Opticl CoM 6.pptx
- 3. Wavelength Division Multiplexing (WDM) Optical signals of different wavelength (1300-1600 nm) can propagate without interfering with each other. The scheme of combining a number of wavelengths over a single fiber is called wavelength division multiplexing (WDM).
- 4. • Each input is generated by a separate optical source with a unique wavelength. • An optical multiplexer couples light from individual sources to the transmitting fiber • At the receiving station, an optical demultiplexer is required to separate the different carriers before photodetection of individual signals • To prevent spurious signals to enter into receiving channel, the demultiplexer must have narrow spectral operation with sharp wavelength cut-offs. • The acceptable limit of crosstalk is – 30 dB
- 5. Features of WDM • Important advantages or features of WDM are as mentioned below – 1. Capacity upgrade : Since each wavelength supports independent data rate in Gbps. 2. Transparency : Each optical channel can carry any transmission format (different asynchronous bit rates, analog or digital) 3. Wavelength routing : 4. Wavelength switching :
- 6. Wavelength Division Multiplexing Each wavelength is like a separate channel (fiber)
- 7. TDM Vs WDM Ex: SONET
- 8. Wavelength Division Multiplexing • Passive/active devices are needed to combine, distribute, isolate and amplify optical power at different wavelengths
- 9. WDM Standards 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
- 10. WDM Standards 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
- 11. WDM Standards ITU-T Standard Transmission DWDM windows
- 12. WDM Standards 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 Ex. 10.1
- 13. Nortel OPTERA 640 System 64 wavelengths each carrying 10 Gb/s
- 15. 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
- 16. WDM USES MANY WAVELENGTHS ON SINGLE OPTIC FIBER
- 17. WDM v/s DWDM • 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 • 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
- 18. Dense Wavelength Division Multiplexing (DWDM) 1) DWDM is a data transmission technology having very large capacity and efficiency. 2) Multiple data channels of optical signals are assigned different wavelengths, and are multiplexed onto one fiber. 3) DWDM system consist of transmitters, multiplexers, optical amplifer and demultiplexer. 4) DWDM used single mode fiber to carry multiple light waves of different frequencies. 5) In DWDM system to overcome the effects of dispersion and attenuation channel spacing of 100 GHz is used. 6) The channel spacing is about .4nm or .8nm or 1.6 nm, corresponding to 50GHz, 100 GHz or 200GHz
- 20. 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)
- 21. Filter, Multiplexer and Router
- 22. Basic Star Coupler • Can be wavelength selective/nonselective • Up to N =M = 64, typically N, M < 10 May have N inputs and M outputs
- 23. 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
- 24. Fused Biconical Tapered 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
- 27. 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
- 28. 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
- 29. 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
- 30. 8x8 bi-directional star coupler by cascading 3 stages of 3-dB Couplers (12 = 4 X 3) Try Ex. 10.5 λ1, λ2 λ1, λ2 λ1, λ2 λ5, λ6 λ3, λ4 λ7, λ8
- 32. Fiber Bragg Grating • This is invented at Communication Research Center, Ottawa, Canada • The FBG has changed the way optical filtering is done • The FBG has so many applications • The FBG changes a single mode fiber (all pass filter) into a wavelength selective filter
- 33. Fiber Brag Grating (FBG) • Basic FBG is an in-fiber passive optical band reject filter • FBG is created by imprinting a periodic perturbation in the fiber core • The spacing between two adjacent slits is called the pitch • Grating play an important role in: – Wavelength filtering – Dispersion compensation – Optical sensing – EDFA Gain flattening – Single mode lasers and many more areas
- 35. FBG Theory Exposure to the high intensity UV radiation changes the fiber core n(z) permanently as a periodic function of z z: Distance measured along fiber core axis ∧: Pitch of the grating ncore: Core refractive index δn: Peak refractive index
- 37. Simple De-multiplexing Function Peak Reflectivity Rmax = tanh2(kL)
- 39. Dispersion Compensation Longer wavelengths take more time Shorter wavelengths take more time Reverse the operation of dispersive fiber
- 40. ADD/DROP MUX FBG Reflects in both directions; it is bidirectional
- 42. FBG for DFB Laser • Only one wavelength gets positive feedback 🡪 single mode Distributed Feed Back laser
- 44. FBG Properties Advantages • Easy to manufacture, low cost, ease of coupling • Minimal insertion losses – approx. 0.1 db or less • Passive devices Disadvantages • Sensitive to temperature and strain. • Any change in temperature or strain in a FBG causes the grating period and/or the effective refractive index to change, which causes the Bragg wavelength to change.
- 45. Unique Application of FBG
- 46. Resonance Cavity with FBG
- 49. • What is the wavelength separation when RF separation 50 MHz?
- 50. Interferometers
- 51. 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)
- 52. Basic Mach-Zehnder Interferometer Phase shift of the propagating wave increases with ΔL, Constructive or destructive interference depending on ΔL
- 53. 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, Try Ex. 10-6
- 54. Four-Channel Wavelength Multiplexer • By appropriately selecting ΔL, wavelength multiplexing/de-multiplexing can be achieved
- 56. Arrayed Wave Guide Filters Each waveguide has slightly different length
- 57. Phase Array Based WDM Devices • The arrayed waveguide is a generalization of 2x2 MZI multiplexer • The lengths of adjacent waveguides differ by a constant ΔL • Different wavelengths get multiplexed (multi-inputs one output) or de- multiplexed (one input multi output) • For wavelength routing applications multi- input multi-output routers are available
- 58. Diffraction Gratings source impinges on a diffraction grating ,each wavelength is diffracted at a different angle Using a lens, these wavelengths can be focused onto individual fibers. Less channel isolation between closely spaced wavelengths.
- 59. Generating Multiple Wavelength for WDM Networks • Discrete DFB lasers –Straight forward stable sources, but expensive • Wavelength tunable DFB lasers • Multi-wavelength laser array –Integrated on the same substrate –Multiple quantum wells for better optical and carrier confinement • Spectral slicing – LED source and comb filters
- 60. Discrete Single-Wavelength Lasers • Number of lasers into simple power coupler; each emit one fixed wavelength • Expensive (multiple lasers) • Sources must be carefully controlled to avoid wavelength drift
- 61. Frequency Tuneable Laser • Only one (DFB or DBR) laser that has grating filter in the lasing cavity • Wavelength is tuned by either changing the temperature of the grating (0.1 nm/OC) • Or by altering the injection current into the passive section (0.006 nm/mA) • The tuning range decreases with the optical output power
- 62. Tunable Laser Characteristics Typically, tuning range 10-15 nm, Channel spacing = 10 X Channel width
- 63. Tunable Filters (Important) • Tunable filters are made by at least one branch of an interferometric filter has its – Propagation length or – Refractive index altered by a control mechanism • When these parameters change, phase of the propagating light wave changes (as a function of wavelength) • Hence, intensity of the added signal changes (as a function of wavelength) • As a result, wavelength selectivity is achieved
- 64. Tunable Optical Filters (EXPLAIN ABOUT using MZI tunable filter and FBG tunable filter)
- 65. Tuneable Filter Considerations • Tuning Range (Δν): 25 THz (or 200nm) for the whole 1330 nm to 1500 nm. With EDFA normally Δλ = 35 nm centered at 1550 nm • Channel Spacing (δν): the min. separation between channels selected to minimize crosstalk (30 dB or better) • Maximum Number of Channels (N = Δν/ δν): • Tuning speed: Depends on how fast switching needs to be done (usually milliseconds)
- 66. OTDR
- 67. OTDR