Nonlinear Optics in Silicon Applications in Optical Communication Systems

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Nonlinear Optics in Silicon - Applications in
Optical Communication Systems

• APS-2006-W6.00004
• 418 PM 454 PM
• Thomas E. Murphy
• Department of Electrical and Computer Engineering

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Collaborators and Colleagues

• Research Assistants
• Reza Salem (UMD)
• Paveen Apiratikul (UMD)
• Undergraduate Students
• Amir Ali Ahmadi (UMD/MIT)
• Postdoctoral Researchers
• Gaston E. Tudury (UMBC)
• Anthony S. Lenihan (UMBC)
• Faculty
• Gary M. Carter (UMBC)
• Government
• Timothy U. Horton (LPS)

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Outline

• Introduction Optical Signal Processing
• Nonlinear Effects in Silicon
• Two-Photon Absorption
• Polarization Dependence
• Applications in Communication Systems
• Future Directions

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Comparison of WDM and OTDM
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Optical Signal Processing

• (Controlling light with light)

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Why Optical Signal Processing?

• Speed
• Optical nonlinear processes can respond on a
  timescale faster than the fastest electronics
• Short optical pulses are easier to create than
  short electrical pulses
• Cost / Simplicity
• Avoid costly O-E-O conversion(Optical
  Electrical Optical)
• Only convert O-E or E-O at the edges of the
  network

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Some Practical Constraints
WHAT WILL IT TAKE TO REPLACE ELECTRONICS WITH
OPTICS?

• Wavelength independence
• Must work over entire C-band (1530-1580 nm)
• Polarization independence
• Cannot depend upon the input polarization state
• Attainable optical power levels
• Conventional EDFAs Pavg lt 200 mW
• Speed
• Faster than electronics (gt 40 Gb/s)

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Nonlinear Optical Effects in Silicon

• c(2) 0 (symmetry)
• Real c(3)
• Self/cross- phase modulation
• Intensity-dependent refraction (n2)
• Four-wave mixing
• Self-focusing
• Imag c(3)
• Two-photon absorption
• Raman Effect (also c(3))
• Free-carrier plasma dispersion

NON RESONANT ELECTRONIC
(W6.00002)
(W6.00003)
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Linear vs. Nonlinear Absorption
Two-Photon Absorption
Linear Absorption
Quadratic nonlinearity Similar to
second-harmonic generation
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Two-Photon Absorption in Silicon Photodiode
TPA is observed when 1100 nm lt ? lt 2200 nm
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How Fast is Two-Photon Absorption?
Autocorrelation of 20 fs optical pulse using
two-photon absorption in silicon detector

• Non-resonant effect
• Phase matching is not required
• Very broad bandwidth (1100-2200 nm)

D. J. Ripin et al., Opt. Lett. 27(1) 6163, 2002.
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How Sensitive is Two-Photon Absorption?
Fringe-resolved autocorrelation of 1.7 ps pulse
measured in silicon APD detector
Pavg 7.2 µW Ppeak 220 µW
C. Xu et al., Electron. Lett. 38(2) 8688, 2002.
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Is TPA Polarization Independent?
LINEAR POLARIZATIONS
ELLIPTICAL POLARIZATIONS

• No dependence for linear polarization

• Decreases by 2/3 for circular pol.

R. Salem and T. E. Murphy, Opt. Lett. 29(13),
1524-1526 (2004).
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Polarization Dependence
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Average Power Linear vs. Circular

• These two signals have same intensity

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Two-Photon Absorption Linear vs. Circular

• Nonlinear current from two-photon absorption is
  proportional to ltE (t)4gt, not ltE (t)2gt

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Polarization-Dependence in Cross-Correlation

• In most cases, we have two different inputs
• Cross-correlation depends on both polarization
  states
• Observation In almost all cases, one
  polarization state is fixed (locally generated)

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Polarization Sensitivity Experiment
R. Salem et al., Opt. Lett. 29(13) 15241526,
2004.
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Polarization Sensitivity

• If one polarization state is fixed CIRCULAR, the
  cross-correlation is independent of the other
  state

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Applications of Two-Photon Absorption

• Autocorrelation
• Pattern / Address Recognition
• Synchronization and Clock Recovery
• Temporal Demultiplexing
• Optical Sampling
• Quality Monitoring

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Electrical Clock Recovery

• Advantages polarization and wavelength
  independent
• Disadvantages limited speed (lt40 Gb/s), not
  scalable

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Optical Clock Recovery

• Does not require high-speed electrical detector
  or mixer
• Nonlinear medium acts as phase detector
• Nonlinear Medium Two-photon absorption

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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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80 Gb/s Transmitter and Receiver
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80 Gb/s Clock Recovery System

• PCLK 6 mW, PDATA 3 mW
• Closed-Loop Bandwidth 6 kHz

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Frequency Response of Phase-Locked Loop
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Timing Jitter Spectral Domain
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Timing Jitter of Recovered Clock

• Enables measurement of low-frequency jitter
  (drift) below 100 Hz
• Limited by electronic jitter of instrument

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Jitter Measurement
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Effect of Polarization Fluctuations
tmin lt t0 lt ?max
Zero-crossing time
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Agreement with Experimental Results

• Statistical distribution of the zero-crossing
  time, measured on sampling oscilloscope

Polarization Scrambling OFF
50 ps
Polarization Scrambling ON

• srms sqrt(343)2 (290)2 190 fs

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Wavelength-Independent Operation
Eye Diagrams of Recovered Data
l 1530 nm
l 1550 nm
l 1570 nm

• Wavelength limited only by EDFA

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80 Gb/s Transmission over 1000 km

• No control of polarization is needed

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80 Gb/s Transmission over 1000 km
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Challenges

• Off-the-shelf devices are not optimized for
  two-photon absorption (AR coatings, depletion
  region thickness, etc.)
• Interaction length is constrained by diffraction
• Compact devices are desired
• Efficiency need devices that require less
  optical power

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The Future Waveguide Detection
WAVEGUIDE PHOTODIODE
BULK PHOTODIODE

• Overcome focusing limit
• Increase interaction length

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Resonant Cavity Detection

• Most applications do not require fs resolution
• Resonant Cavity trades speed for sensitivity

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Summary

• Nonlinear detection in Silicon (TPA) is
• Ultrafast
• Sensitive
• Inexpensive
• Broadband
• Polarization Insensitive (if youre careful!)
• Potential Applications in High-Speed Networks
• Clock recovery (described here)
• Pattern / Address Recognition
• Demultiplexing / Sampling

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Related Work

• R. Salem and T. E. Murphy, "Polarization-Insensiti
  ve Cross-Correlation Using Two-Photon Absorption
  in a Silicon Photodiode", Opt. Lett. 29(13),
  1524-1526 (2004).
• 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).
• G. E. Tudury, R. Salem, G. M. Carter and T. E.
  Murphy, "Transmission of 80 Gbit/s over 840 km in
  standard fibre without polarisation control",
  Electron. Lett. 41(25) 1394-1395 (2005).
• 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.

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Dithering Phase Detection
CROSSCORRELATION
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Electrical Dithering vs. Optical Dithering