EE 230: Optical Fiber Communication Lecture 15

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EE 230 Optical Fiber Communication Lecture 15
WDM Components
From the movie Warriors of the Net
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ITU Grid

• Wavelengths for CWDM and frequencies for DWDM
  defined by International Telecommunication Union,
  a part of the United Nations located in Geneva
• Central frequency is 193.1 THz, equivalent to
  1552.52 nm
• Frequencies for 50 GHz channel spacings are thus
  defined as 193.1 0.05n THz where n is a
  positive or negative integer

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Active vs. Passive Devices

• Passive requires no electrical power and
  transfer function cannot be modified by user
• Active allows user to manipulate what it does
  to light pulses. Requires power.

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Platforms for WDM components

• Discrete optics thin-film filters,
  microelectromechanical systems (MEMS), isolators,
  circulators
• All-fiber components couplers, Mach-Zehnder
  interferometers
• Planar lightwave circuits (PLC)
  arrayed-waveguide gratings (AWG), couplers, MZs,
  etc.

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Coupler parameters

• Splitting ratio P2/(P1P2)
• Excess loss 10 log (P0/P1P2)
• Insertion loss 10 log (Pin/Pout)
• Crosstalk 10 log (P3/P0)

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Coupling as function of length
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Mach-Zehnder Interferometer

• where neff is determined from the Pcore/P graphs

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Multiplexing/demultiplexing criterion

• where ?L is the path length difference between
  the two arms

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Wavelength dependence of MZ output

• For wavelengths ?1 entering at input port 1, and
  ?2 entering at input port 2,

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Wavelength adjustment (trim)

• Coarse adjustment possible with fiber MZs by
  heating and pulling shorter arm to increase
  channel spacing
• Fine adjustment for both fiber and PLCs done with
  UV irradiation to line transmission peaks up with
  ITU grid

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Example

• To multiplex four wavelengths separated by 50 GHz
  (0.4 nm)
• How many stages needed?
• (log2 W). How many total MZs?
• Two in one stage, one in the next.
• What is ?L for each stage?

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Example, continued

• If first frequency is ITU center, what are other
  three, and their wavelengths?
• 193.10, 193.15, 193.20, and 193.25 THz
• 1552.52, 1552.12, 1551.72, and 1551.32 nm
• If neff1.45, determine ?L values

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Example, continued

• First stages have 100 GHz channel spacing, one
  for even-numbered wavelengths and one for odd.
  ?L equals c/2n(100x109)1.0 mm
• Second stages have 50 GHz channel spacing. ?L
  c/2n(50x109)2.1 mm
• As channel spacing gets smaller, it gets easier
  to make MZs (larger ?L)!

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General MZ expression

• For a multiplexer or demultiplexer with N
  wavelengths, you need nlog2N stages where the
  path length difference for stage i is

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Arrayed-Waveguide Grating
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AWG channel spacing

• where nsinput/output waveguide index, nccentral
  waveguide array index, and

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Tuning an AWG

• Each input waveguide corresponds to a different
  center wavelength and channel spacing. Several
  waveguides around the center one will correspond
  to the correct channel spacing within the
  tolerance, and the peak wavelengths will vary
  from one waveguide to another.

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WDM Muxes and Demuxes
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Grating Based Demultiplexer
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Optical Filters
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Interference Filter Based WDM
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Thermal drift in waveguide devices

• ?n/?T for silica7.5x10-6 per degree
• for silicon2.63 ppm per degree
• d?/dT 12 pm per degree (red shift)
• 2/3 due to thermooptic effect, 1/3 to CTE

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Effect of thermal drift

• Channel spacing100 GHz0.8 nm800 pm
• DWDM device completely transparent every 800 pm,
  opaque between
• Silica-on-silicon drifts 12 pm/?
• Device becomes a beam stop if temperature changes
  by ?
• 33?! Passive devices routinely T stabilized
  customers unhappy

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Athermalization Techniques

• Mechanical compensation flex entire chip,
  adjust point at which signal injected into device
• Materials compensation design waveguide to be
  inherently athermal