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High-Speed Broadband Polarization-High-Speed Broadband Polarization-Independent Optical Clock Recovery in Independent Optical Clock Recovery in

a Silicon Detectora Silicon Detector

OFC 2006, OWW4March 8, 2006

Amir Ali AhmadiReza 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 < 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 < λ < 2200 nm

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Cross-CorrelationBackground Level

Optical Crosscorrelation using TPA

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)

PROBLEM: Background level depends on power, wavelength and polarization

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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.)

τmin < t < max

zero-crossing time

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Possible Solution: Differential Detection

Produces bipolar error signalRequires two identical nonlinear detectors

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Dithering Phase Detection

CROSSCORRELATION:

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Electrical Dithering vs. Optical Dithering

Our Approach: Optical Dithering

Phase Detection

Phillips et al., Opt. Lett. 22(17), 1326–1328 (1997).Sakamoto et al., IEEE Photon.Technol.Lett. 16(2), 563-565 (2004).

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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 PLLFeatures of the optical clock recovery system:

– Based on TPA in an inexpensive silicon photodiode– Polarization independent operation– Wavelength insensitive performance (experimentally

verified from λ=1534 nm to λ=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

For more information, please visit us online

http://www.photonics.umd.edu/

• 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)