Shivkumar KalyanaramanRensselaer Polytechnic Institute 1 ECSE-6660 Optical Networking Components: Part I http://www.pde.rpi.edu/ Or http://www.ecse.rpi.edu/Homepages/shivkuma/ Shivkumar Kalyanaraman Rensselaer Polytechnic Institute shivkuma@ecse.rpi.edu Based in part on textbooks of S.V.Kartalopoulos (DWDM) and H. Dutton (Understanding Optical communications), and slides of Partha Dutta Shivkumar KalyanaramanRensselaer Polytechnic Institute 2 Overview Couplers, Splitters, Isolators, Circulators Filters, Gratings, Multiplexors Optical Amplifiers, Regenerators Light Sources, Tunable Lasers, Detectors Modulators Chapter 2 and 3 of Ramaswami/Sivarajan Shivkumar KalyanaramanRensselaer Polytechnic Institute 3 Couplers, Splitters Shivkumar KalyanaramanRensselaer Polytechnic Institute 4 Optical 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: 3-dB coupler Tap if close to 1 -selective if depends upon (used in EDFAs) Shivkumar KalyanaramanRensselaer Polytechnic Institute 5 Couplers (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 KalyanaramanRensselaer Polytechnic Institute 6 Couplers (Contd) Shivkumar KalyanaramanRensselaer Polytechnic Institute 7 8-port Splitter Made by Cascading Y- Couplers Shivkumar KalyanaramanRensselaer Polytechnic Institute 8 8x8 Star Coupler Power from all inputs equally split among outputs Shivkumar KalyanaramanRensselaer Polytechnic Institute 9 Isolators and Circulators Extension of coupler concept Non-reciprocal => 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. Recall: Polarization · Polarization: Time course of the direction of the electric field vector - Linear, Elliptical, Circular, Non-polar · 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) Shivkumar KalyanaramanRensselaer Polytechnic Institute 11 Polarizing Filters Done using crystals called dichroics Shivkumar KalyanaramanRensselaer Polytechnic Institute 12 Rotating Polarizations Crystals called "Faraday Rotators" can rotate the polarization without loss! Optical Isolator Shivkumar KalyanaramanRensselaer Polytechnic Institute 14 Polarization-dependent Isolators Limitation: Requires a particular SOP for input light signal Shivkumar KalyanaramanRensselaer Polytechnic Institute 15 Polarization-independent Isolators SWP: Spatial Walk-off Polarizer (using birefringent crystals) Splits signal into orthogonally polarized components Shivkumar KalyanaramanRensselaer Polytechnic Institute 16 Multiplexers, Filters, Gratings Wavelength selection technologies... Shivkumar KalyanaramanRensselaer Polytechnic Institute 17 Applications Wavelength (band) selection, Static wavelength crossconnects (WXCs), OADMs Equalization of gain Filtering of noise Ideas used in laser operation Dispersion compensation modules Shivkumar KalyanaramanRensselaer Polytechnic Institute 18 Characteristics of Filters Low insertion (input-to- 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 KalyanaramanRensselaer Polytechnic Institute 19 Gratings 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 a[sin(i) - sin(d)] = m m = order of the grating Shivkumar KalyanaramanRensselaer Polytechnic Institute 20 Transmission 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 KalyanaramanRensselaer Polytechnic Institute 21 Diffraction Gratings Shivkumar KalyanaramanRensselaer Polytechnic Institute 22 Grating principles (contd) Blazing: concentrating the refracted energies at a different maxima other than zero-th order Reflecting slits are inclined at an angle to the grating plane. Shivkumar KalyanaramanRensselaer Polytechnic Institute 23 Bragg Gratings Periodic perturbation (eg: of RI) "written" in the propagation medium Bragg condition: Energy is coupled from incident to scattered wave if wavelength is 0 = 2 neff where is period of grating If incident wave has wavelength 0, this wavelength is reflected by Bragg grating Shivkumar KalyanaramanRensselaer Polytechnic Institute 24 Bragg Grating Principles Shivkumar KalyanaramanRensselaer Polytechnic Institute 25 Bragg Gratings (contd) Uniform vs apodized index profile Apodized: side lobes cut off, but width of main lobe increased Reflection spectrum is the F-transform 