Optical Transmission Systems GOAL of the presentation • Overview of Optical Component Technologies • Basic understanding of certain key issues in Component Technologies Outline • Set the Context • Describe existing technologies • Explain fundamental limits • Describe practical considerations/tradeoffs Light’s Dual Nature Particles Waves Conduction band Rays n0 Bandgap n1 n0 Valence band Absorption Emission Interference Refraction Reflection Why Optical ? • • • • • • • • High Bandwidth ( 500 Tbps) Low Attenuation (.25 dB/km) Low BER (10 -13) Light and occupies lesser space Flexible and Reliable Less crosstalk due to neutral photons Hard to eavesdrop Environmentally sound. Bandwidth Comparison Bandwidth Evolutionary Landmarks All-Optical Network (Terabits  Petabits) TDM (Gb/s) 80l @ 40Gb/s 40 176l @OC-192 35 25 20 15 32l @OC-192 EDFA + Raman Amplifier 16l @OC-192 8l @OC-48 4l @OC-192 EDFA 10 Gb/s 10 2l @1.2Gb/s (1310 nm, 1550 nm) 5 4l @OC-48 2l @OC-48 2.4 Gb/s 565 Mb/s 1.2 Gb/s 0 TDM DWDM 2006 2004 2002 2000 1998 1996 1994 1992 1990 1988 810 Mb/s 1.8 Gb/s 1986 1984 405 Mb/s 1982 Bandwidth 30 Enablers EDFA + Raman Amplifier Dense WDM/Filter High Speed Opto-electronics Advanced Fiber 40 Gb/s Shannon’s Limit Fiber Optic Transmission Bands 193 Frequency Wavelength (vacuum) Longhaul Telecom Regional Telecom 229 353 461 THz Near Infrared 1.8 1.6 1.4 1.2 UV 1.0 0.8 0.6 0.4 HeNe Lasers 633 nm 1550 nm CD Players 780 nm 1310 nm Local Area Networks 850 nm 0.2 µm Optical Components 1. The Fiber A. Total Internal Reflection B. Multi Mode Fiber C. Single Mode Fiber D. Transmission Impairments Loss a. Rayleigh scattering b. Reflection c. Absorption Dispersion a. Chromatic Dispersion 1. Material Dispersion 2. Waveguide Dispersion b. Polarization Mode Dispersion Non-Linearities a. Stimulated Raman Scattering b. Stimulated Brillouin Scattering c. Four Wave Mixing d. Self Phase Modulation e. Cross Phase Modulation Optical Components (Contd) 2. Couplers 3. Isolators 4. Circulators 5. Filters A. Diffraction Grating B. Reflection Grating C. Fabry Perot Filter D. Thin Film Dielectric Filter. E. Bragg Grating a. Short Period fiber bragg grating b. Long Period fiber bragg grating F. Mac Zhender Interferometer G. Arrayed Waveguide Grating H. Acoustoptic Tunable Filter Optical Components (Contd) 6. Optical Amplifers A. Erbium Doped Fiber Amplifiers (EDFA) B. Raman Amplifiers (RA) C. Semiconductor Optical Amplifier (SOA) 7. Lasers A. MLM Fabry Perot B. Single Mode Lasers a. Distributed Bragg Reflector Laser (DBR) b. Distributed Feedback Laser (DFB) c. External Cavity Lasers 8. Modulators 9. Detectors A. PIN B. APD Optical Components (Contd) 10. Optical Switches Photonic Switches A. Mechanical switches B. Electrooptic switches C. Thermooptic switches D. Semiconductor optical Amplifiers Lambda Switches A. Optoelectronic Method B. Semiconductor Optical Amplifier a. Cross Gain Modulation b. Cross Phase Modulation C. Four Wave Mixing An Optical Network 3000Km - 5000Km Tunable DCM DCF Tunable DCM Dynamic PMD Compensation Tx Tx Tx W D M OA Dispersion Managed Fiber Medium Dispersion Fiber New Modulation Formats Forward Error Correction OA C+L Band OA S Band OA Raman Amplifier Dynamic OADM .... . D W D M Broadband PMD Compensation PMDC Rx PMDC Rx PMDC Rx Impairment Tolerant Receiver 1. The Fiber Light at this angle is refracted Glass n = 1.5 Light at this angle is reflected back into glass Air n = 1.0 1 = 2 For Total Internal Reflection The Acceptance Cone Multimode fiber Pulse broadening due to multi-path transmission. Bitrate x Distance product is severely