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Optical Networking Components:

Optical Networking Components: 5
Optical Networking Components: Part I Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 1Overview  Couplers, Splitters, Isolators, Circulators  Filters, Gratings, Multiplexors  Optical Amplifiers, Regenerators  Light Sources, Tunable Lasers, Detectors  Modulators  Chapter 2 and 3 of Ramaswami/Sivarajan Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 2Couplers, Splitters Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 3Optical Couplers  Combines splits signals  Wavelength independent or selective  Fabricated using waveguides in integrated optics  = coupling ratio  Power(Output1) =  Power(Input1)  Power(Output2) = (1) Power(Input1)  Power splitter if =1/2: 3dB coupler  Tap if  close to 1 selective if  depends upon  (used in EDFAs) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 4Couplers (contd)  Light couples from one waveguide to a closely placed waveguide because the propagation mode overlaps the two waveguides  Identical waveguides = complete coupling and back periodically (“coupled mode theory”)  Conservation of energy constraint: Possible that electric fields at two outputs have same magnitude, but will be 90 deg out of phase Lossless combining is not possible Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 5Couplers (Contd) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 68port Splitter Made by Cascading Y Couplers Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 78x8 Star Coupler Power from all inputs equally split among outputs Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 8Isolators and Circulators  Extension of coupler concept  Nonreciprocal = will not work same way if inputs and outputs reversed  Isolator: allow transmission in one direction, but block all transmission (eg: reflection) in the other  Circulator: similar to isolator, but with multiple ports. Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 9Recall: Polarization • Polarization: Time course of the direction of the electric field vector Linear, Elliptical, Circular, Nonpolar • Polarization plays an important role in the interaction of light with matter Amount of light reflected at the boundary between two materials Light Absorption, Scattering, Rotation Refractive index of anisotropic materials depends on polarization (Brewster’s law)Polarizing Filters Done using crystals called dichroics Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 11Rotating Polarizations Crystals called “Faraday Rotators” can rotate the polarization without loss Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 12Optical IsolatorPolarizationdependent Isolators Limitation: Requires a particular SOP for input light signal Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 14Polarizationindependent Isolators SWP: Spatial Walkoff Polarizer (using birefringent crystals) Splits signal into orthogonally polarized components Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 15Multiplexers, Filters, Gratings Wavelength selection technologies… Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 16Applications  Wavelength (band) selection,  Static wavelength crossconnects (WXCs), OADMs  Equalization of gain  Filtering of noise  Ideas used in laser operation  Dispersion compensation modules Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 17Characteristics of Filters  Low insertion (inputto output) loss  Loss independent of SOP: geometry of waveguides  Filter passband independent of temperature  Flat passbands  Sharp “skirts” on the passband crosstalk rejection  Cost: integrated optic waveguide manufacture  Usually based upon interference or diffraction Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 18Gratings  Device using interference among optical signals from same source, but with diff. relative phase shifts (I.e. different path lengths)  Constructive interference at wavelength  and grating pitch, a, if asin( ) sin( ) = m  i d  m = order of the grating Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 19Transmission vs Reflection Grating  Narrow slits (tx) vs narrow reflection surfaces (rx)  Majority of devices are latter type (rx)  Note: etalon is a device where multiple optical signals generated by repeated traversals of a single cavity Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 20Diffraction Gratings Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 21Grating principles (contd)  Blazing: concentrating the refracted energies at a different maxima other than zeroth order  Reflecting slits are inclined at an angle to the grating plane. Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 22Bragg Gratings  Periodic perturbation (eg: of RI) “written” in the propagation medium  Bragg condition: Energy is coupled from incident to scattered wave if wavelength is  = 2 n 0 eff where  is period of grating  If incident wave has wavelength  , this wavelength is 0 reflected by Bragg grating Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 23Bragg Grating Principles Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 24Bragg Gratings (contd)  Uniform vs apodized index profile  Apodized: side lobes cut off, but width of main lobe increased  Reflection spectrum is the Ftransform of RI distribution  B/w of grating (1 nm) inversely proportional to grating length (few mm)  Note: Lasers use Bragg gratings to achieve a single frequency Shivkumar Kalyanaraman Rensselaer Polytechnic Institute operation 25Fiber Gratings  