High-Speed Broadband Polarization-Independent Optical Clock Recovery in a Silicon Detector

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High-Speed Broadband Polarization-Independent
Optical Clock Recovery in a Silicon Detector

• OFC 2006, OWW4
• March 8, 2006
• Amir Ali Ahmadi
• Reza Salem
• Thomas E. Murphy
• Department of Electrical and Computer Engineering

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Optical Clock Recovery

• Electrical clock recovery (conventional method)
• Polarization- and wavelength-independent
• Limited speed (usually lt 40 Gb/s)
• Optical clock recovery
• Uses optical nonlinear process
• Higher speed
• Can be polarization- and wavelength-sensitive

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Features of Our Clock Recovery System

• Optical clock recovery in a Phase-Locked Loop
  (PLL)
• Phase detection based on Two-Photon Absorption
  in a silicon detector
• Novel optical dithering scheme
• Eliminates polarization dependence
• Improves tolerance to wavelength variations
• Improves tolerance to power variations

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Two-Photon Absorption in a Silicon Photodiode

• TPA for silicon 1100 nm lt ? lt 2200 nm

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Optical Crosscorrelation using TPA
Cross-Correlation
Background Level

• Background level TPA occurs even when pulses do
  not overlap.

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Clock Recovery using Two Photon Absorption
Salem et al., IEEE Photon.Technol.Lett. 17(9),
1968-1970 (2005) S. Takasaka et al, ECOC, Th
1.3.6 (2005)
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Polarization Dependence

• Contrary to popular belief TPA depends on
  polarization
• Salem et al. Opt. Lett. 29(13), 1524-1526 (2004).
  
• Change in data polarization produces a DC offset
  in the cross-correlation

Two extreme cases
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Polarization Dependence (Contd.)
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Possible Solution Differential Detection

• Produces bipolar error signal
• Requires two identical nonlinear detectors

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Dithering Phase Detection
CROSSCORRELATION
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Electrical Dithering vs. Optical Dithering
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Our Approach Optical Dithering
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Optical Dithering

• 18 meters of PM fiber 25 ps of DGD
• In principle, fdith can be as high as 10 GHz

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10 GHz Dithering Clock Recovery System
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Results RF Spectrum of Clock

• Dither tones suppressed by 68 dB (9 fs
  contribution to jitter)
• Note 25 ps electrical dither would produce only
  8 dB suppression

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10 dB Dynamic Range

• PLL error signal exhibits zero-crossing for any
  input power

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Polarization-Independent Operation

• Data eye diagram measured using recovered clock
  in the presence of polarization fluctuations

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Summary

• New dithering system for phase-locked loop clock
  recovery
• Dithering is done in the optical domain
• Dithering frequency can be as high as 10 GHz and
  does not limit speed of PLL
• Features of the optical clock recovery system
• Based on TPA in an inexpensive silicon photodiode
• Polarization independent operation
• Wavelength insensitive performance
  (experimentally verified from ?1534 nm to ?1568
  nm)
• 10 dB dynamic range
• Provides access to un-dithered optical and
  electrical clock (68 dB dither suppression)

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Related Work

• R. Salem, A. A. Ahmadi, G. E. Tudury and T. E.
  Murphy, "Two-Photon Absorption for Optical Clock
  Recovery in OTDM Networks", submitted to J.
  Lightwave Techonol. (2005)
• R. Salem, G. E. Tudury, T. U. Horton, G. M.
  Carter and T. E. Murphy, "Polarization-Insensitive
  Optical Clock Recovery at 80 Gb/s using a
  Silicon Photodiode", IEEE Photon. Technol. Lett.
  17(9), 1968-1970, (2005)

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