![Definitions
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Excess Lo [
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ion Loss ]
in out
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WDM.ppt
- 1. WDM Concept and Components 18ECC302J: Microwave & Optical Communications
- 2. Part 1: WDM Concept
- 3. Why WDM? • Capacity upgrade of existing fiber networks (without adding fibers) • Transparency - Each optical channel can carry any transmission format (different asynchronous bit rates, analog or digital) • Scalability - Buy and install equipment for additional demand as needed • Wavelength Routing and Switching: Wavelength is used as another dimension to time and space
- 4. Wavelength Division Multiplexing Each wavelength is like a separate channel (Fiber)
- 5. Evolution of the Technology
- 6. TDM Vs WDM Ex: SONET
- 7. Wavelength Division Multiplexing • Passive/active devices are needed to combine, distribute, isolate and amplify optical power at different wavelengths.
- 8. WDM, CWDM and DWDM • WDM technology uses multiple wavelengths to transmit information over a single fiber • Coarse WDM (CWDM) has wider channel spacing (20 nm) – low cost • Dense WDM (DWDM) has dense channel spacing (0.8 nm) which allows simultaneous transmission of 16+ wavelengths – high capacity
- 9. WDM and DWDM • First WDM networks used just two wavelengths, 1310 nm and 1550 nm • Today's DWDM systems utilize 16, 32,64,128 or more wavelengths in the 1550 nm window • Each of these wavelength provide an independent channel (Ex: each may transmit 10 Gb/s digital or SCMA analog) • The range of standardized channel grids includes 50, 100, 200 and 1000 GHz spacing • Wavelength spacing practically depends on: – laser linewidth – optical filter bandwidth
- 10. ITU-T Standard Transmission DWDM windows 2 c
- 11. Principles of DWDM • BW of a modulated laser: 10-50 MHz 0.001 nm • Typical Guard band: 0.4 – 1.6 nm • 80 nm or 14 THz @1300 nm band • 120 nm or 15 THz @ 1550 nm • Discrete wavelengths form individual channels that can be modulated, routed and switched individually • These operations require variety of passive and active devices 2 c Ex. 10.1
- 12. Nortel OPTERA 640 System 64 wavelengths each carrying 10 Gb/s
- 13. DWDM Limitations Theoretically large number of channels can be packed in a fiber For physical realization of DWDM networks we need precise wavelength selective devices Optical amplifiers are imperative to provide long transmission distances without repeaters
- 14. Part II: WDM Devices
- 15. Key Components for WDM Passive Optical Components Wavelength Selective Splitters Wavelength Selective Couplers Active Optical Components Tunable Optical Filter Tunable Source Optical amplifier Add-drop Multiplexer and De-multiplexer
- 16. Photo Detector Responsivity Photo detectors are sensitive over wide spectrum (600 nm). Hence, narrow optical filters needed to separate channels before the detection in DWDM systems
- 17. Passive Devices • These operate completely in the optical domain (no O/E conversion) and does not need electrical power • Split/combine light stream Ex: N X N couplers, power splitters, power taps and star couplers • Technologies: - Fiber based or – Optical waveguides based – Micro (Nano) optics based • Fabricated using optical fiber or waveguide (with special material like InP, LiNbO3)
- 18. Passive Components Coupler: versatile device used as a building block for several other optical devices Isolator: used in systems at the output of amplifiers and lasers to prevent reflections Filter: to multiplex and demultiplex wavelengths in a WDM system, and to provide equalization of the gain and filtering of noise in optical amplifier MUX & DEMUX: MUX combines signals at different wavelengths on its input ports onto a common output port, DEMUX performs the opposite function
- 19. Filter, Multiplexer and Router
- 20. Basic Star Coupler • Can be wavelength selective/nonselective • Up to N =M = 64, typically N, M < 10 May have N inputs and M outputs
