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CSC/ECE 778: Optical Networks Rudra Dutta, Fall 2007 Fiber-Optical Communication and Switching Outline We want/need to understand effect on networking – What components are possible, limitations Quick overview of representative technology – Optical Connection and Power Budget – Fundamentals of Fiber Optic Transmission – Transmission Impairments and Solutions – Lasers and Photodetectors – Other Optical Components (Couplers, Filters, Multiplexers, Switches, OADMs, Amplifiers) Copyright Rudra Dutta, NCSU, Fall, 2007 2 Layering and Optical Services Generalized protocol layering can create complicated multi-layer networks In this context, “optical layer” is another layer close to physical layer, but possibly implementing network semantics of its own Network Data Link Network Data Link Physical Network Physical Physical Data Link Physical Copyright Rudra Dutta, NCSU, Fall, 2007 User Apps IP ATM SONET Optical 3 Why Fiber? Huge bandwidth: 30-50 THz Low losses (intrinsic): 0.2 db/Km Low bit error rates (BER): 10-11 Low power requirements: 100 photons/bit Immunity to electromagnetic interference (EMI) Low cross-talk Repeater-less amplification (EDFAs) Low cost, maintenance Copyright Rudra Dutta, NCSU, Fall, 2007 4 Optical Endpoint Copyright Rudra Dutta, NCSU, Fall, 2007 5 Optical Power Budget Finite power available at source (laser) Minimum detectable receiver power Must account for all losses between source and receiver Optical networks are power-budget limited, not bandwidth limited Copyright Rudra Dutta, NCSU, Fall, 2007 6 Optical Power Budget (cont'd) Copyright Rudra Dutta, NCSU, Fall, 2007 7 Wavelengths of Importance Copyright Rudra Dutta, NCSU, Fall, 2007 8 Optical Fiber Optical waveguide Cylindrical core surrounded by cladding (+ protective covering) – – Single-mode vs. multimode fiber – – made of same transparent material (glass, plastic) difference is value of refractive index n = c / v single-mode: core diameter 8-12µm, link length > 2Km multimode: core diameter 50µm, link length < 2Km Step-index vs. graded-index fiber – – step-index: refractive index constant across core diameter graded-index: refractive index varies along core diameter Copyright Rudra Dutta, NCSU, Fall, 2007 9 Refractive Index Profiles Copyright Rudra Dutta, NCSU, Fall, 2007 10 Geometric Optics: Snell's Law n1 sin i = n2 sin t Copyright Rudra Dutta, NCSU, Fall, 2007 11 Geometric Optics: Total Reflection Critical angle: c = sin-1 (n2 ÷ n1) Copyright Rudra Dutta, NCSU, Fall, 2007 12 Maximum Cone of Acceptance Copyright Rudra Dutta, NCSU, Fall, 2007 13 Transmitter-to-Fiber Coupling Copyright Rudra Dutta, NCSU, Fall, 2007 14 Modes: The Wave Picture Copyright Rudra Dutta, NCSU, Fall, 2007 15 Allowed Ray Angles Only allowed ray angles result in guided modes AB = d sin m = m /2 leads to half wavelength in the core – m : integer, : optical wavelength in the core Mode: one possible path that a guided ray can take Copyright Rudra Dutta, NCSU, Fall, 2007 16 Transmission Impairments Factors affecting transmission distance and bandwidth: – – – attenuation dispersion non-linear effects Must minimize their effects for high performance – – improvement and redesign of fiber itself compensating for these factors Attenuation problem solved dispersion effects significant Dispersion effects reduced non-linear effects dominant Copyright Rudra Dutta, NCSU, Fall, 2007 17 Attenuation Decrease in optical power along the length of the fiber Varies with wavelength Attenuation coefficient: adB = - 10/L log10 (PR÷PT) (dB/Km) – – – L : length of fiber PT : power launched into the fiber PR : power received at end of fiber Copyright Rudra Dutta, NCSU, Fall, 2007 18 Power Losses Material absorption: due to – – Rayleigh scattering: medium is not absolutely uniform – – resonances of silica molecules impurities -- most serious is peak at 1390 nm due to OH ions refractive index fluctuates light is scattered scattering proportional to -4 dominant at < 800 nm Waveguide imperfections: relatively small component – – nonideal fiber geometries due to bending, manufacturing imperfections Copyright Rudra Dutta, NCSU, Fall, 2007 19 Low Loss Region of An Optical Fiber Copyright Rudra Dutta, NCSU, Fall, 2007 20 Erbium-Doped Fiber Amplifiers Copyright Rudra Dutta, NCSU, Fall, 2007 21 EDFA Principle of Operation Ei : energy level Ni : population of erbium ions at energy level Ei – – normally (no pump/signal): N1 > N2 > N3 pump/signal present: population inversion N2 > N1 Copyright Rudra Dutta, NCSU, Fall, 2007 22 EDFA Properties Emission: – – stimulated amplification spontaneous noise amplified spontaneous emission limit on number of EDFAs along the fiber Energy levels are narrow bands each transition associated w/ a band of wavelengths amplify wide band around 1550nm Replace expensive and complicated electronic units Signal remains in optical form transparency “Distributed” amplifiers Copyright Rudra Dutta, NCSU, Fall, 2007 23 Semiconductor Optical Amplifiers (SOAs) Similar to semiconductor laser Consist of active medium (p-n junction) Energy levels of electrons confined to 2 bands EDFA E1, E2 Mobile carriers (holes, electrons) play the role of erbium ions Has several disadvantages compared to EDFAs Useful when combined with other components into optoelectronic integrated circuits (OEICs) – – preamplifier in optical receiver power amplifier in optical transmitter Copyright Rudra Dutta, NCSU, Fall, 2007 24 Dispersion A narrow pulse spreads out as it propagates along the fiber Intersymbol interference: – – pulse overlaps neighboring pulses sharply increases the BER Dispersion imposes a limit on the bit rate that can be supported Intermodal vs. chromatic dispersion Copyright Rudra Dutta, NCSU, Fall, 2007 25 Intermodal Dispersion Most serious form of dispersion Occurs in multimode fibers Different modes of a wavelength travel at different speeds Multimode fibers limited to low bitrate-distance products Solutions: – – use single-mode fibers for large bitrate-distance products (8 µm < 2a < 10 µm only one mode is guided) use graded-index fibers Copyright Rudra Dutta, NCSU, Fall, 2007 26 Graded Index Fibers Copyright Rudra Dutta, NCSU, Fall, 2007 27 Propagation in Graded Index Fibers Rays are bent as they approach the cladding Rays further from core travel faster (due to lower n) Intermodal dispersion reduced by several orders of magnitude Copyright Rudra Dutta, NCSU, Fall, 2007 28 Chromatic Dispersion Two sources of chromatic dispersion: – – material dispersion, DM waveguide dispersion, DW Chromatic dispersion: D = DM + DW Copyright Rudra Dutta, NCSU, Fall, 2007 29 Material Dispersion The physical effect that allows raindrops to form rainbow Refractive index of a material changes with wavelength different wavelengths travel at different speeds along the fiber Different delays cause spreading of output pulse, depending on: – wavelength span of source – length of fiber Copyright Rudra Dutta, NCSU, Fall, 2007 30 Waveguide Dispersion DW is a function of fiber geometry Dispersion-shifted fibers: – – DW causes zero-dispersion point to shift to 1550 nm range min dispersion range