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Optical Fiber Communications
Chapter 10
WDM Concepts and Components
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Overview – Chapter 10
10.1 Overview of WDM
10.2 Passive Optical Couplers
10.3 Isolators and Circulators
10.4 Fiber Grating Filters
10.5 Dielectric Thin-Film Filters
10.6 Phased-Array-Based Devices
10.7 Diffraction Gratings
10.8 Active Optical Components
10.9 Tunable Light Sources
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Overview of WDM
• A characteristic of WDM is that the discrete wavelengths
form an orthogonal set of carriers that can be separated,
routed, and switched without interfering with each other.
• WDM networks require a variety of passive and active
devices to combine, distribute, isolate, and amplify optical
power at different wavelengths.
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WDM Spectral Bands
• Many independent narrowband regions in the O- through Lbands can be used simultaneously.
• These regions are designated either in terms of spectral
width or optical bandwidth.
• The optical bandwidth Δν related to a particular spectral
width Δλ is found by differentiating c = λν; for Δλ << λ2
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WDM Standards
• ITU-T Recommendation G.694.1 specifies DWDM operation in the S-, C-,
and L-bands for frequency spacing of 100 to 12.5 GHz (or, equivalently,
0.8 to 0.1 nm at 1550 nm).
• The number NM is used by ITU-T to designate a specific 19N.M-THz Cband 100-GHz channel, e.g., the frequency 194.3 THz is ITU channel 43.
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10.2 Passive Optical Couplers
• Passive devices operate completely in the optical domain to
split and combine light streams.
• They include N  N couplers (with N ≥ 2), power splitters,
power taps, and star couplers.
• They can be fabricated either from optical fibers or by means
of planar optical waveguides using material such as LiNbO3,
InP, silica, silicon oxynitride, or various polymers.
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The 2  2 Fiber Coupler
•
•
P0 is the input power, P1 is the throughout power, and P2 is the power coupled
into the second fiber.
P3 and P4 are extremely low signal levels (-50 to -70 dB below the input level)
resulting from backward reflections and scattering in the device
The evanescent tail
from one fiber core
couples into
another closely
spaced fiber core
Optical power
coupling
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Performance of an Optical Coupler
• 3-dB coupler: P1 = P2 = 0.5 P0
• Tap coupler: P2 = 0.005 P0 (- 23 dB)
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Example Coupler Performance
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Star Couplers
• In general, an N  M coupler has N inputs and M outputs
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N  N Star Coupler
• Can construct star couplers by cascading 3-dB couplers
• The number of 3-dB couplers needed to construct an N  N
star is
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Mach-Zehnder Interferometer Multiplexers
• By splitting the input beam and introducing a phase shift in one of the
paths, the recombined signals will interfere constructively at one output
and destructively at the other.
• In the central region, when the signals in the two arms come from the
same light source, the outputs from these two guides have a phase
difference
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Cascaded MZIs
• Using basic 2  2 MZIs, any size N  N
multiplexer (with N = 2n) can be constructed.
• Each module i has a different ΔLi in order to
have all wavelengths exit at port C
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Optical Isolators
Optical isolators allow light to pass in only one direction.
• This prevents scattered or reflected light from traveling in
the reverse direction.
• E.g., can keep backward-traveling light from entering a laser
diode and possibly causing instabilities in the optical output.
Polarizationindependent isolator
made of three
miniature optical
components
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Optical Circulators
• An optical circulator is a
nonreciprocal multiport passive
device that directs light
sequentially from port to port in
only one direction.
• In the 3–port example, an input on
port 1 is sent out on port 2, an
input on port 2 is sent out on port
3, and an input on port 3 is sent
out on port 1.
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Isolator and Circulator Parameters
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Fiber Bragg Grating (FBG)
Example formation:
Two ultraviolet beams will
create a permanent
interference pattern in a
GeO2-doped silica fiber to
form a periodic index
variation along the axis.