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 operation Shivkumar KalyanaramanRensselaer Polytechnic Institute 26 Fiber Gratings Very low-cost, 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) Short-period (aka Bragg, 0.5m) or long-period gratings (upto a few mm) Short-period (Fiber Bragg): low loss (0.1dB), - accuracy (0.05nm) Long-period fiber gratings used in EDFAs to provide gain compensation Shivkumar KalyanaramanRensselaer Polytechnic Institute 27 Fiber Bragg Grating Shivkumar KalyanaramanRensselaer Polytechnic Institute 28 OADM Elements with F-B Gratings Shivkumar KalyanaramanRensselaer Polytechnic Institute 29 Fiber Bragg Chirped Grating Used in dispersion compensation (it tightens the pulse width) Shivkumar KalyanaramanRensselaer Polytechnic Institute 30 Long-period 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 KalyanaramanRensselaer Polytechnic Institute 31 Fabry-Perot (FP) Filters Fabry-Perot filter also called F-P 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 KalyanaramanRensselaer Polytechnic Institute 32 Fabry-Perot (FP) Interferometer The outgoing s for which d = k /2, add up in phase (resonant s) Shivkumar KalyanaramanRensselaer Polytechnic Institute 33 Interferometer Sharpness & Line Width Different DWDM s can coincide with the passbands. FSR = free-spectral-range between the passbands Shivkumar KalyanaramanRensselaer Polytechnic Institute 34 Filter Parameters Shivkumar KalyanaramanRensselaer Polytechnic Institute 35 Spectral Width, Linewidth, Line Spacing Shivkumar KalyanaramanRensselaer Polytechnic Institute 36 Thin-Film Multilayer Filters (TFMF) TFMF is an FP etalon where mirrors are realized using a multiple reflective dielectric thin-film layers (I.e. multiple cavities >= 2) Shivkumar KalyanaramanRensselaer Polytechnic Institute 37 Mux/Demux Using Cascaded TFMFs Each filter passes one and reflects the other s Very flat top and sharp skirts Shivkumar KalyanaramanRensselaer Polytechnic Institute 38 Cascaded TFMFs (contd) Shivkumar KalyanaramanRensselaer Polytechnic Institute 39 Mach-Zehnder Filter/Interferometer (MZI) Shivkumar KalyanaramanRensselaer Polytechnic Institute 40 Mach-Zehnder (Contd) Reciprocal device Phase lag + interference Used for broadband filtering Crosstalk, non-flat spectrum, large skirts... Tunability: by varying temperature (~ few ms) Shivkumar KalyanaramanRensselaer Polytechnic Institute 41 Thermo-Tunable M-Z Filter Shivkumar KalyanaramanRensselaer Polytechnic Institute 42 Multi-stage MZI Transfer Function Shivkumar KalyanaramanRensselaer Polytechnic Institute 43 Arrayed 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 KalyanaramanRensselaer Polytechnic Institute 44 Arrayed Waveguide Grating Shivkumar KalyanaramanRensselaer Polytechnic Institute 45 Acousto-Optic 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 KalyanaramanRensselaer Polytechnic Institute 46 Dynamic 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 KalyanaramanRensselaer Polytechnic Institute 47 High Channel Count Multiplexers Multi-stage Banded multiplexers Shivkumar KalyanaramanRensselaer Polytechnic Institute 48 Multi-stage Interleaving Filters in the last stage can be much wider than each channel width Shivkumar KalyanaramanRensselaer Polytechnic Institute 49 Amplifiers, Regenerators Shivkumar KalyanaramanRensselaer Polytechnic Institute 50 Amplification Shivkumar KalyanaramanRensselaer Polytechnic Institute 51 Optical Amplifiers vs Regenerators 40-80 km Terminal Regenerator - 3R (Reamplify, Reshape and Retime) Terminal 120 km TerminalTerminal EDFA - 1R (Reamplify) Terminal EDFA amplifies all s Terminal Terminal Terminal Terminal Terminal Shivkumar KalyanaramanRensselaer Polytechnic Institute 52 OEO Regenerator Shivkumar KalyanaramanRensselaer Polytechnic Institute 53 1R, 2R and 3R Regeneration Shivkumar KalyanaramanRensselaer Polytechnic Institute 54 Regenerators vs O-Amplifiers Regenerators specific to bit rate and modulation format used; O-Amps 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 KalyanaramanRensselaer Polytechnic Institute 55 EDFA Enables DWDM! ... ... 