coincides with min loss range Dispersion-flattened fibers: dispersion profile close to zero for a wide spectral range Copyright Rudra Dutta, NCSU, Fall, 2007 31 Dispersion Profile of Single-Mode Fiber Copyright Rudra Dutta, NCSU, Fall, 2007 32 Non-Linear Effects Stimulating Raman Scattering (SRS): – – – – Stimulating Brillouin Scattering (SBS): – light interacts with fiber medium inelastic collisions not important in single-channel systems (thresh. about 500mW) involves transfer of power: hi freq. wave lo freq. wave introduces cross-talk in multiwavelength systems no cross-talk, low threshold power (few mW for 20-Km fiber) Four-Wave Mixing – – three signals present at neighboring freq: f1, f2, f3 new signal produced, e.g., f4 = f1 + f2 - f3 Copyright Rudra Dutta, NCSU, Fall, 2007 33 Solitons Distortion, non-linearities: distort, broaden a propagating pulse Right combination of distortion, non-linearity: – – – – compensate each other produce a narrow, stable pulse (soliton) solitons travel over long distances without any distortion solitons in opposite directions pass thru transparently Ideal situation for long-distance communication EDFAs needed to maintain solitons over long distances Copyright Rudra Dutta, NCSU, Fall, 2007 34 Lasers Light amplification by stimulated emission of radiation Schawlow and Townes, 1958 First solid-state laser by Maiman, 1960 Today, lasers exist in myriad forms Copyright Rudra Dutta, NCSU, Fall, 2007 35 Semiconductor Energy State Diagrams Copyright Rudra Dutta, NCSU, Fall, 2007 36 Fabry-Perot Cavity Part of light leaves cavity through right facet, part is reflected Resonant wavelengths: L = m /2 Copyright Rudra Dutta, NCSU, Fall, 2007 37 Single-Wavelength Operation FP laser cavity supports many modes/wavelengths of operation Monochromatic light needed for high bitratedistance products Geometry is modified to achieve singlewavelength operation Distributed Bragg Reflector (DBR) lasers Distributed Feedback (DFB) lasers Expensive, widely used in long-distance communication Copyright Rudra Dutta, NCSU, Fall, 2007 38 Tunability Laser tunability important in WDM network applications: – – slow tunability (ms range): set up lightpaths in wavelength routing networks fast tunability (µs or ns range): multiple access (TWDMA) applications Copyright Rudra Dutta, NCSU, Fall, 2007 39 Tunability (cont'd) Mechanically tuned: change FP cavity length – Injection current tuned: change refr. index in DFB/DBR lasers – (tuning range: 10-20 nm, tuning time: 100-500 ms) (tuning range: 4 nm, tuning time: 10s of ns) Multiwavelength laser arrays – – – built in single chip one or more lasers can be activated simultaneously light from each laser fed to star coupler Copyright Rudra Dutta, NCSU, Fall, 2007 40 Optical Receivers Copyright Rudra Dutta, NCSU, Fall, 2007 41 Photodetectors Copyright Rudra Dutta, NCSU, Fall, 2007 42 Filters Various technologies: – – – – – Fabry-Perot filters Multilayer interference (MI) filters Mach-Zehnder interferometers Arrayed waveguide grating Acousto-optic tunable filter Tunability important Can be used as MUX/DEMUX, wavelength routers Copyright Rudra Dutta, NCSU, Fall, 2007 43 MI Filters Bandpass filter Passes thru particular wavelength, reflects all other Cascade multiple filters to create a MUX/DEMUX Copyright Rudra Dutta, NCSU, Fall, 2007 44 MI Filters as MUX/DEMUX Copyright Rudra Dutta, NCSU, Fall, 2007 45 MUX/DEMUX: Logical View Copyright Rudra Dutta, NCSU, Fall, 2007 46 Directional Couplers Coupling possible when waveguides placed close together Coupling ratio controlled by voltage Copyright