PMD Compensation at Ultra-High Bit Rates or Optical Spectral Processing / All-Order PMD Technology: Compensation, Sensing, Emulation A.M. Weiner Purdue University amw@ecn.purdue.edu http://ece.www.ecn.purdue.edu/~amw Funding: PURDUE UNIVERSIT Y UL T RAFAST O PT IC S & O PT IC AL FIBER C O MMUNIC AT IO NS L A BO RAT O RY A.M. We ine r (O FC 2008)
Outline • Introduction to PMD (focus on all-order PMD) • Optical spectral processing (pulse shaping etc.) • Sub-ps pulse all-order PMD compensation experiments • Extending to DWDM via hyperfine-resolution spectral dispersers • Spectral polarization sensor (parallel sensing at under 1 ms) • All-order PMD emulation (generation) PURDUE UNIVERSIT Y UL T RAFAST O PT IC S & O PT IC AL FIBER C O MMUNIC AT IO NS L A BO RAT O RY A.M. We ine r (O FC 2008)
Polarization Mode Dispersion (PMD) “Anatomy of a real fiber” Poole and Nagel, in Optical Fiber Telecommunications IIIA, Academic Press (1997). See also Kogelnik, Jopson, and Nelson, in Optical Fiber Telecommunications IVB, Academic Press (2002). For broadband inputs, random birefringences Δτ lead to wavelength- dependent polarization scrambling and wavelength- and polarization-dependent delays. PURDUE UNIVERSIT Y UL T RAFAST O PT IC S & O PT IC AL FIBER C O MMUNIC AT IO NS L A BO RAT O RY A.M. We ine r (O FC 2008)
First Order PMD Narrowband inputs S 3 Poincare ŝ ( ω 2) ŝ ( ω 1) ŝ ( ω 3) sphere 1 st Order PMD: Differential θ Small distortion – group delay Small bandwidth limit (DGD) S 2 Ω ( ω 2) S 1 ∂ �� ˆ s = Ω × out ˆ s ∂ω out �� �� Poole and Giles, Opt. Lett. 13 , 155 (1988) PSP ω = Ω ω Ω ω ( ) ( ) / ( ) 2 2 2 �� θ ω = Ω ω ≈ ω − ω ( ) ( ) DGD 2 2 3 1 • Fiber characterized by two principal states of polarization (PSPs), in general elliptical • For input light launched along a PSP, output SOP is constant to first order in ω • Two PSPs have a differential group delay (DGD) – Maxwellian distribution • Valid only for small DGD (compared to pulse width) PURDUE UNIVERSIT Y UL T RAFAST O PT IC S & O PT IC AL FIBER C O MMUNIC AT IO NS L A BO RAT O RY A.M. We ine r (O FC 2008)
M. Duelk and P. Winzer, IEEE 802. 3 High Speed Study Group, Nov. 2006 PURDUE UNIVERSIT Y UL T RAFAST O PT IC S & O PT IC AL FIBER C O MMUNIC AT IO NS L A BO RAT O RY A.M. We ine r (O FC 2008)
M. Duelk and P. Winzer, IEEE 802.3 High Speed Study Group, Nov. 2006 • Appropriate modulation format and FEC suggests impressive inroads against PMD • For very speed systems or higher PMD fibers, PMD issues likely to remain important PURDUE UNIVERSIT Y UL T RAFAST O PT IC S & O PT IC AL FIBER C O MMUNIC AT IO NS L A BO RAT O RY A.M. We ine r (O FC 2008)
PMD Compensation • Electrical compensation • Impairment resistant modulation format • Optical compensation (bit-rate and format independent) Polarization Polarization First-order optical compensator splitter combiner Distorted input Compensated Split into PSPs, delay, Polarization output and recombine! controller (or similar) Adjustable delay • Applies only to small DGD • less than a few tenths of pulse duration for RZ • less than a few tenths of bit period for NRZ •Already challenging in view of: • time-dependent, random PMD variations • requirements for low outage probability (e.g., <10 -5 ) PURDUE UNIVERSIT Y UL T RAFAST O PT IC S & O PT IC AL FIBER C O MMUNIC AT IO NS L A BO RAT O RY A.M. We ine r (O FC 2008)
Limitations to First Order PMD Approximation 0.64 The autocorrelation bandwidth of the � B PMD PSP vectors is inversely proportional DGD to the mean differential group delay. Shtaif, Mecozzi, and Nagel, IEEE Phot. Tech. Lett. 12 , 53 (2000). Foschini, Jopson, Nelson, and Kogelnik, Journal of Lightwave Technology 17 , 1560 (1999) PURDUE UNIVERSIT Y UL T RAFAST O PT IC S & O PT IC AL FIBER C O MMUNIC AT IO NS L A BO RAT O RY A.M. We ine r (O FC 2008)
All-Order PMD Effects 800 fs pulse distorted by PMD emulator with mean DGD ~ 5.5 ps H. Miao, et, Opt. Lett. 32 , 2360 (2007) • Complicated frequency-dependent polarization scrambling • Frequency- and polarization-dependent delays • Will occur whenever the distortion approaches the pulse width or bit period PURDUE UNIVERSIT Y UL T RAFAST O PT IC S & O PT IC AL FIBER C O MMUNIC AT IO NS L A BO RAT O RY A.M. We ine r (O FC 2008)
