Components for WDM Networks

1. Components for WDM Networks Xavier Fernando ADROIT Group Ryerson University

2. 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)

3. 10.2 Passive Components • Operate completely in optical domain • N x N couplers, power splitters, power taps, star couplers etc.

4. Fig. 10-3: Basic Star Coupler May have N inputs and M outputs • Can be wavelength selective/nonselective • Up to N =M = 64, typically N, M < 10

5. Fig. 10-4: Fused-fiber coupler / 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

6. Definitions Try Ex. 10.2

7. Coupler characteristics : Coupling Coefficient

8. Coupler Characteristics • By adjusting the draw length of a simple fused fiber coupler, • power ratio can be changed • Can be made wavelength selective

9. 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

10. Filter, Multiplexer and Router

11. A Static Wavelength Router

12. Fig. 10-11: Fused-fiber star coupler Splitting Loss = -10 Log(1/N) dB Excess Loss = 10 Log (Total Pin/Total Pout) Fused couplers have high excess loss

13. Fig. 10-12: 8x8 bi-directional star coupler by cascading 3 stages of 3-dB Couplers 1, 2 1, 2 5, 6 1, 2 3, 4 7, 8 (12 = 4 X 3) Try Ex. 10.5

14. 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

15. 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 and many more areas

16. Fig. 10-16: Bragg grating formation

17. FBG Theory Exposure to the high intensity UV radiation, the refractive index of the fiber core (n) permanently changes to a periodic function of z z: Distance measured along fiber core axis : Pitch of the grating ncore: Core refractive index

18. Reflection at FBG

19. Fig. 10-17: Simple de-multiplexing function Peak ReflectivityRmax = tanh2(kL)

20. Wavelength Selective DEMUX

21. Dispersion Compensation using FBG Longer wavelengths take more time Reverse the operation of dispersive fiber Shorter wavelengths take more time

22. ADD/DROP MUX FBG Reflects in both directions; it is bidirectional

23. Fig. 10-27: Extended add/drop Mux

24. Advanced Grating Profiles

25. 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.

26. Interferometers

27. 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) 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)

28. Fig. 10-13: Basic Mach-Zehnder interferometer Phase shift of the propagating wave increases with L, Constructive or destructive interference depending on L

29. 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

30. Mach-Zehnder interferometer

31. Fig. 10-14: Four-channel wavelength multiplexer

32. Mach-Zehnder interferometer

33. Mach-Zehnder interferometer

34. MZI- Demux Example

35. Fiber Grating Filters • Grating is a periodic structure or perturbation in a material • Transmitting or Reflecting gratings • The spacing between two adjacent slits is called the pitch • Grating play an important role in: • Wavelength filtering • Dispersion compensation • EDFA Gain flattening and many more areas

36. Reflection grating Different wavelength can be separated/added

37. Arrayed wave guide grating

38. Phase Array Based WDM Devices • The arrayed waveguide is a generalization of 2x2 MZI multiplexer • The lengths of adjacent waveguides differ by a constant L • 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

39. 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.

40. Arrayed Waveguide Grating -- good performance -- more cost effective -- quicker design cycle time --- higher channel count

41. Multi wavelength sources • Series of discrete DFB lasers • Straight forward, but expensive stable sources • Wavelength tunable lasers • By changing the temperature (0.1 nm/OC) • By altering the injection current (0.006 nm/mA) • 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

42. Tunable Filters • At least one branch of the coupler has its length or ref. index altered by a control mechanism • Parameters: tuning range (depends on amplifier bandwidth), channel spacing (to minimize crosstalk), maximum number of channels (N) and tuning speed

43. Fig. 10-23: Tunable optical filter

44. Fig. 10-21: Tunable laser characteristics Typically, tuning range 10-15 nm, Channel spacing = 10 X Channel width

45. Summary • DWDM plays an important role in high capacity optical networks • Theoretically enormous capacity is possible • Practically wavelength selective (optical signal processing) components decide it • Passive signal processing elements are attractive • Optical amplifications is imperative to realize DWDM networks