980 Pump Laser WDM Coupler WDM Coupler EDF DCF Optical Isolator 1480 Pump Laser Optical Filter Optical Isolator EDF EDFAs amplify all s in 1550 window simultaneously Key performance parameters include Saturation output power, noise figure, gain flatness/passband Shivkumar KalyanaramanRensselaer Polytechnic Institute 56 Optical Amplifier Varieties Shivkumar KalyanaramanRensselaer Polytechnic Institute 57 Optical Amplifier Flat Gain Region Shivkumar KalyanaramanRensselaer Polytechnic Institute 58 Principles: Stimulated Emission Transitions between discrete energy levels of atoms accompanied by absorption or emission of photons E2 E1 can be stimulated by an optical signal 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" (N2 > N1) through a pumping process Shivkumar KalyanaramanRensselaer Polytechnic Institute 59 Spontaneous Emission E2 E1 transitions can be spontaneous (I.e. independent 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 KalyanaramanRensselaer Polytechnic Institute 60 Optical Amplification: mechanics Shivkumar KalyanaramanRensselaer Polytechnic Institute 61 Erbium-Doped Fiber Amplifier (EDFA) Length of fiber: core doped with (rare earth) erbium ions Er3+ 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 KalyanaramanRensselaer Polytechnic Institute 62 EDFA success factors 1. Availability of compact and reliable high-power semiconductor pump lasers 2. EDFA is an all-fiber device => polarization- independent & easy to couple light in/out 3. Simplicity of device 4. No crosstalk introduced while amplifying! Shivkumar KalyanaramanRensselaer Polytechnic Institute 63 EDFA: Operation When Er3+ ions introduced in silica, electrons disperse into an energy band around the lines E1, E2, E3 (Stark splitting) Within each band, the ion distribution is non-uniform (thermalization) Due to these effects, a large range (50 nm) can be simultaneously amplified & luckily it is in the 1530nm range Shivkumar KalyanaramanRensselaer Polytechnic Institute 64 EDFA: Operation (Contd) 980 nm or 1480nm pumps are used to create a population inversion between E2 and E1 980 nm pump => E1 E3 (absorption) & E3 E2 (spontaneous emission) 1480 nm pump => E1 E2 (absorption, less efficient) Lifetime in E3 is 1s, whereas in E2 it is 10ms Shivkumar KalyanaramanRensselaer Polytechnic Institute 65 EDFA 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" (E3 E4) Shivkumar KalyanaramanRensselaer Polytechnic Institute 66 Towards Flat EDFA Gain Long period fiber-grating used to add some "loss" in the peaks of the curve (see ) Shivkumar KalyanaramanRensselaer Polytechnic Institute 67 Reducing EDFA Gain Ripples Shivkumar KalyanaramanRensselaer Polytechnic Institute 68 EDFA: Summary Shivkumar KalyanaramanRensselaer Polytechnic Institute 69 Semiconductor Optical Amplifiers (SOA) SOAs have severe crosstalk problems, besides others But used in switches etc Shivkumar KalyanaramanRensselaer Polytechnic Institute 70 Recall: SRS and Raman Amplifiers Power transferred from lower- to higher- channels (about 100nm) Eg: 1460-1480nm 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 ultra-long-haul systems Shivkumar KalyanaramanRensselaer Polytechnic Institute 71 Raman Amplification Shivkumar KalyanaramanRensselaer Polytechnic Institute 72 Raman Amplification (contd) Shivkumar KalyanaramanRensselaer Polytechnic Institute 73 Counter-pumped Raman Amplification Shivkumar KalyanaramanRensselaer Polytechnic Institute 74 Distributed Raman Amplifiers Complement EDFAs in ultra-long-haul systems Challenge: need high-power pumps Pump power fluctuation => crosstalk noise! Counter-pumping: (dominant design) pump power fluctuations are averaged out over the propagation time of fiber; other crosstalk sources also reduced Shivkumar KalyanaramanRensselaer Polytechnic Institute 75 Practical Raman Pumps Use a conveniently available (eg: 1100 nm) pump and use Raman effect itself, in combination with a series of FP-resonators (created through -selective mirrors, I.e. matched Bragg gratings) Eg: 1100nm 1155nm 1218nm 1288nm 1366nm 1455 nm The final stage (1455nm) has low-reflectivity=> output pump at 1455nm which produces gain at 1550nm! 80% of the power comes to the output! Shivkumar KalyanaramanRensselaer Polytechnic Institute 