1. WDM Concept and Components
EE 8114
Course Notes

2. Part 1: WDM Concept

3. Evolution of the Technology

4. 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

5. Wavelength Division Multiplexing
Each wavelength is like a separate channel (fiber) 6

6. Ex: SONET
TDM Vs WDM

7. Wavelength Division Multiplexing
Passive/active devices are needed to combine, distribute, isolate and amplify optical power at different wavelengths 2

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 windows1

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 nm band
120 nm or 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

12. Nortel OPTERA 640 System
64 wavelengths each carrying 10 Gb/s

14. 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

15. Part II: WDM Devices

16. 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

17. 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

18. 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)

19. Filter, Multiplexer and Router

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

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 4

22. 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

23. Definitions
Try Ex. 10.2

24. Coupler characteristics
: Coupling Coefficient 5

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 11

28. 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

29. Fiber Bragg Grating

30. 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

31. 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

32. Bragg Grating formation

33. 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

34. Reflection at FBG

35. Simple De-multiplexing Function
Peak Reflectivity Rmax = tanh2(kL)

36. Wavelength Selective DEMUX

37. Dispersion Compensation
Longer wavelengths
take more time
Reverse the operation of
dispersive fiber
Shorter wavelengths
take more time

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

39. Extended Add/Drop Mux

40. FBG for DFB Laser
Only one wavelength gets positive feedback  single mode Distributed Feed Back laser

41. Advanced Grating Profiles

42. 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.

43. Unique Application of FBG

44. Resonance Cavity with FBG

45. Transmission Characteristics

46. Experimental Set-Up

47. What is the wavelength separation when RF separation 50 MHz?

48. Interferometers

49. 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)

50. Basic Mach-Zehnder Interferometer
Phase shift of the propagating wave increases with L,
Constructive or destructive interference depending on L

51. 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

52. Four-Channel Wavelength Multiplexer
By appropriately selecting ΔL, wavelength multiplexing/de-multiplexing can be achieved

53. MZI- Demux Example

54. Arrayed Wave Guide Filters
Each waveguide has
slightly different length

55. 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

56. 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.

57. 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

58. 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

59. 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

60. Tunable Laser Characteristics
Typically, tuning range nm,
Channel spacing = 10 X Channel width

61. Tunable Filters
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

62. Tunable Optical Filters

63. 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)

64. Issues in WDM Networks
Nonlinear inelastic scattering processes due to interactions between light and molecular or acoustic vibrations in the fibre
Stimulated Raman Scattering (SRS)
Stimulated Brillouin Scattering (SBS)
Nonlinear variations in the refractive index due to varying light intensity
Self Phase Modulation (SPM)
Cross Phase Modulation (XPM)
Four Wave Mixing (FWM)

66. Summary DWDM plays an important role in high capacity optical networks
Theoretically enormous capacity is possible
Practically wavelength selective (optical signal processing) components and nonlinear effects limit the performance
Passive signal processing elements like FBG, AWG are attractive
Optical amplifications is imperative to realize DWDM networks