Polarization-Independent Techniques for Optical Signal Processing

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2. Acknolwedgements. CollaboratorsGary Carter (Professor of CSEE, UMBC)Gaston Tudury (Research Associate, UMBC)Anthony Lenihan (Research Assistant Professor, UMBC)Timothy Horton (Laboratory for Physical Sciences)Amir Amadi (Undergraduate Student, ECE, UMD). OthersPaveen Apiratikul, Arash Komae

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Polarization-Independent Techniques for Optical Signal Processing

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1. 1 Polarization-Independent Techniques for Optical Signal Processing Reza Salem Electrical and Computer Engineering Dept. University of Maryland, College Park PhD Dissertation Defense August 29th, 2006 Committee Members: Dr. Thomas Murphy (chair, academic advisor) Dr. Christopher Davis Dr. Julius Goldhar Dr. Ping-Tong Ho Dr. Wendell Hill (dean’s representative)

2. 2 Acknolwedgements Collaborators Gary Carter (Professor of CSEE, UMBC) Gaston Tudury (Research Associate, UMBC) Anthony Lenihan (Research Assistant Professor, UMBC) Timothy Horton (Laboratory for Physical Sciences) Amir Amadi (Undergraduate Student, ECE, UMD)

3. 3 Outline Introduction Optical vs. Electrical Signal Processing Polarization Dependence Two-Photon Absorption in Silicon Photodiodes Polarization Independent Clock Recovery Cross-phase modulation in Fiber Polarization Independent Demultiplexing

4. 4 Simplified Diagram of OTDM Network

5. 5

6. 6

7. 7 The Problem of Polarization Dependence Optical fiber is (nominally) symmetric No preferred polarization axis Even when input polarization is known The output polarization is unpredictable and it can vary on a µs timescale Polarization evolution depends on bending, temperature, manufacturing imperfections, etc. Photodetectors are polarization insensitive

8. 8 Signal Processing Tasks Clock Recovery Demultiplexing Regeneration Performance Monitoring (Sampling, PMD Measurement, OSNR Measurement, etc.) Logic Gate Wavelength Conversion

9. 9 OTDM Clock Recovery and Demultiplexing

10. 10

11. 11 Linear vs. Nonlinear Absorption

12. 12 Two-Photon Absorption in Silicon Photodiode

13. 13

14. 14 Optical Cross-Correlation using TPA

15. 15 Clock Recovery using Two Photon Absorption

16. 16 Details of the Clock Recovery System

17. 17 80 Gb/s Transmission Experiment

18. 18 Bit Error Rate Measurements Measured with recovered clock, after 110 km transmission

19. 19 Effect of OSNR on Clock Recovery Q is measured as a function of OSNR OSNR is controlled by injecting noise

20. 20 Effect of OSNR on Clock Recovery

21. 21 Longer Transmission Distances Longer distances can be achieved by recirculating fiber loops Up to 840 km error free (Q>6) transmission is achieved at 80 Gb/s No polarization control is needed

22. 22 Polarization Dependence of TPA (One input)

23. 23

24. 24 Polarization Sensitivity of Cross-Correlation

25. 25 Polarization Sensitivity If one polarization state is fixed CIRCULAR, the cross-correlation is independent of the other state

26. 26 Effect of Polarization Fluctuations

27. 27 Timing Jitter of Recovered Clock Enables measurement of low-frequency jitter (drift) below 100 Hz Limited by electronic jitter of instrument

28. 28 Jitter Measurement

29. 29 Effect of Polarization Fluctuation

30. 30 System Improvement An offset-free scheme for the clock recovery can make the system even less sensitive to polarization Such a scheme also increases the power dynamic range

31. 31

32. 32 Cross-Phase Modulation in Optical Fiber Cross-Phase Modulation (XPM) in optical fibers is an ultra-fast process Spectral filtering of the XPM spectrum can be used for wavelength conversion and demultiplexing at speeds beyond 40 Gb/s

33. 33 Polarization Independent XPM Polarization-independent XPM can be achieved if pump is circularly polarized Requires twisted or spun fibers to maintain circular polarization Lou et. al., IEEE Photonics Technology Letters, 12(12), p. 1701, 2001 Circular polarization is difficult to maintain in highly-nonlinear fibers (non-zero birefringence)

34. 34 Bismuth-Oxide-Based Highly Nonlinear Fiber

35. 35 Demultiplexing Using XPM

36. 36 XPM-Induced Wavelength Shift

37. 37 Blue Shift In 10 GHz Probe Spectrum

38. 38

39. 39

40. 40

41. 41

42. 42 Photonic Crystal Fiber (Crystal-Fiber Co.)

43. 43 XPM Polarization Dependence in Birefringent Fiber

44. 44 Because of birefringence: clock and data polarization states evolve at different rates Polarization-Independent XPM in Birefringent Fiber

45. 45 Pol. Dependence of XPM, Measurement and Simulation

46. 46 80 Gb/s Optical Demultiplexing

47. 47 Polarization Independent Demultiplexing To achieve polarization-independent performance: Must use the right CLOCK polarization (Data polarization scrambled in both cases)

48. 48 Receiver Performance Power penalty is about 2.5 dB for all channels (compared to back to back) Polarization scrambled in all cases

49. 49 What is the best CLOCK Polarization? Programmable polarization controller & analyzer on CLOCK Measure BER as a function of CLOCK polarization (Data polarization is still scrambled)

50. 50 Optimal Polarization States (Measured) Plotted points correspond to BER < 10–9 (Data polarization is still scrambled) Agrees with the theoretical analysis (circle of states)

51. 51 Summary GOAL: Replace high-speed electronics with nonlinear optical signal processing PROBLEM: Most nonlinear optical processes are polarization dependent Optical Clock Recovery based on Two-Photon Absorption in Silicon Detector Inexpensive, ultrafast, wavelength- and polarization independent Optical Demultiplexing in Highly Nonlinear Fibers Lengths as short as 2 meters Up to 160 Gb/s speeds Two new ways to overcome polarization dependence of XPM

52. 52 Awards and Publications

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