76 Recall: Optical Amplifier Varieties Shivkumar KalyanaramanRensselaer Polytechnic Institute 77 Raman vs OFAs Shivkumar KalyanaramanRensselaer Polytechnic Institute 78 Long-Haul All-optical Amplification Shivkumar KalyanaramanRensselaer Polytechnic Institute 79 Optical Regenerator Shivkumar KalyanaramanRensselaer Polytechnic Institute 80 Regenerator Shivkumar KalyanaramanRensselaer Polytechnic Institute 81 Regen w/ Dispersion Compensation and Gain Equalization Shivkumar KalyanaramanRensselaer Polytechnic Institute 82 Light Sources: LEDs, Lasers, VCSELs, Tunable Lasers Shivkumar KalyanaramanRensselaer Polytechnic Institute 83 Lasers: Key Target Characteristics Laser: an optical amplifier enclosed in a reflective cavity that causes it to oscillate via positive feedback High output power (1-10 mW normal, 100-200mW 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 Side-mode 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 KalyanaramanRensselaer Polytechnic Institute 84 Recall: Energy Levels & Light Emission Shivkumar KalyanaramanRensselaer Polytechnic Institute 85 Spontaneous Emission, Meta-Stable States Shivkumar KalyanaramanRensselaer Polytechnic Institute 86 Recall:Stimulated Emission Shivkumar KalyanaramanRensselaer Polytechnic Institute 87 Recall: Fabry-Perot Etalon Shivkumar KalyanaramanRensselaer Polytechnic Institute 88 Laser vs LEDs LED: Forward-biased pn-junction (~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 Multi-longitudinal mode (MLM): larger spectrum (10s of nm) with discrete lines (unlike LEDs) Shivkumar KalyanaramanRensselaer Polytechnic Institute 89 Simple LEDs: p-n junction, bandgap Shivkumar KalyanaramanRensselaer Polytechnic Institute 90 Double 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 e-hole recombinations) Shivkumar KalyanaramanRensselaer Polytechnic Institute 91 Effect of Temperature on and I Shivkumar KalyanaramanRensselaer Polytechnic Institute 92 LED: Temperature-dependent Wavelength Drift Shivkumar KalyanaramanRensselaer Polytechnic Institute 93 LEDs: Useful in Free-space- Optical Communication · Output Optical Power IP 24.1 = · P -- Output Optical Power · -- wavelength · I -- Input Electrical Current · Output Optical Spectral Width Shivkumar KalyanaramanRensselaer Polytechnic Institute 94 Lasers 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 Radiation" Shivkumar KalyanaramanRensselaer Polytechnic Institute 95 Lasing Shivkumar KalyanaramanRensselaer Polytechnic Institute 96 Modes, Spectral Width and Linewidth Shivkumar KalyanaramanRensselaer Polytechnic Institute 97 Fabry-Perot Laser Sources Shivkumar KalyanaramanRensselaer Polytechnic Institute 98 Laser: Output Behavior vs Applied Power Shivkumar KalyanaramanRensselaer Polytechnic Institute 99 Directing the Light in a Fabry-Perot Laser Shivkumar KalyanaramanRensselaer Polytechnic Institute 100 Longitudinal 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 multiple-longitudinal mode (MLM) laser (Large spectral width (10 nm or ~1.3 Thz!) Desired: single-longitudinal mode (SLM): Add a filter to suppress other s by 30dB+ Shivkumar KalyanaramanRensselaer Polytechnic Institute 101 Multi-mode output of Laser Cavity Shivkumar KalyanaramanRensselaer Polytechnic Institute 102 Recall: History of SLM/MLM Usage Shivkumar KalyanaramanRensselaer Polytechnic Institute 103 Distributed Feedback (DFB) Lasers Idea: Provide a distributed set of reflections (feedback) by a series of closely-spaced 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 KalyanaramanRensselaer Polytechnic Institute 104 Bragg Laser Shivkumar KalyanaramanRensselaer Polytechnic Institute 105 In-Fibre Laser using FBGs Shivkumar KalyanaramanRensselaer Polytechnic Institute 106 External 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 anti-reflection coating outside of the primary cavity facet Used in test equipment: cannot modulate at high speed Shivkumar KalyanaramanRensselaer Polytechnic Institute 107 VCSELs: Vertical Cavity Surface-Emitting 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 high-index dielectrics (I.e. dielectric mirrors) are used Issues: Large ohmic resistance: heat dissipation problem Room-temperature 