Very lowcost, low loss, ease of coupling (to other fibers), polarization insensitivity, low temp coeff and simple packaging  “Writing” Fiber Gratings:  Use photosensitivity of certain types of fibers (eg: Silica doped with Ge, hit with UV light = RI change)  Use a “phase mask” (diffractive optical element)  Shortperiod (aka Bragg, 0.5m) or longperiod gratings (upto a few mm)  Shortperiod (Fiber Bragg): low loss (0.1dB),  accuracy (0.05nm)  Longperiod fiber gratings used in EDFAs to provide gain compensation Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 26Fiber Bragg Grating Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 27OADM Elements with FB Gratings Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 28Fiber Bragg Chirped Grating  Used in dispersion compensation (it tightens the pulse width) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 29Longperiod Fiber Gratings  Principle of operation slightly different from fiber Bragg  Energy after grating interaction is coupling into other forward propagating modes in the cladding  …instead of being fully reflected as in Fiber Bragg  Cladding modes very lossy and quickly attenuated  = Couple energy OUT of a desired wavelength band Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 30FabryPerot (FP) Filters  FabryPerot filter also called FP interferometer or etalon  Cavity formed by parallel highly reflective mirrors  Tunable: w/ cavity length or RI within cavity  Eg: Piezoelectric material can “compress” when voltage is applied Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 31FabryPerot (FP) Interferometer  The outgoing s for which d = k /2, add up in phase (resonant s) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 32Interferometer Sharpness Line Width  Different DWDM s can coincide with the passbands.  FSR = freespectralrange between the passbands Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 33Filter Parameters Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 34Spectral Width, Linewidth, Line Spacing Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 35ThinFilm Multilayer Filters (TFMF)  TFMF is an FP etalon where mirrors are realized using a multiple reflective dielectric thinfilm layers (I.e. multiple cavities = 2) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 36Mux/Demux Using Cascaded TFMFs  Each filter passes one  and reflects the other s  Very flat top and sharp skirts Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 37Cascaded TFMFs (contd) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 38MachZehnder Filter/Interferometer (MZI) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 39MachZehnder (Contd)  Reciprocal device  Phase lag + interference  Used for broadband filtering  Crosstalk, nonflat spectrum, large skirts…  Tunability: by varying temperature ( few ms) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 40ThermoTunable MZ Filter Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 41Multistage MZI Transfer Function Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 42Arrayed Waveguide Grating (AWG)  Generalization of MZI: several copies of signal, phase shifted differently and combined = 1xn, nx1 elements  Lower loss, flatter passband compared to cascaded MZI  Active temperature control needed Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 43Arrayed Waveguide Grating Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 44AcoustoOptic Tunable Filter (AOTF)  Interaction between sound and light: Sound is used to create a Bragg grating in a waveguide  Acoustic wave in opposite direction to optical signal  Density variations depend on acoustic RF freq lead to RI variations: RF frequency can be easily tuned  Polarization dependent or independent designs… Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 45Dynamic Wavelength Crossconnects  Multiple acoustic waves can be launched simultaneously  The Bragg conditions for multiple s can be satisfied simultaneously  = Dynamic crossconnects  Lots of crosstalk wide passbands Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 46High Channel Count Multiplexers  Multistage Banded multiplexers Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 47Multistage Interleaving  Filters in the last stage can be much wider than each channel width Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 48Amplifiers, Regenerators Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 49Amplification Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 50Optical Amplifiers vs Regenerators 4080 km Terminal Terminal Regenerator 3R (Reamplify, Reshape and Retime) 120 km Terminal Terminal EDFA 1R (Reamplify) Terminal Terminal Terminal Terminal Terminal Terminal EDFA amplifies all s Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 51OEO Regenerator Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 521R, 2R and 3R Regeneration Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 53Regenerators vs OAmplifiers  Regenerators specific to bit rate and modulation format used; OAmps are insensitive (I.e. transparent)  A system with optical amplifiers can be more easily upgraded to higher bit rate w/o replacing the amplifiers  Optical amplifiers have large gain bandwidths = key enabler of DWDM  Issues:  Amplifiers introduce additional noise that accumulates  Spectral shape of gain (flatness), output power, transient behavior need to be carefully designed Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 54EDFA Enables DWDM ... EDF EDF WDM WDM ... Coupler Coupler Optical Optical