Optical Fibers And Cables - University Of Houston

ECE 6323

Introduction Fundamental of optical amplifiers Types of optical amplifiers Erbium-doped fiber amplifiers Semiconductor optical amplifier Others: stimulated Raman, optical parametric Advanced application: wavelengthconversion Advanced application: optical regeneration

What is optical amplification? What use is optical amplification?

Review: stimulated emission Through a population, netgain occurs when there aremore stimulated photonsthan photons absorbed Require populationinversion from input pump Amplified output light iscoherent with input light

Optical amplificationEnergy pumpPinPout Pin PCoherent zdP gPdzIf g 0: Optical gain(else, loss) P Pin ( N 2 N1 ) zOptically amplified signal:coherent with input: temporally,spatially, and with polarization

What is optical amplification? What use is optical amplification? The most obvious: to strengthen a weakened signal(compensate for loss through fibers) But why not just detect the signal electronically andregenerate the signal? System advantage: boosting signals of many wavelengths: key toDWDM technology System advantage: signal boosting through many stages withoutthe trouble of re-timing the signal There are intrinsic advantages with OA based on noiseconsiderations Can even be used for pre-amplification of the signal beforedetected electronically

Introduction Fundamental of optical amplifiers Types of optical amplifiers Erbium-doped fiber amplifiers Semiconductor optical amplifier Others: stimulated Raman, optical parametric Advanced application: wavelengthconversion Advanced application: optical regeneration

Optical amplification fundamental

Summary of OA fundamentalKey concepts in optical amplification process:1- The signal is amplified with gain as in the following equation:( d I[z ])/(d z) g[z] I[z]but gain g[z] can be saturated:g[z] g0/(1 I(z) /Isat)where g0 is a characteristic value, and Isat , the saturation intensityis:Isat ( spont/(2 stim)) h nwhere spont and stim are the amplifier stimulated emission andspontaneous emission coefficients, respectively.The main features are:1.a power (or intensity) gain is exponential at small signal1.b but becomes linear when the power (or intensity) islarge

Summary of OA fundamental (cont.)2- Just like a laser has certain spectralrange, gain can occur only over certainrange of wavelength that depends on themedium. The variation of gain vs.wavelength is called gain spectrum, g[l]3- The light coming out of an OA is not just the amplified inputsignal (what if we don't have any input?) but also includesAmplified Spontaneous Emission (ASE): very important effect onthe OA noise characteristics,4- There are 2 key noise terms of an OA:- Signal--spontaneous beat noise- Spontaneous--spontaneous beat noiseBoth are critical to the performance of OA.

Example of optical amplifier research

Optical amplifier can be: Booster: boosts the signal power that is lossthrough transmission Pre-amplifier: enhance the signal to overcomedetector noise Gain behavior is: Linear for booster application (high power gain) Exponential for small signals (pre-amp) Noise source from ASE: Signal-ASE beat noise ASE-ASE beat noise

Some Receiver Performance Data

Introduction Fundamental of optical amplifiers Types of optical amplifiers Erbium-doped fiber amplifiers Semiconductor optical amplifier Others: stimulated Raman, optical parametric Advanced application: wavelengthconversion Advanced application: optical regeneration

Gain (dB) – relative onlySpectra of various type of amplifiers

Different OA types serve different operationalneeds: Some for high power boosterSome for mid-range powerSome for pre-amplificationSome for processing on chip (to compensate forloss) Some for optical signal processing in photoniccircuit For optical communication applications, virtuallyall are in waveguide form. Cost-performance are also major factor

Erbium doped fiber amplifier (EDFA)Energy structure of Er3 ion in glass

EDFA gain spectrum characteristics Intrinsic gain is NOT flat overthe C and L band Must be engineered to flattenthe effective gain System application sometimesrequire equalizer

Typical EDFA module

Most versatile: spectrum, gain, size, andintegratability Remember the semiconductor laser: anysemiconductor laser structure withoutoptical cavity can function as an OA: Edge emitting ridge waveguide Vertical amplification, multiple pass design Cost competitive, especially for pre-amp

R1Max gainR21R1R2 e 20 dB(not included inputcoupling loss)gL0.01Real useful gain 10-11 dB withcoupling loss

Why BraggOutputGratingCoupled OPA?amplified radiationInput Surface emitting BG is alarge-loss element:Amplifier waveguide Lasing suppressionBragg grating coupler AR coating tolerance Low gain FP ripple Wavelength dispersion WDM, multi-spectralapplications ASE filtering for low noise Low numerical apertureoutput (flat wavefront);distributed output for highpower 5-0.000750.30.350.4

Device Structure and FabricationWG ridge (17 m)BG sectionGain sectionBG filter and couplerBG sectionDevice schematicGrating couplerOPA GainBG: 1.42 m, 50% dc, 0.5 m deepDevice: width 17 m; Gain length 1.8mm;BG length 1.5mm.(can be tilted, but insignificantimprovement)

Some photonic circuit designs with SOA

Stimulated Raman emission: can be usedfor gain Lower gain, requires higher pump power thanEDFA and SOA But offer wider gain spectrum than EDFA Specialized application OPA: A nonlinear process, requirematerials with high optical nonlinearity.Require very high peak power. Lesspractical

Wavelength Conversion For network management, asignal may need to change itswavelength from one segmentto another There are many ways toachieve this, some preferredways is NOT to convert thesignal back to electrical: directoptical conversion There are a number ofapproaches, but all with somelimitsSometimes, it’s useful tochange the carrierwavelength of a signall1l2

Some wavelength converter concepts

Signal RegenerationInputOutput A signal can be degraded in different ways: weaker (amplitude) – need re-amplification distorted (shape) – need re-shaping jittered – need re-timing This is the concept of 3R Sometimes, there is one more R to make 4R: needwavelength conversion: re-allocation of wavelength

Signal regeneration conceptsAmplification toovercome lossReshape toovercomedistortion dueto dispersionand lossRe-time toovercome jitter dueto dispersion andrandom fibervibrationRe-allocationofwavelengths(for networkefficiency)

While it is highly desirable, it is still a bigtechnical challenge to come up with anefficient device for 3R and 4R, a fewexists but not quite widespread usage Nevertheless, it is likely will be integratedin future system, depending on thedemand It would make network more efficient

Fundamental of optical amplifiers Types of optical amplifiers Erbium-doped fiber amplifiers Semiconductor optical amplifier Others: stimulated Raman, optical parametric Advanced application: wavelength conversion Advanced application: optical regeneration