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 3

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 4

6. Definitions
Try Ex. 10.2

7. Coupler characteristics
: Coupling Coefficient 5

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 11

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 12

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 Reflectivity Rmax = 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. Different wavelength can be separated/added
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 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