Optical Filter Isolator Isolator DCF 1480 980 Pump Pump Laser Laser  EDFAs amplify all s in 1550 window simultaneously  Key performance parameters include  Saturation output power, noise figure, gain flatness/passband Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 55Optical Amplifier Varieties Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 56Optical Amplifier Flat Gain Region Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 57Principles: Stimulated Emission  Transitions between discrete energy levels of atoms accompanied by absorption or emission of photons  E E can be stimulated by an optical signal 2 1  Resulting photon has same energy, direction of propagation, phase, and polarization (a.k.a coherent)  If stimulated emission dominates absorption, then we have amplification of signal  Need to create a “population inversion” (N N ) through 2 1 a pumping process Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 58Spontaneous Emission  E E transitions can be spontaneous (I.e. independent 2 1 of external radiation)  The photons are emitted in random directions, polarizations and phase (I.e. incoherent)  Spontaneous emission rate (or its inverse, spontaneous emission lifetime) is a characteristic of the system  Amplification of such incoherent radiation happens along with that of incident radiation  A.k.a. amplified spontaneous emission (ASE): appears as noise  ASE could saturate the amplifier in certain cases Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 59Optical Amplification: mechanics Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 60ErbiumDoped Fiber Amplifier (EDFA)  Length of fiber: core doped with (rare earth) erbium ions 3+ Er  Fiber is pumped with a laser at 980 nm or 1480nm.  Pump is coupled (in and out) using a selective coupler  An isolator is placed at the end to avoid reflections (else this will convert into a laser) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 61EDFA success factors  1. Availability of compact and reliable highpower semiconductor pump lasers  2. EDFA is an allfiber device = polarization independent easy to couple light in/out  3. Simplicity of device  4. No crosstalk introduced while amplifying Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 62EDFA: Operation 3+  When Er ions introduced in silica, electrons disperse into an energy band around the lines E , E , E (Stark splitting) 1 2 3  Within each band, the ion distribution is nonuniform (thermalization)  Due to these effects, a large  range (50 nm) can be simultaneously amplified luckily it is in the 1530nm range Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 63EDFA: Operation (Contd)  980 nm or 1480nm pumps are used to create a population inversion between E and E 2 1  980 nm pump = E E (absorption) E E 1 3 3 2 (spontaneous emission)  1480 nm pump = E E (absorption, less efficient) 1 2  Lifetime in E is 1s, whereas in E it is 10ms 3 2 Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 64EDFA Pumping Issues  Higher power 1480nm pumps easily available compared to 980 nm pumps  Higher power 1480nm pumps may be used remotely  Degree of population inversion with 1480nm is less = more noise  Fluoride fiber (EDFFAs) produce flatter spectrum than EDFAs, but they must be pumped at 1480nm (see pic earlier) due to “excited state absorption” (E E ) 3 4 Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 65Towards Flat EDFA Gain  Long period fibergrating used to add some “loss” in the peaks of the curve (see ) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 66Reducing EDFA Gain Ripples Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 67EDFA: Summary Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 68Semiconductor Optical Amplifiers (SOA)  SOAs have severe crosstalk problems, besides others Shivkumar Kalyanaraman  But used in switches etc Rensselaer Polytechnic Institute 69Recall: SRS and Raman Amplifiers  Power transferred from lower to higher channels (about 100nm)  Eg: 14601480nm pump = amplification at 1550 1600nm  Gain can be provided at ANY wavelength (all you need is an appropriate pump )  Multiple pumps can be used and gain tailored  Lumped or distributed designs possible  Used today to complement EDFAs in ultralonghaul systems Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 70Raman Amplification Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 71Raman Amplification (contd) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 72Counterpumped Raman Amplification Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 73Distributed Raman Amplifiers  Complement EDFAs in ultralonghaul systems  Challenge: need highpower pumps  Pump power fluctuation = crosstalk noise  Counterpumping: (dominant design) pump power fluctuations are averaged out over the propagation time of fiber; other crosstalk sources also reduced Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 74Practical Raman Pumps  Use a conveniently available (eg: 1100 nm) pump and use Raman effect itself, in combination with a series of FPresonators (created through selective mirrors, I.e. matched Bragg gratings)  Eg: 1100nm 1155nm  1218nm 1288nm  1366nm  1455 nm  The final stage (1455nm) has lowreflectivity= output pump at 1455nm which produces gain at 1550nm  80 of the power comes to the output Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 75Recall: Optical Amplifier Varieties Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 76Raman vs OFAs Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 77LongHaul Alloptical