Operating Principle: Incident optical wave at l0 will be reflected back
if the following grating condition is met: l0 = 2neffL, where neff is
average weighting of n1 and n2 and L = grating period (periodicity of
index variation)
Incident l0
n2
Reflected l0
n1
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Fiber Bragg Grating Application
Demultiplexing (wavelength dropping) process:
• Consider 4 wavelengths entering a circulator at port 1.
• All wavelengths exit from port 2.
• The fiber Bragg grating is designed to reflect λ2 and pass all
other wavelengths.
• After reflection, λ2 enters port 2 and comes out of port 3.
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Multiplexing of Four Wavelengths
• One needs to cascade N-1 FBGs and N-1 circulators for combining or
separating N wavelengths.
• Example for multiplexing four wavelengths using three FBGs and three
circulators (labeled C2, C3, and C4). The fiber grating filters labeled
• FBG2, FBG3, and FBG4 are constructed to reflect wavelengths λ2, λ3, and
λ4, respectively, and to pass all others.
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Etalon Theory
• A dielectric thin-film filter (TFF) is used as an optical bandpass flter.
• It allows a very narrow wavelength band to pass straight through it and
reflects all other wavelengths.
• The basis of these devices is a reflective mirror surfaces called a FabryPerot interferometer or an etalon.
• The transmission T of an ideal etalon in which there is no light
absorption by the mirrors is an Airy function
• The periodicity of the device is called the free spectral range or FSR
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Dielectric reflector stacks
Throughput
lk
Input: l1 … lN
Reflection:
l1 … lk-1 ,
lk+1 … lN
Dielectric cavity layers
Glass
substrate
Filter transmission (dB)
Dielectric Thin-Film Filter
0
-10
3 cavities
Steep
rolloff
2 cavities
-20
1 cavity
-30
-40
0.996
0.998
1
1.002
1.004
Relative wavelength lc/l
A thin-film dielectric resonant cavity filter is a Fabry-Perot interferometer
Mirrors surrounding cavity are multiple reflective dielectric thin-film layers
Cavity length determines a particular wavelength to pass & reflects all others
Flat passbands with steep rolloffs
Low insertion loss: 0.5 dB at peak and < 3.5 dB at center frequency ± 10 GHz
High optical return loss (> 45 dB)
Thin-film filters with a 50-GHz passband are commercially available
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Arrayed Waveguide Grating
4
3
Lens
region
2
l1, l2, l3, l4
1
Lens
region
5
l2
l3
l1
6
At (4): DF = 2p neff DL / lc
neff = effective index
lc = center wavelength
At (6): DlFSR = lc2/ (DL neff)
FSR-A
FSR-B
l1A ·· l4A l1B ·· l4B
l4
l4
l
The input waveguides (1) enter a lens region (2)
(2) divides the power among the different waveguides in the grating array (3)
Each grating waveguide has a precise length difference DL with its neighbors
Light in each waveguide emerges with different phase delays DF at (4)
The second lens region (5) refocuses the light from all array waveguides onto the output
waveguide array (6) [DlFSR = free spectral range = AWG periodicity]
Each wavelength is focused into a different output waveguide in region (6)
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FSR Example
• The FSR specifies the spectral width that will be separated across the
output waveguides of an AWG
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Diffraction-Grating Couplers
• Diffraction gratings spatially separate ls in a beam
• Reflection gratings are ruled or etched fine parallel lines on a
reflective surface
• Transmission gratings have periodic index variations
• Each wavelength will reflect or refract at a different angle
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Active Optical Components
• Active components require some type of external energy either to
perform their functions or to be used over a wider operating range than
a passive device, thereby offering greater application flexibility
• Many active optical components use micro-electrical-mechanical systems
or MEMS technology
• A simple example of a MEMS actuation method.
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Examples of Active Devices
A tunable optical filter can be
varied to select a specific
narrow spectral band within a
much wider optical band.
A dynamic gain equalizer (DGE)
equalizes the gain profile of an
erbium-doped fiber amplifier (EDFA)
An optical add/drop multiplexer
(OADM) inserts (adds) or extracts
(drops) wavelengths at a designated
point in an optical network.
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