limited! Gradient-Index (GI) Fiber Doping profile designed to minimize “race” conditions (“outer” modes travel faster due to lower refractive index!) Most common designs: 62.5/125 or 50/125 m, NA ~ 0.2 Bitrate x Distance product: ~ 1 Gb/s • km r Single-Mode Fiber (SMF) Step-Index type with very small core n 1.465 1.460 r Transmission Impairments Rayleigh Scattering Reflection Absorption Transmission Impairments Attenuation (dB/km) 2.5 “Optical Windows” 2.0 3 4 5 2 OH Absorption 1 1.5 AllWaveTM eliminates the 1385nm water peak 1.0 0.5 900 1100 1300 1500 1700 Wavelength (nm) Main cause of attenuation: Rayleigh scattering in the fiber core Loss - st 1 Order Chromatic Dispersion Cause of Chromatic Dispersion Material Dispersion • β = nω/c • n2 (ω ) = 1 + χ(ω) • P(r, ω) = έ χ(r,ω)E (r,ω) Waveguide Dispersion Power distribution between core and cladding Dispersion Limits Dispersion Compensation • Dispersion Compensation Fiber • Fiber Bragg Gratings Polarization Mode Dispersion PMD – The Networking Killer • • • • 20 % of installed fiber useless => 10 G 75 % of installed fiber useless => 40 G Leads to increased PDL Limits transmission to 25 km!! Two PMD Solutions 1. Optical mitigation 2. Electrical mitigation PMD Compensation Non-Linearities - SBS and SRS Cause of Non-Linearity P = έ [ χ1. E + χ 2. E.E + χ3.E.E.E +…] For high values of E, the third order term becomes significant. Scattering Compensation Ways to reduce SBS penalty • Power below threshold • Increase line width of the source • Use Phase modulation schemes Ways to reduce SRS penalty • Keep the channels densely packed • Keep power below threshold. Four Wave Mixing FWM Compensation • • • • Unequal channel spacing Increased channel spacing Reduced power below threshold Use spatial walk off – introduce time delay Non-Linearties (Contd) • Self Phase Modulation Refractive index dependance on the power of a signal • Cross Phase Modulation Refractive index dependance on the power of another signal Effects of Nonlinearites Multiple Channels vs Single Channel Tools to combat Impairments • • • • Power / Channel Dispersion Compensation Channel Spacing Wavelength / Frequency Choice 2. Couplers Couplers (Contd.) Applications: 1. 2. 3. 4. 5. 6. 3 dB couplers – for Broadcast and Select networks Taps for monitoring purposes Optical Switches Mac Zhender Interferometers Lambda selective multiplexers and demultiplexers Combine pump and signals in EDFAs 3. Isolators 4. Circulators 5. Filters Fabry Perot Filter Diffraction Grating Bragg Grating Mac Zhender Grating Thin Film Dielectric Grating Arrayed Waveguide Grating Acuostooptic Tunable Filter 6. Optical Amplifiers EDFA EDFA (Contd.) EDFA (Contd.) SNR All Optical System Capacity Raman Amplification Raman Amplifiers Used as Pre-amplifier in the reverse direction • Same pump laser as EDFA • Higher gains • Larger Bandwidth • Distributed and hence lower noise figure’ • Lower Plaunched – reduced nonlinearities Benefits of Raman Semiconductor Optical Amplifier 7. Lasers MLM Fabry Perot Laser SLM Lasers External Cavity Lasers External Cavity Lasers (Contd. ) 8. External Modulators 9. Photodetectors A Photodiode 10. Optical Switches • Photonic Switches (MEMS) Electrooptic Switch Lamda Switch An Optical Network - Revisited 3000Km - 5000Km Tunable DCM DCF Tunable DCM Dynamic PMD Compensation Tx Tx Tx W D M OA Dispersion Managed Fiber Medium Dispersion Fiber New Modulation Formats Forward Error Correction OA C+L Band OA S Band OA Raman Amplifier Dynamic OADM .... . D W D M Broadband PMD Compensation PMDC Rx PMDC Rx PMDC Rx Impairment Tolerant Receiver Conclusion