This document outlines Wavelength Division Multiplexing (WDM) concepts and operations, highlighting its features, advantages, and technology evolution, including Coarse and Dense WDM. It discusses the principles of WDM, including multiplexers and demultiplexers, as well as components like optical isolators and circulators. The text serves as a comprehensive guide for understanding WDM technology in optical fiber communications.

1. OFC Systems
Module 3
WDM CONCEPTS AND COMPONENTS
By
Dr. Venkateswara Rao Kolli
Associate Professor
ECE,MCE.

2. Syllabus
1.WDM concepts,
2.Overview of WDM operation principles,
3.WDM standards,
4.Mach-Zehender Interferometer
Multiplexer,
6.Isolators and circulators,
7.Direct thin film filters,
8.Active optical components

3. Evolution of the Technology
1.WDM concepts

4. Features of WDM
Important advantages or features of WDM are as
mentioned below –
1. Capacity upgrade : Each wavelength supports
independent data rate in Gbps.
2. Transparency : WDM can carry fast asynchronous,
slow synchronous, synchronous analog and digital data.
3. Wavelength routing : Link capacity and flexibility
can be increased by using multiple wavelength.
4. Wavelength switching : WDM can add or drop
multiplexers, cross connects and wavelength converters
Why WDM?

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

6. WDM
concepts(contd…)
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

7. 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 line width
• optical filter bandwidth

8. Principles of
WDM
• 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 2
c
 

 
  
 
 

9. 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

10. •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).
•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.
2.Overview of WDM operation
principles

11. 3.ITU-T Standard Transmission DWDM
windows

12. 4.Mach-Zehnder
Interferometer Multiplexer
Phase shift of the propagating wave increases
Constructive or destructive interference depe
These
devices can
be either
active or
passive.
A layout of
2 x 2
passive MZI
is shown in
Fig.
It consists
of three
stages
a) 3-dB
splitter
b) Phase
shifter
c)3-dB
Combiner.

13. MZI Multiplexer (contd…)
Mach-Zehnder interferometry is used to make
wavelength dependent multiplexers.
•Initially a 3 dB directional coupler is
used to split input signals.
•The middle stage, in which one of waveguide
is longer by ΔL to given a wavelength
dependent phase shift between the two arms.
•The third stage is a dB coupler which
recombines the signals at output.
•Thus, input beam is splitted an phase shift
it introduced in one of the paths, the
recombined signals will be in phase at one
output and out of phase at other output.
•The output will be available in only one
port

14. MZI Multiplexer (contd…)

15. MZI Multiplexer (contd…)

16. 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,
1 2
1 1
2 eff
n L
 
 
 
  
 
 
2 eff
n
L



  
MZI Multiplexer
(contd…)

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

18. MZI- Demux Example

20. 5.Optical Add/Drop Multiplexer
• As add/drop multiplexer is essentially a
form of a wavelength router with one input
port and one output port with an additional
local port where wavelengths are added
to/dropped from incoming light signal.
• It is an application of optical filter in
optical networks.
• Fiber grating devices are used for add/drop
functions.
• Many variations of add/drop element can be
realized by using gratings in combination

21. 6.Isolator
• An isolator is a passive non-reciprocal device.
• It allows transmission in one direction through it and
blocks all transmission in other direction.
• Isolator are used in systems before optical amplifiers
and lasers mainly to prevent reflections from entering
these devices otherwise performance will degrade.
• Important parameters of an isolator are its insertion
loss (in forward direction) and isolation (in reverse
direction).
• The insertion loss should be as small as possible
while isolation should be as large as possible.
• The typical insertion loss is around 1 dB and
isolation is around 40 to 50 dB.

22. Principle of
operation
• Isolator works on the principle of state
of polarization (SOP) of light in a
single mode fibers.
• The state of polarization (SOP) refers to
the orientation of its electric field
vector on a plane that is orthogonal to
its direction of propagation.
• The electric field can be expressed as
linear combination of two orthogonal
linear polarization supported by fiber.
• These two polarization modes are
horizontal and vertical modes.

23. Principle of operation(contd…)
• Let input light signal has vertical state of
polarization (SOP) and blocks energy in horizontal SOP.
• The polarizer is followed by Faraday rotator.
• Faraday rotator is an asymmetric device which rotates
the SOP clockwise by 45o in both direction of
propagation.
• The polarizer after Faraday rotator passes only SOPs
with 45o orientation.
• In this way light signal from left to right is passed
through the device without any loss.
• Light entering the device from right due to reflection,

24. 7.Circulator
• A three-part circulator is shown in Fig. 7.5.1.
Signals of different wavelengths are entered at a
port and sends them out at next port.
• All the wavelengths are passed to port-2.
• If port-2 absorbs any specific wavelength then
remaining wavelengths are reflected and sends them to

26. Circulators are used to implement demultiplexer using
∂ fiber Bragg grating forextracting a desired
wavelength.
The wavelength satisfying the Bragg condition of
grating gets reflected and exits at next port.
Fig. 7.5.2 illustrates the concept of demultiplexer
function using a fiber grating and an optical
circulator.
Here, from all the wavelengths only λ3 is to be
extracted.
The circulator takes four wavelengths λ1, λ2, λ3 and
Circulator(contd…)

27. 8.Dielectric Thin-Film Filter
(TFF)

28. Dielectric Thin-Film Filter
(TFF)(contd…)

29. References
TEXT BOOKS:
1. Optical Fiber Communication – Gerd Keiser, 4th
Ed., MGH, 2008.
2. Optical Fiber Communications– – John M. Senior,
Pearson Education. 3rd Impression, 2007.