WDM Concept and
Components
EE 8114
Course Notes
Part 1: WDM Concept
Evolution of the Technology
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
Wavelength Division Multiplexing
Each wavelength is like a separate channel (fiber)
Ex: SONET
TDM Vs WDM
Wavelength Division Multiplexing
• Passive/active devices are needed to
combine, distribute, isolate and amplify
optical power at different wavelengths
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
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
ITU-T Standard Transmission DWDM windows
 c 
   2  
 
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
 c 
   2  
 
Ex. 10.1
Nortel OPTERA 640 System
64 wavelengths each carrying 10 Gb/s
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
Part II: WDM Devices
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
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
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)
Filter, Multiplexer and Router
Basic Star Coupler
May have N inputs and M outputs
• Can be wavelength selective/nonselective
• Up to N =M = 64, typically N, M < 10
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
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
Definitions
Splitting (Coupling) Ratio = P2 ( P1  P2 )
Excess Loss =10 Log[ P0 ( P1  P2 )]
Insertion Loss =10 Log[ Pin Pout ]
Crosstalk = 10 Log( P3 P0 )
Try Ex. 10.2
P1  P0 cos 2 (z )
P2  P0 sin 2 (z )
Coupler
characteristics
: Coupling Coefficient
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
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
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
8x8 bi-directional star coupler by cascading 3
stages of 3-dB Couplers
 1,  2
 1,  2
 1,  2  5,  6
 3,  4  7,  8
N
Number of 3-dB Couplers N c = log 2 N
2
(12 = 4 X 3)
Try Ex. 10.5
Fiber Bragg Grating
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
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
Bragg Grating formation
2 sin(  / 2)  uv
FBG Theory
Exposure to the high intensity UV radiation
changes the fiber core n(z) permanently as a
periodic function of z
n( z )  ncore  n[1  cos( 2z / )]
z:
:
ncore:
δn:
Distance measured along fiber core axis
Pitch of the grating
Core refractive index
Peak refractive index
Reflection at FBG
Simple De-multiplexing Function
Reflected Wavelength B  2neff
Peak Reflectivity Rmax = tanh2(kL)
Wavelength Selective DEMUX
Dispersion Compensation
Longer wavelengths
take more time
Reverse the operation of
dispersive fiber
Shorter wavelengths
take more time
ADD/DROP MUX
FBG Reflects in both directions; it is bidirectional
Extended Add/Drop Mux
FBG for DFB Laser
• Only one wavelength gets positive feedback 
single mode Distributed Feed Back laser
Advanced Grating Profiles
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.
neff
neff
neff 
T 

T

Unique Application of FBG
Resonance Cavity with FBG
Transmission Characteristics
Experimental Set-Up
• What is the wavelength separation
when RF separation 50 MHz?
Interferometers
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)
Basic Mach-Zehnder Interferometer
Phase shift of the propagating wave increases with L,
Constructive or destructive interference depending on L
Mach-Zehnder Interferometer
Phase shift at the output due to the propagation
path length difference:
2 neff
 
L

If the power from both inputs (at different
wavelengths) to be added at output port 2, then,
1 1 
  2 neff    L
 1 2 
Try Ex. 10-6
Four-Channel Wavelength Multiplexer
• By appropriately selecting ΔL, wavelength
multiplexing/de-multiplexing can be achieved
MZI- Demux Example
Arrayed Wave Guide Filters
Each waveguide has
slightly different length
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 multiinput multi-output routers are available
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.
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
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
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
Tunable Laser Characteristics
Typically, tuning range 10-15 nm,
Channel spacing = 10 X Channel width
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
Tunable Optical Filters
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)
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)
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