Lecture Note on Optical Components

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Lecture Note on Optical Components
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Optical Couplers

• Combines splits signals
• Light couples from one waveguide to a closely
  placed waveguide because the propagation mode
  overlaps the two waveguides
• Wavelength independent or selective
• Fabricated using waveguides in integrated optics
• ? coupling ratio
• Power (Output1) ? Power (Input1)
• Power (Output2) (1- ?) Power (Input1)
• Power splitter if ?1/2 3-dB coupler
• Tap if ? close to 1
• ?-selective if ? depends upon ?
• Lossless combining is not possible

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8-Port Splitter by Cascading Y-Couplers
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8x8 Star Coupler
Power from all inputs equally split among
outputs
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Isolators and Circulators

• Extension of coupler concept
• Non-reciprocal type will not work same way if
  inputs and outputs reversed
• Isolator allows transmission in one direction,
  but blocks all transmission (e.g., reflection) in
  the other.
• Circulator is similar to isolator, but with
  multiple ports.

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Filters and Multiplexers
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Filters

• Low insertion (input-to-output) loss
• Filter passband independent of temperature
• Flat passband
• Sharp skirts on the passband crosstalk rejection

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Gratings

• Device using interference among optical signals
  from same source, but with different relative
  phase shifts (i.e. different path lengths)

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Transmission versus Reflection Grating
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Diffraction Gratings
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Mux/Demux Using Cascaded Filters

• Each filter passes one ? and reflects the other
  ?s
• Very flat top and sharp skirts

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Amplification
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Optical Amplifiers versus Regenerators
EDFA amplifies all ls
EDFA is a common optical amplifier.
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OEO Regenerator
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1R, 2R and 3R Regenerations
1R Reamplify
2R Reamplify, Reshape
3R Reamplify, Reshape, Retime
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Regenerators versus Optical Amplifiers

• Regenerators specific to bit rate and modulation
  format used optical amplifiers are insensitive.
• A system with optical amplifiers can be more
  easily upgraded to higher bit rate without
  replacing the amplifiers.
• Optical amplifiers have large gain bandwidths gt
  key enabler of DWDM
• Issues
• Amplifiers introduce additional noise that
  accumulates.
• Spectral shape of gain (flatness), output power,
  transient behavior need to be carefully designed.

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Optical Regenerator
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Optical Receivers
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Electronic versus Photonic Coding
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Optical Modulation Methods
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Eye Diagrams
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WDM Multiplexing

• TDM 10Gb/s upper limit
• WDM Use multiple carrier frequencies to transmit
  data simultaneously

B b/s
1 2 N
NB b/s
1 2 N
B b/s
...
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Multiplexers, Filters, Routers

• Filter selects one wavelength and rejects all
  others
• Multiplexer combines different wavelengths
• Router exchanges wavelengths from one input to a
  different output

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Considerations in Switch

• Number of switch elements complexity of switch
• Loss uniformity different losses to different
  outputs (especially for large switches)
• Number of crossovers waveguide crossovers
  introduce power loss and crosstalk
• Blocking any unused input port can be connected
  to any unused output port

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Crossbar Switch

• Non-blocking
• Shortest path length 1
• Longest path length 2n-1
• No crossover

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Clos Switch
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Benes Switch
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MEMS Mirror Switching Component
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NxN Switching with MEMS Mirror Arrays