) Fiber Raman and Brillouin amplifiers Rare earth doped fiber amplifiers (erbium – EDFA 1500 nm, praseodymium – PDFA 1300 nm) The most practical optical amplifiers to date include the SOA and EDFA types. New pumping methods and materials are also improving the performance of Raman amplifiers. 3
optical networks have been made possible by the optical amplifier. Optical amplifiers can be divided into two classes: optical fibre amplifiers (OFA) and semiconductor optical amplifiers (SOAs). The former has tended to dominate conventional system applications such as in-line amplification used to compensate for fibre losses.
Optical Amplifiers vs Regenerators (1 of 2) Transparent: Regenerators specific to bit rate and modulation format used; O-Amps are insensitive Easily upgraded: A system with optical amplifiers can be more easily upgraded to higher bit rate without replacing the amplifiers Optical amplifiers
Semiconductor Optical Amplifiers (SOAs) have mainly found application in optical telecommunication networks for optical signal regeneration, wavelength switching or wavelength conversion. The objective of this paper is to report the use of semiconductor optical amplifiers for optical sensing taking into account their optical bistable properties .
RF/IF Differential Amplifiers 9 Low Noise Amplifiers 9 Gain Blocks and Driver Amplifiers 10 Wideband Distributed Amplifiers 11 Linear and Power Amplifiers 12 GaN Power Amplifiers 13 Active Bias Controllers 13. Variable Gain Amplifiers 14. Analog Controlled VGAs 14 Digitally Controlled VGAs 14 Baseband Programmable VGA Filters 14. Attenuators 15
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
pile resistances or pile resistances calculated from profiles of test results into characteristic resistances. Pile load capacity – calculation methods 85 Case (c) is referred to as the alternative procedure in the Note to EN 1997-1 §7.6.2.3(8), even though it is the most common method in some countries. Characteristic pile resistance from profiles of ground test results Part 2 of EN 1997 .
