Optical%20Components

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Optical Components
Ajmal Muhammad, Robert Forchheimer Information
Coding Group ISY Department
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Outline

• Types of optical components
• Passive (reciprocal non-reciprocal)
• Lens, couplers, isolators, circulators, filters,
  multiplexer, demultiplexer
• Active
• Modulator, switch, optical amplifier, wavelength
  converter, gain equalizer
• Wavelength Selectivity
• Fixed
• Tunable
• Parameters
• Temperature dependency, insertion loss
  (input?output loss)
• inter-channel cross-talks, fast tunability,
  stability and polarization dependency

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Requirements

• 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

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Optical Component Platform

• 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.
• Integrated optics components
• Uses planar manufacturing techniques to develop
  devices such as Array Waveguide Grating (AWG),
  Variable Optical Attenuators (VOA), Electro-optic
  Modulators, etc.
• Fiber based optical components
• Devices made of fibers such as fused optical
  couplers, fused WDMs, Fiber-Bragg gratings (FBG),
  etc.
• Hybrid type

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Passive Components

• 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

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Couplers
Couplers

• Structure
• NxN (e.g., 2x2)
• a is proportional to l (a is coupling ratio, l is
  coupling length)
• Parameters of interest
• Coupling ratio
• Coupling length
• Excess loss (beyond a)
• Type
• Wavelength dependent (a has wavelength-dependency)
  
• Wavelength independent (wavelength flat)
• Splitting ratio
• 3dB (splitting the power evenly) - a0.5
• Taps (e.g., a 1 thus, a very small portion is
  dropped)

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Couplers - Passive Reciprocal Device

• They can combine or separate different
  wavelengths
• The lights (different wavelengths) are coupled
  together
• Example 8x8 3-dB couplers

1310 (signal)
Amplified Signal
1550 nm (pump)
Wavelength-dependent coupler
Multiple signals combined and broadcast to many
outputs
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Couplers
6x6 coupler
1x2 coupler
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Isolators - Passive Non-Reciprocal Device

• 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

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Operation of Isolators
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Isolators
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Passive Components

• 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 Variety of technologies are available

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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)

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Gratings

• Transmission gratings
• The incident light is transmitted through the
  slits
• Due to diffraction (narrow slits) the light is
  transmitted in all direction
• 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

Transmission gratings
Reflective gratings
Diffraction gratings
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Fiber Bragg Gratings
Any periodic perturbation in the propagation
medium serves as a Bragg gratings
Diffractive optical element
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Optical Add/Drop Using Fiber Bragg Grating
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Fabry-Perot Filters

• A cavity with highly reflective mirrors parallel
  to each other (Bragg structure)
• Acts like a resonator
• Also called FP Interferometer
• Used in lasers

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Tunability of Fabry-Perot

• 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

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Multilayer Dielectric Thin Film Filters

• 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.
• The primary considerations in DTF design are
• Low-pass-band loss ( 0.3 dB)
• Good channel spacing (gt 10 nm)
• Low inter-channel cross-talk (gt -28 dB)

MUX/DEMUX using DTF filters
DTF filters
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Mach-Zehnder Interferometer

• Uses two couplers
• The coupling ratio can be different
• A phase difference between two optical paths may
  be artificially induced
• Adjusting ?L changes the phase of the received
  signal
• 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
• each output port supports the one of the two
  wavelengths that satisfies a certain phase
  condition
• Note
• ?fC/2n?L
• ?F2pf.?L.(n/c)

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Tunability

• Can be achieved by altering n or L

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Arrayed Waveguide Grating (AWG)

• AWG is a generalization of the Mach-Zehnder
  interferometer

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AWG as DEMUX and Cross-Connect
Input coupler
Arrayed guides
Output coupler
Static Wavelength Cross-connect
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Multiplexer/Demultiplexer
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Multiplexer/Demultiplexer
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Active Components

• Modulator, switch, and router
• Optical amplifier (fiber amplifier, semiconductor
  amplifier)
• Wavelength converter
• Gain equalizer

Optical switch can be used for1) Light
modulation(phase intensity)2) Routing optical
data
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Type of Optical Modulators/Switches

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Micro-Electro-Mechanical (MEMS) Switch




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Electro-Optic Modulator
Need material with high electro-optic
effectElectro-optic refractive index change is
proportional to applied electric field
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Wavelength Converter
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Different types of Wavelength Converter

• 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
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Gain Equalizers
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Gain/Power Equalizers
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Gain/Power Equalizers
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