1.3um VCSELs recently shown Shivkumar KalyanaramanRensselaer Polytechnic Institute 108 VCSELs Shivkumar KalyanaramanRensselaer Polytechnic Institute 109 VCSEL Structure Shivkumar KalyanaramanRensselaer Polytechnic Institute 110 Wavelength-Selective VCSEL Array High array packing densities possible with VCSELs compared to edge-emitting 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 KalyanaramanRensselaer Polytechnic Institute 111 Combining VCSELs Shivkumar KalyanaramanRensselaer Polytechnic Institute 112 Mode-locked Lasers Match the phase of the longitudinal modes => regular pulsing in time- domain (aka "mode locking") Used in O-TDM Achieved by using longer cavities (eg: fiber laser) or modulating the gain of cavity Shivkumar KalyanaramanRensselaer Polytechnic Institute 113 Mode Locking by Amplitude Modulation of Cavity Gain Shivkumar KalyanaramanRensselaer Polytechnic Institute 114 Gaussian Beams Shivkumar KalyanaramanRensselaer Polytechnic Institute 115 Tunable Lasers Tunable lasers: key enabler of re-configurable 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: Electro-optical: changing RI by injecting current or applying an E-field (approx 10-15 nm) Temperature tuning: (1 nm range) may degrade lifetime of laser Mechanical tuning: using MEMS => compact Shivkumar KalyanaramanRensselaer Polytechnic Institute 116 Tunable Two- & Three-section DBR Lasers Shivkumar KalyanaramanRensselaer Polytechnic Institute 117 Tunable DBR Lasers (Contd) Shivkumar KalyanaramanRensselaer Polytechnic Institute 118 Sampled Grating DBR Goal: larger tuning range by combining tuning ranges at different peaks (aka "combs") Shivkumar KalyanaramanRensselaer Polytechnic Institute 119 Sampled Grating DBR (contd) Shivkumar KalyanaramanRensselaer Polytechnic Institute 120 Photodetectors Shivkumar KalyanaramanRensselaer Polytechnic Institute 121 Optical Receivers: Basic Ideas Shivkumar KalyanaramanRensselaer Polytechnic Institute 122 Photoconductive Detector * Application of external bias => absorbed photons lead to electron/hole pairs and a current (aka "photo-current") · Energy of incident photon at least the bandgap energy => largest = cutoff · Si, GaAs cannot be used; InGaAs, InGaAsP used Shivkumar KalyanaramanRensselaer Polytechnic Institute 123 Practical Photoconductors Responsivity 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) Shivkumar KalyanaramanRensselaer Polytechnic Institute 126 Responsivity vs Responsivity is dependent upon the choice of wavelength Shivkumar KalyanaramanRensselaer Polytechnic Institute 127 Photoconductor 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 conduction-band electrons rapidly OUT of the semiconductor Better: use a pn-junction and reverse-bias it: positive bias to n-type A.k.a. photo-diode Drift current: e-h pairs in the depletion region: rapidly create external current Diffusion: e-h pairs created OUTSIDE the depletion region move more slowly and may recombine, reducing efficiency Shivkumar KalyanaramanRensselaer Polytechnic Institute 128 Reversed-biased PN photodiode Photodiodes Reverse biased p-n or p-i-n junctions Photodiodes are faster than photoconductors Shivkumar KalyanaramanRensselaer Polytechnic Institute 130 P-I-N Photodiode To improve efficiency, use a lightly doped intrinsic semiconductor between the p- and n-type semiconductors Much of light absorption takes place in the I-region: increases efficiency and responsivity Better: make the p- and n-type transparent (I.e. above cuttoff ) to desired : double heterojunction Eg: cuttoff for InP is 0.92 um (transparent in 1.3-1.6 um range), and cuttoff for InGaAs is 1.65um Shivkumar KalyanaramanRensselaer Polytechnic Institute 131 Avalanche Photodiode Photo-generated 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 KalyanaramanRensselaer Polytechnic Institute 132 Avalanche Process Shivkumar KalyanaramanRensselaer Polytechnic Institute 133 Electric Field Strengths in APD Shivkumar KalyanaramanRensselaer Polytechnic Institute 134 Modulators Shivkumar KalyanaramanRensselaer Polytechnic Institute 135 Electronic vs Photonic Regime Cannot go negative in the photonic regime Shivkumar KalyanaramanRensselaer