1 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

2 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

3 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

4 Two-Photon Absorption in a Silicon Photodiode  TPA for silicon: 1100 nm < λ < 2200 nm

5 Cross-Correlation Background Level Optical Crosscorrelation using TPA  Background level: TPA occurs even when pulses do not overlap.

6 Clock Recovery using Two Photon Absorption Salem et al., IEEE Photon.Technol.Lett. 17(9), (2005) S. Takasaka et al, ECOC, Th (2005) PROBLEM: Background level depends on power, wavelength and polarization

7 Polarization Dependence  Contrary to popular belief: TPA depends on polarization Salem et al. Opt. Lett. 29(13), (2004).  Change in data polarization produces a DC offset in the cross-correlation: Two extreme cases

8 Polarization Dependence (Contd.) τ min < t <  max zero-crossing time

9 Possible Solution: Differential Detection  Produces bipolar error signal  Requires two identical nonlinear detectors

10 Dithering Phase Detection CROSSCORRELATION:

11 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), (2004).

12 Our Approach: Optical Dithering

13 Optical Dithering  18 meters of PM fiber = 25 ps of DGD  In principle, f dith can be as high as 10 GHz

14 10 GHz Dithering Clock Recovery System

15 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

16 10 dB Dynamic Range  PLL error signal exhibits zero-crossing for any input power

17 Polarization-Independent Operation  Data eye diagram measured using recovered clock in the presence of polarization fluctuations

18 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 λ =1534 nm to λ =1568 nm) –10 dB dynamic range –Provides access to un-dithered optical and electrical clock (68 dB dither suppression)

19 Related Work For more information, please visit us online 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), , (2005)