Rudra Dutta, NCSU, Fall, 2007 47 Couplers: Logical View P1’ = a11 P1 + a12 P2, P2’ = a21 P1 + a22 P2 For ideal symmetric couplers: a11 = a22 = a, a12 = a21 = 1-a Copyright Rudra Dutta, NCSU, Fall, 2007 48 Couplers Star Coupler: – – Power Splitter: – P2 = 0, a = 1/2 Switches: – – a = 1/2, 2x2 star coupler (3-dB coupler) Cascade 2x2 couplers to build NxN star coupler a = 0,1; 2x2 switch cascade 2x2 switches to build NxN switch Real devices are lossy: – a11 + a12 < 1, a21 + a22 < 1 Copyright Rudra Dutta, NCSU, Fall, 2007 49 Internal Structure of Star Coupler Copyright Rudra Dutta, NCSU, Fall, 2007 50 Gratings Copyright Rudra Dutta, NCSU, Fall, 2007 51 Gratings: Principle of Operation Multiple narrow slits spaced equally apart on the grating plane Light incident on one side of grating transmitted through slits Diffraction: light through each slit spreads out in all directions Different s interfere constructively at different points of imaging plane separate WDM signal into constituent wavelengths Copyright Rudra Dutta, NCSU, Fall, 2007 52 Bragg Gratings Bragg grating: any periodic pertrubation in propagating medium Perturbation is usually periodic variation of refractive index Bragg gratings used in many photonic devices: – – – DBR lasers: Bragg gratings written in waveguides Fiber Bragg gratings (FBG): written in fiber Acousto-optic tunable filters: Bragg grating formed by propagation of an acoustic wave in the medium Copyright Rudra Dutta, NCSU, Fall, 2007 53 FBG as Add-Drop Multiplexers Copyright Rudra Dutta, NCSU, Fall, 2007 54 OADM: Logical View Copyright Rudra Dutta, NCSU, Fall, 2007 55 Optical Switches Mechanical switches – – Bubble-Based switches – couplers, ratio modified by changing refr. index (ns range) Thermo-Optic switches – bubbles in optical fluid reflect beam (10s of ms range) Electro-Optic switches – directional couplers, ratio modified by bending (ms range) MEMS mirrors moved in and out of path (100s of ns range) refractive index function of temperature (ms range) Semiconductor Optical Amplifier (SOA) switches – SOA, change in voltage to use as on-off switch (ns range) Copyright Rudra Dutta, NCSU, Fall, 2007 56 MEMS Optical Switching MEMS = micro-electro-mechanical system Movable mirrors to reflect light 2D MEMS: a 2-state pop-up MEMS mirror – – state ``0'': popped up position light reflected state ``1'': flat (folded) position light passes through Copyright Rudra Dutta, NCSU, Fall, 2007 57 2D MEMS Switches Copyright Rudra Dutta, NCSU, Fall, 2007 58 Analog Beam-Steering Mirror Mirror can be freely rotated on two axes to reflect a light beam Copyright Rudra Dutta, NCSU, Fall, 2007 59 3D MEMS Switch Copyright Rudra Dutta, NCSU, Fall, 2007 60 Static Optical Switches Copyright Rudra Dutta, NCSU, Fall, 2007 61 Reconfigurable Optical Switches Copyright Rudra Dutta, NCSU, Fall, 2007 62 Wavelength Converters Copyright Rudra Dutta, NCSU, Fall, 2007 63 Spectrum Partitioning c = f, f - c/2 100 Ghz is about .8 nm at 1,550 nm range 10-Ghz spacing: – – – – 100 Ghz spacing OK for optical switches – very dense by current standards can accommodate 1 Gbps digital bit rates can accomodate 1 Ghz analog bandwidths OK for receivers, but too close for wavelength routing WDM limit today Waveband routing alleviates throughput loss – – But better switching technology nullifies advantage However, continue to be useful because needs “coarser” filters Copyright Rudra Dutta, NCSU, Fall, 2007 64 Spectrum Partitioning (cont'd) Copyright Rudra Dutta, NCSU, Fall, 2007 65 Waveband vs. Wavelength Copyright Rudra Dutta, NCSU, Fall, 2007 66