Amplification Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 78Optical Regenerator Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 79Regenerator Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 80Regen w/ Dispersion Compensation and Gain Equalization Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 81Light Sources: LEDs, Lasers, VCSELs, Tunable Lasers Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 82Lasers: Key Target Characteristics  Laser: an optical amplifier enclosed in a reflective cavity that causes it to oscillate via positive feedback  High output power (110 mW normal, 100200mW EDFA pumps, few Ws for Raman pumps)  Threshold Current: drive current beyond which the laser emits power  Slope Efficiency: ratio of output optical power to drive current  Narrow spectral width at specified   Sidemode suppression ratio  Tunable laser: operating s stability: drift over lifetime needs to small relative to WDM channel spacing  Modulated lasers: low (accumulated) chromatic dispersion Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 83Recall: Energy Levels Light Emission Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 84Spontaneous Emission, MetaStable States Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 85Recall:Stimulated Emission Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 86Recall: FabryPerot Etalon Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 87Laser vs LEDs  LED: Forwardbiased pnjunction (low R etalon)  Recombination of injected minority carriers by spontaneous emission produces light  Broad spectrum (upto gain b/w of medium)  Low power: 20dBm  Low internal modulation rates: 100s of Mbps max  LED slicing: LED + filter (power loss)  Laser:  Higher power output  Sharp spectrum (coherence):  chromatic dispersion  Internal or External modulation:  distance,  bit rates  Multilongitudinal mode (MLM): larger spectrum (10s of nm) with discrete lines (unlike LEDs) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 88Simple LEDs: pn junction, bandgap Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 89Double Heterojunction LED  Light produced in a more localized area in double heterojunction LEDs  Heterojunction: junction between two semiconductors with different bandgap energies  Charge carriers attracted to lower bandgap (restricts region of ehole recombinations) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 90Effect of Temperature on  and I Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 91LED: Temperaturedependent Wavelength Drift Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 92LEDs: Useful in Freespace Optical Communication • Output Optical Power • P— Output Optical Power 1.24 •— wavelength P I  • I — Input Electrical Current • Output Optical Spectral Width Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 93Lasers vs Optical Amplifiers  As reflectivity of the cavity boundaries (aka facets) , the gain is high only for the resonant s of the cavity  All resonant s add in phase  Gain in general is a function of the  and reflectivity  If reflectivity (R) and gain is sufficiently high, the amplifier will “oscillate” I.e. produce light output even in the absence of an input signal  This lasing threshold is where a laser is no longer a mere amplifier, but an oscillator  W/o input signal, stray spontaneous emissions are amplified and appear as light output  Output is “coherent”: it is the result of stimulated emission  LASER = “Light Amplification by Stimulated Emission of Shivkumar Kalyanaraman RensseR laea r Pd olyiatio technic In nst” itute 94Lasing Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 95Modes, Spectral Width and Linewidth Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 96FabryPerot Laser Sources Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 97Laser: Output Behavior vs Applied Power Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 98Directing the Light in a FabryPerot Laser Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 99Longitudinal Modes: SLM and MLM : within the b/w of the gain medium inside the cavity  Cavity length should be integral multiple of /2  Such s are called “longitudinal modes”  FP laser is a multiplelongitudinal mode (MLM) laser (Large spectral width (10 nm or 1.3 Thz)  Desired: singlelongitudinal mode (SLM):  Add a filter to suppress other s by 30dB+ Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 100Multimode output of Laser Cavity Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 101Recall: History of SLM/MLM Usage Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 102Distributed Feedback (DFB) Lasers  Idea: Provide a distributed set of reflections (feedback) by a series of closelyspaced reflectors  Done using a periodic variation in width of cavity  Bragg condition satisfied for many s; only the  s.t. the corrugation period is /2 is preferentially amplified  Corrugation inside gain region: called DFB laser  Corrugation outside gain region: called DBR (distributed Bragg reflector) laser Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 103Bragg Laser Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 104InFibre Laser using FBGs Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 105External Cavity Lasers  Only those s which are resonant for both primary and external cavities are transmitted  Diffraction grating can be used in external cavity with  selective reflection at grating and antireflection coating outside of the primary cavity facet  Used in test equipment: cannot modulate at high