Optical Components
Ajmal Muhammad, Robert Forchheimer
Information Coding Group
ISY Department
Outline
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Types of optical components
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Passive (reciprocal & non-reciprocal)
Lens, couplers, isolators, circulators, filters, multiplexer, demultiplexer
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Active
Modulator, switch, optical amplifier, wavelength converter, gain equalizer
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Wavelength Selectivity
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Fixed
Tunable
Parameters
Temperature dependency, insertion loss (inputoutput loss)
inter-channel cross-talks, fast tunability, stability and polarization
dependency
Requirements
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Bandwidth
Low insertion loss (inputoutput loss)
High return loss (outputinput loss)
Polarization insensitivity
Low crosstalk
High extinction ratio
Temperature insensitivity
Low control power
Small size
Cost
Optical Component Platform
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Micro-optic components
The assembly of discrete elements are placed together in an optical
component. The elements require precise optical alignment to maximize the
performance.
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Integrated optics components
Uses planar manufacturing techniques to develop devices such as Array
Waveguide Grating (AWG), Variable Optical Attenuators (VOA), Electro-optic
Modulators, etc.
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Fiber based optical components
Devices made of fibers such as fused optical couplers, fused WDMs, FiberBragg gratings (FBG), etc.
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Hybrid type
Passive Components
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Coupler: versatile device used as a building block for
several other optical devices
Isolator: used in systems at the output of amplifiers and
lasers to prevent reflections
Filter: to multiplex and demultiplex wavelengths in a
WDM system, and to provide equalization of the gain
and filtering of noise in optical amplifier
MUX & DEMUX: MUX combines signals at different
wavelengths on its input ports onto a common output
port, DEMUX performs the opposite function
Couplers
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Structure
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Coupling ratio
Coupling length
Excess loss (beyond α)
Type
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NxN (e.g., 2x2)
α is proportional to l (α is coupling ratio, l is coupling length)
Parameters of interest
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Couplers
Wavelength dependent (α has wavelength-dependency)
Wavelength independent (wavelength flat)
Splitting ratio
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3dB (splitting the power evenly) - α=0.5
Taps (e.g., α ∼ 1 – thus, a very small portion is dropped)
Couplers - Passive Reciprocal Device
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They can combine or separate different wavelengths
The lights (different wavelengths) are coupled together
Example: 8x8 3-dB couplers
1310 (signal)
1550 nm
(pump)
Amplified
Signal
Wavelength-dependent
coupler
Multiple signals combined and
broadcast to many outputs
Couplers
1x2 coupler
6x6 coupler
Isolators - Passive Non-Reciprocal Device
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Transmit in one direction only
Avoid reflection of laser – or any reflection
One input, one output or multiple ports
Key parameters are insertion loss and excess loss
Example: circulator
Operation of Isolators
Isolators
Passive Components
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Coupler: versatile device used as a building block for
several other optical devices
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Isolator: used in systems at the output of amplifiers and
lasers to prevent reflections
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Filter: Variety of technologies are available
Gratings
Describe a device involving interference among multiple
optical signals coming from the same source but having
difference phase shift
There are a number of gratings
 Reflective
 Transmission
 Diffraction
 Stimax (same as reflection but integrate with concave
mirrors)
Gratings
Transmission gratings
 The incident light is transmitted through
the slits
 Due to diffraction (narrow slits) the light is
Transmission
transmitted in all direction
gratings
 Each slit becomes a secondary source of
light
 A constructive interference will be created
on the image plane only for specific WLs
that are in phase  high light intensity
 Narrow slits are placed next to each other
 The spacing determines the pitch of the
gratings
 Angles are due to phase shift
Reflective
gratings
Diffraction
gratings
Fiber Bragg Gratings
Any periodic perturbation in the propagation medium serves as a
Bragg gratings
Diffractive optical
element
Optical Add/Drop Using Fiber
Bragg Grating
Fabry-Perot Filters
A cavity with highly reflective mirrors parallel to each other
(Bragg structure)
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Acts like a resonator
Also called FP Interferometer
Used in lasers
Tunability of Fabry-Perot
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Changing the cavity length (l)
Varying the refractive index (n) within the cavity
Mechanical placement of mirrors
Not very reliable
Using piezoelectric material within the cavity
Thermal instability
Multilayer Dielectric Thin Film
Filters
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Dielectric thin-film (DTF) interference filters consist of
alternating quarter-wavelength thick layers of high
refractive index and low refractive index
each layer is a quarter-wavelength thick.
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The primary considerations in DTF design are:
Low-pass-band
loss (« 0.3 dB)
Good
channel spacing (> 10 nm)
Low inter-channel cross-talk (> -28 dB)
DTF filters
MUX/DEMUX using DTF filters
Mach-Zehnder
Interferometer
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Uses two couplers
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Because of the path difference, the two waves arrive at coupler 2
with a phase difference
At coupler 2, the two waves recombine and are directed to two
output ports
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The coupling ratio can be different
A phase difference between two optical paths may be artificially induced
Adjusting ΔL changes the phase of the received signal
each output port supports the one of the two wavelengths that satisfies a certain
phase condition
Note:
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Δf=C/2nΔL
ΔΦ=2πf.ΔL.(n/c)
Tunability
Can be achieved by altering n or L
Arrayed Waveguide Grating (AWG)
AWG is a generalization of the Mach-Zehnder interferometer
AWG as DEMUX and CrossConnect
Input coupler
Arrayed guides Output coupler
Static Wavelength
Cross-connect
Multiplexer/Demultiplexer
Multiplexer/Demultiplexer
Active Components
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Modulator, switch, and router
Optical amplifier (fiber amplifier, semiconductor amplifier)
Wavelength converter
Gain equalizer
Optical switch can be used for:
1) Light modulation(phase & intensity)
2) Routing optical data
Type of Optical Modulators/Switches
Micro-Electro-Mechanical
(MEMS) Switch
Electro-Optic Modulator
Need
material
with
high
electro-optic
effect
Electro-optic: refractive index change is proportional to applied electric
field
Wavelength Converter
Different types of Wavelength
Converter
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OE/EO regeneration
SOA-based
Cross-gain modulation
Cross-phase modulation
Four-Wave mixing
Fiber-based
Cross-phase modulation
Four-Wave mixing
OE/EO
Cross-gain
Cross-phase
Four-Wave mixing
Gain Equalizers
Gain/Power Equalizers
Gain/Power Equalizers