Polytechnic Institute 136 Optical Modulation Methods Shivkumar KalyanaramanRensselaer Polytechnic Institute 137 Issues in Optical Modulation On-Off 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/dispersion-limited regimes Shivkumar KalyanaramanRensselaer Polytechnic Institute 138 External Modulation External modulation can be: one-stage designs (if mode-locked lasers used) or two stage designs Shivkumar KalyanaramanRensselaer Polytechnic Institute 139 External 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: electro-absorption or EA modulators) EA: applying E-field shrinks bandgap => photons absorbed (Stark effect) Shivkumar KalyanaramanRensselaer Polytechnic Institute 140 Lithium 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 (15-20dB) and precise control of chirp, but is polarization dependent Shivkumar KalyanaramanRensselaer Polytechnic Institute 141 External Modulators (contd) Shivkumar KalyanaramanRensselaer Polytechnic Institute 142 Optical Modulators Shivkumar KalyanaramanRensselaer Polytechnic Institute 143 Cross-Gain & Cross-Phase Modulation Shivkumar KalyanaramanRensselaer Polytechnic Institute 144 Eye Diagrams Shivkumar KalyanaramanRensselaer Polytechnic Institute 145 Eye Diagrams (contd) Shivkumar KalyanaramanRensselaer Polytechnic Institute 146 BER Estimation w/ Eye Diagrams Shivkumar KalyanaramanRensselaer Polytechnic Institute 147 BER Estimation (contd) Shivkumar KalyanaramanRensselaer Polytechnic Institute 148 Switches Shivkumar KalyanaramanRensselaer Polytechnic Institute 149 Multiplexing: WDM 1 2 N 1 2 N... B b/s NB b/s1 2 N 1 2 N B b/s TDM: Time Division Multiplexing 10Gb/s upper limit WDM: Wavelength Division Multiplexing Use multiple carrier frequencies to transmit data simultaneously Shivkumar KalyanaramanRensselaer Polytechnic Institute 150 Multiplexers, 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 KalyanaramanRensselaer Polytechnic Institute 151 Switch Parameters Extinction Ratio: ratio of output power in ON state to the power in the OFF state 10-25 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 KalyanaramanRensselaer Polytechnic Institute 152 Switch 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 free-space- switches) Blocking Characteristics: Any unused input port can be connected to any unused output port? Wide-sense non-blocking: without requiring any existing connection to be re-routed => make sure future connections will not block Strict-sense non-blocking: regardless of previous connections Re-arrangeably non-blocking: connections may be re- routed to make them non-blocking Shivkumar KalyanaramanRensselaer Polytechnic Institute 153 Crossbar Switch Wide-sense non- blocking Shortest path length = 1 vs longest = 2n-1 Fabricated w/o any crossovers Shivkumar KalyanaramanRensselaer Polytechnic Institute 154 Clos Architecture * Strict-sense non-blocking; used in large port-count s/ws * N = mk; k (m x p) switches in first/last stages; p (k x k) switches in middle stage; * Non-blocking if p >= 2m - 1 * Lower number of crosspoints than crossbar (n2/3) Shivkumar KalyanaramanRensselaer Polytechnic Institute 155 Spanke Architecture · Strict-sense non-blocking · 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 KalyanaramanRensselaer Polytechnic Institute 156 Benes Architecture · Rearrangeably non-blocking · Efficient in number of 2x2 components · -ves: not WS-non-blocking and requires waveguide crossovers Shivkumar KalyanaramanRensselaer Polytechnic Institute 157 Spanke-Benes Architecture · Rearrangeably non-blocking · Efficient in number of 2x2 components · Eliminates waveguide crossovers: n-stage planar... Shivkumar KalyanaramanRensselaer Polytechnic Institute 158 MEMS Mirror Switching Component Shivkumar KalyanaramanRensselaer Polytechnic Institute 159 NxN Switching with MEMS Mirror Arrays Shivkumar KalyanaramanRensselaer Polytechnic Institute 160 Analog Beam Steering Mirror Shivkumar KalyanaramanRensselaer Polytechnic Institute 161 Planar Waveguide Switch Shivkumar KalyanaramanRensselaer Polytechnic Institute 162 Planar Waveguide Switch Shivkumar KalyanaramanRensselaer Polytechnic Institute 163 1x2 Liquid Crystal Switch