Wavelength division multiplexing (WDM) is a technique that allows multiple optical signals of different wavelengths to propagate simultaneously over a single optical fiber. In WDM, each input is generated by a separate optical source with a unique wavelength. An optical multiplexer couples the individual signals to the transmitting fiber, and at the receiving end an optical demultiplexer separates the signals before photodetection. Dense WDM (DWDM) utilizes dense channel spacing of 0.8 nm or less to transmit 16 or more wavelengths simultaneously over a single fiber, enabling high network capacity. Passive devices like fused fiber couplers and arrayed waveguide gratings, and active devices like tunable lasers and filters, are used to combine, separate

1. WDM- Wavelength
Division Multiplexing

2. WAVELENGTH DIVISION
MULTIPLEXING

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

25. Definitions
Try Ex. 10.2

26. Coupler
characteristics
κ: Coupling Coefficient

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

31. Fiber Bragg Grating

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

34. Bragg Grating formation

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

36. Reflection at FBG

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

38. Wavelength Selective DEMUX

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

41. Extended Add/Drop Mux

42. FBG for DFB Laser
• Only one wavelength gets positive feedback 🡪
single mode Distributed Feed Back laser

43. Advanced Grating Profiles

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

47. Transmission Characteristics

48. Experimental Set-Up

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

55. MZI- Demux Example

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