- 21. Fused-Biconical Coupler or Directional Coupler • P3, P4 extremely low ( -70 dB below Po) • Coupling / Splitting Ratio = P2/(P1+P2) • If P1=P2 It is called 3-dB coupler
- 22. Coupler • Fabricated by twisting together, melting and pulling together two single mode fibers • They get fused together over length W; tapered section of length L; total draw length = L+W • Significant decrease in V-number in the coupling region; energy in the core leak out and gradually couples into the second fibre
- 23. Definitions 2 1 2 Splitting (Coupling) Rat = ) i ( o P P P 0 1 2 =10 Log Excess Lo [ ss ( ] ) P P P =1 In 0 sert Log[ ion Loss ] in out P P 3 0 = 10 Lo Crosstalk g(P P )
- 24. Coupler Characteristics ) ( sin 2 0 2 z P P ) ( cos2 0 1 z P P : Coupling Coefficient
- 25. Coupler Characteristics • power ratio between both output can be changed by adjusting the draw length of a simple fused fiber coupler • It can be made a WDM de-multiplexer: • Example, 1300 nm will appear output 2 (p2) and 1550 nm will appear at output 1 (P1) • However, suitable only for few wavelengths that are far apart, not good for DWDM
- 26. Wavelength Selective Devices These perform their operation on the incoming optical signal as a function of the wavelength Examples: • Wavelength add/drop multiplexers • Wavelength selective optical combiners/splitters • Wavelength selective switches and routers
- 27. Fused-Fiber Star Coupler Splitting Loss = -10 Log(1/N) dB = 10 Log (N) dB Excess Loss = 10 Log (Total Pin/Total Pout) Fused couplers have high excess loss
- 28. 8x8 Bi-directional Star Coupler by Cascading 3-stages of 3-dB Couplers c 2 Number of 3-dB Cou N N = log N 2 plers (12 = 4 X 3) Try Ex. 10.5 1, 2 1, 2 1, 2 5, 6 3, 4 7, 8
- 29. Passive Non-Reciprocal Devices • Types – Isolators – Faraday Rotators – Circulators
- 30. 30 Isolators and Circulators (Nonreciprocal devices) Isolators are for transmitter, circulators are for add and drop or others. The insertion loss should be small ~ 1dB A circulator is similar to an isolator except it has multiple ports.
- 31. Isolators • Transmit in one direction only • Avoid reflection of laser – or any reflection • One input, one output or multiple ports • Key parameters are insertion loss and excess loss • Example of circulators:
- 32. Isolators - Passive Non-Reciprocal Device Transmit in one direction only Avoid reflection of laser – or any reflection One input, one output or multiple ports Key parameters are insertion loss and excess loss Example: circulator
- 33. Operation of Isolators Only Ex exists State of polarization is fixed (SOP) Rotator rotates by 45 degree
- 34. Operation of Isolators – more realistic Polarization Independent Isolator Half-wavelength plates are used to rotate 45 degree The Spatial-walk-off polarizer splits the signal into two orthogonally polarized signals
- 36. Interferometers
- 37. Interferometer An interferometric device uses 2 interfering paths of different lengths to resolve wavelengths Typical configuration: two 3-dB directional couplers connected with 2 paths having different lengths Applications: — wideband filters (coarse WDM) that separate signals at1300 nm from those at 1550 nm — narrowband filters: filter bandwidth depends on the number of cascades (i.e. the number of 3-dB couplers connected)
- 38. Basic Mach-Zehnder Interferometer Phase shift of the propagating wave increases with L, Constructive or destructive interference depending on L
- 39. Mach-Zehnder Interferometer Phase shift at the output due to the propagation path length difference: If the power from both inputs (at different wavelengths) to be added at output port 2, then, 1 2 1 1 2 eff n L 2 eff n L
- 40. Four-Channel Wavelength Multiplexer • By appropriately selecting ΔL, wavelength multiplexing/de-multiplexing can be achieved
- 42. 42 Fabry-Perot Filters This filter is called Fabry-Perot interferometer or etalon. Principle of Operation The wavelengths for which the cavity length is an integral multiple of half the wavelength in the cavity are called resonant wavelengths.