speed Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 106VCSELs: Vertical Cavity SurfaceEmitting Lasers  Frequency (longitudinal mode) spacing = c/2nl  If l is made small, mode spacing increases beyond cutoff of gain region bandwidth = SLM  Thin active layer: deposited on a semiconductor substrate = “vertical cavity” “surface emitting”  For high mirror reflectivity, a stack of alternating low and highindex dielectrics (I.e. dielectric mirrors) are used  Issues: Large ohmic resistance: heat dissipation problem  Roomtemperature 1.3um VCSELs recently shown Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 107VCSELs Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 108VCSEL Structure Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 109WavelengthSelective VCSEL Array  High array packing densities possible with VCSELs compared to edgeemitting lasers (silicon fabrication)  Used a tunable laser by turning on required laser  Harder to couple light into fiber  Yield problems: if one laser does not meet spec, the whole array is wasted Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 110Combining VCSELs Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 111Modelocked Lasers  Match the phase of the longitudinal modes = regular pulsing in time domain (aka “mode locking”)  Used in OTDM  Achieved by using longer cavities (eg: fiber laser) or modulating the gain of cavity Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 112Mode Locking by Amplitude Modulation of Cavity Gain Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 113Gaussian Beams Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 114Tunable Lasers  Tunable lasers: key enabler of reconfigurable optical networks  Tunability characteristics:  Rapid ( ms ranges)  Wide and continuous range of over 100 nm  Long lifetime and stable over lifetime  Easily controllable and manufacturable  Methods:  Electrooptical: changing RI by injecting current or applying an Efield (approx 1015 nm)  Temperature tuning: (1 nm range) may degrade lifetime of laser Shivkumar Kalyanaraman  Mechanical tuning: using MEMS = compact Rensselaer Polytechnic Institute 115Tunable Two Threesection DBR Lasers Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 116Tunable DBR Lasers (Contd) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 117Sampled Grating DBR  Goal: larger tuning range by combining tuning ranges at different peaks (aka “combs”) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 118Sampled Grating DBR (contd) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 119Photodetectors Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 120Optical Receivers: Basic Ideas Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 121Photoconductive Detector Application of external bias = absorbed photons lead to electron/hole pairs and a current (aka “photocurrent”) • Energy of incident photon at least the bandgap energy = largest  = cutoff  • Si, GaAs cannot be used; InGaAs, InGaAsP used Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 122Practical Photoconductors Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 123Responsivity Ratio of electric current flowing in the device to the incident optical power Photoelectric detectors responds to photon flux rather than optical power (unlike thermal detectors)Responsivity vs  Responsivity is dependent upon the choice of wavelength Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 126Photoconductor vs Photodiode  Photoconductor (I.e. a single semiconductor slab) is not very efficient:  Many generated electrons recombine with holes before reaching the external circuit  Need to “sweep” the generated conductionband electrons rapidly OUT of the semiconductor  Better: use a pnjunction and reversebias it: positive bias to ntype  A.k.a. photodiode  Drift current: eh pairs in the depletion region: rapidly create external current  Diffusion: eh pairs created OUTSIDE the depletion region move more slowly and may recombine, reducing efficiency Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 127Reversedbiased PN photodiode Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 128Photodiodes Reverse biased pn or pin junctions Photodiodes are faster than photoconductorsPIN Photodiode  To improve efficiency, use a lightly doped intrinsic semiconductor between the p and ntype semiconductors  Much of light absorption takes place in the Iregion: increases efficiency and responsivity  Better: make the p and ntype transparent (I.e. above cuttoff ) to desired : double heterojunction  Eg: cuttoff for InP is 0.92 um (transparent in 1.31.6 um range), and cuttoff for InGaAs is 1.65um Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 130Avalanche Photodiode  Photogenerated electron subjected to high electric field (I.e. multiplication region) may knock off more electrons (I.e. force ionization)  Process = “avalanche multiplication”  Too large a gain G can lead to adverse noise effects Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 131Avalanche Process Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 132Electric Field Strengths in APD Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 133Modulators Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 134Electronic vs Photonic Regime Cannot go negative in the photonic regime Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 135Optical Modulation Methods Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 136Issues in Optical Modulation  OnOff keying (OOK) is the simplest  Direct modulation vs External modulation  Extinction ratio: ratio of output power for bit=1 to output power for bit=0  Some lasers cannot be directly modulated  Direct modulation adds “chirp,” I.e., time variation of frequency within the pulse  Chirped pulses are more susceptible to chromatic dispersion  Combat chirp by increasing the power of bit=0, so that lasing threshold is not lost Reduction of extinction ratio (down to 7dB)  Solution: external modulation for higher speeds, longer distance/dispersionlimited regimes Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 137External Modulation  External modulation can be:  onestage designs (if modelocked lasers used) or  two stage designs Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 138External Modulation (contd)  Light source is continuously operated (I.e. not modulated)  External modulation turns light signal ON or OFF  They can be integrated in same package as laser (eg: electroabsorption or EA modulators)  EA: applying Efield shrinks bandgap = photons absorbed (Stark effect) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 139Lithium Niobate External Modulators  MZI or directional coupler configuration  Voltage applied = change RI and determine coupling (or invert phase in MZI)  MZI design gives good extinction ratio (1520dB) and precise control of chirp, but is polarization dependent Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 140External Modulators (contd) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 141Optical Modulators Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 142CrossGain CrossPhase Modulation Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 143Eye Diagrams Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 144Eye Diagrams (contd) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 145BER Estimation w/ Eye Diagrams Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 146BER Estimation (contd) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 147Switches Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 148Multiplexing: WDM  TDM: Time Division Multiplexing B b/s 10Gb/s upper limit NB b/s 1  WDM: Wavelength 2 Division Multiplexing N Use multiple carrier frequencies B b/s  1 1 to transmit data  2 ... 1 2 N 2 simultaneously  N N Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 149Multiplexers, Filters, Routers  Filter selects one wavelength and rejects all others  Multiplexor combines different wavelengths  Router exchanges wavelengths from one input to a different output Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 150Switch Parameters  Extinction Ratio: ratio of output power in ON state to the power in the OFF state  1025 dB in external modulators  Insertion loss: fraction of power lost  Different losses to different outputs = larger dynamic range = may need to equalize (esp. for large switches)  Crosstalk: ratio of power at desired vs undesired output  Low polarization dependent loss (PDL)  Latching: maintain switch state even if power turned off  Readout capability: to monitor current state  Reliability: measured by cycling the switch through its states a few million times Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 151Switch Considerations  Number of switch elements: complexity of switch  Loss uniformity:different losses to different outputs (esp for large switches)  Number of crossovers: waveguide crossovers introduce power loss and crosstalk (not a problem for freespace switches)  Blocking Characteristics: Any unused input port can be connected to any unused output port  Widesense nonblocking: without requiring any existing connection to be rerouted = make sure future connections will not block  Strictsense nonblocking: regardless of previous connections  Rearrangeably nonblocking: connections may be re routed to make them nonblocking Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 152Crossbar Switch Widesense non blocking Shortest path length = 1 vs longest = 2n1 Fabricated w/o any crossovers Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 153Clos Architecture Strictsense nonblocking; used in large portcount s/ws N = mk; k (m x p) switches in first/last stages; p (k x k) switches in middle stage; Nonblocking if p = 2m 1 2/3 Lower number of crosspoints than crossbar (n ) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 154Spanke Architecture • Strictsense nonblocking • Only 2 stages: 1xn and nx1 switches used instead of 2x2 • Switch cost scales linearly with n • Lower insertion loss and equal optical path lengths Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 155Benes Architecture • Rearrangeably nonblocking • Efficient in number of 2x2 components • ves: not WSnonblocking and requires waveguide Shivkumar Kalyanaraman Rensselaer Polytechnic Institute crossovers 156SpankeBenes Architecture • Rearrangeably nonblocking • Efficient in number of 2x2 components • Eliminates waveguide crossovers: nstage planar… Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 157MEMS Mirror Switching Component Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 158NxN Switching with MEMS Mirror Arrays Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 159Analog Beam Steering Mirror Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 160Planar Waveguide Switch Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 161Planar Waveguide Switch Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 1621x2 Liquid Crystal Switch Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 163
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