All-Order Optical PMD Compensation Complex - Generate frequency-dependent frequency-dependent inverse matrix? vector field - Operate on frequency-dependent vector field? Compensator RX TX Link All-order PMD (frequency-dependent complex transfer matrix) Sensor Controller - Spectral polarimetry? -Complexity? - Frequency-dependent -Requirements on TX? delay or phase? - Compensator synthesis in the time-domain (digital filter approach) - Compensator synthesis in the optical frequency domain PURDUE UNIVERSIT Y UL T RAFAST O PT IC S & O PT IC AL FIBER C O MMUNIC AT IO NS L A BO RAT O RY A.M. We ine r (O FC 2008)
All-Order Optical PMD Compensation Digital filter (time-domain) approach C.K. Madsen, Opt. Lett. 25 , 878 (2000) Cascaded all-pass filter elements – e.g., cascaded first-order compensator elements Examples - Cascaded polarization mode coupling in birefringent LiNbO 3 – R. Noe, et al, Elec. Lett. 35 , 652 (1999) [Univ. Paderborn] - Cascaded ring resonators in silica PLCs - C.K. Madsen, et al, JLT 22 , 1041 (2004) [Lucent] Challenges - Large number of stages for all-order PMD - Complexity of control problem grows with number of stages - Compensation of various orders of PMD is coupled and must be considered simultaneously PURDUE UNIVERSIT Y UL T RAFAST O PT IC S & O PT IC AL FIBER C O MMUNIC AT IO NS L A BO RAT O RY A.M. We ine r (O FC 2008)
Parallel, Optical Spectral Processing - Pulse shaping -Dynamic spectral equalizers -Dynamic wavelength processing Spectral Spectral combiner disperser Processed output Broadband input - Ultrashort pulse - CW plus modulation - Multiple wavelengths Spatial light modulator Control of phase, intensity, polarization … Frequency-by-frequency, independently, in parallel PURDUE UNIVERSIT Y UL T RAFAST O PT IC S & O PT IC AL FIBER C O MMUNIC AT IO NS L A BO RAT O RY A.M. We ine r (O FC 2008)
Femtosecond Pulse Shaping Fourier synthesis via parallel spatial/spectral modulation A.M. Weiner, Rev. Sci. Instr. 71 , 1929 (2000) Examples: Phase encoded O-CDMA waveform; square pulse Weiner et al, Opt. Lett. 15 , 326 (1990); IEEE JQE 28 , 908 (1992) Basic 4-f optical system, plus spectral masking: Liquid crystal modulator (LCM) arrays: •Long pulses (Nd:YAG), fixed mask: •Originally phase-only, then independent C. Froehly et al, Progress in Optics 20 , 65 (1983) phase and intensity, now polarization •100 fs pulses, fixed mask: •Down to ~msec response, hundreds of pixels Weiner, Heritage, and Kirschner, JOSA B 5 , 1563 (1988) • Diverse applications: fiber communications, coherent quantum control, few femtosecond pulse compression, nonlinear optical microscopy, RF photonics ... PURDUE UNIVERSIT Y UL T RAFAST O PT IC S & O PT IC AL FIBER C O MMUNIC AT IO NS L A BO RAT O RY A.M. We ine r (O FC 2008)
“Pulse Shaping” in WDM: Intensity Control Manipulation on a wavelength-by-wavelength basis No concern for phase or for coherence between channels Wavelength selective add-drop multiplexer (and wavelength selective switches) Ford et al, J. Lightwave Tech. 17 , 904 (1999) [Lucent] Spectral gain equalizer Ford et al, IEEE JSTQE 10 , 579 (2004) [Lucent] PURDUE UNIVERSIT Y UL T RAFAST O PT IC S & O PT IC AL FIBER C O MMUNIC AT IO NS L A BO RAT O RY A.M. We ine r (O FC 2008)
Programmable Fiber Dispersion Compensation Using a Pulse Shaper: Subpicosecond Pulses Spectral phase equalizer • Coarse dispersion compensation using matched lengths of SMF and DCF • Fine-tuning and higher-order dispersion compensation using a pulse shaper as a programmable spectral phase equalizer • Similar ideas apply to DWDM tunable dispersion compensation and few femtosecond pulse compression. ( ) −∂ψ ω ( ) τ ω = ∂ω A.M. Weiner, U.S. patent 6,879,426 PURDUE UNIVERSIT Y UL T RAFAST O PT IC S & O PT IC AL FIBER C O MMUNIC AT IO NS L A BO RAT O RY A.M. We ine r (O FC 2008)
Higher-Order Phase Equalization Using LCM Input and output pulses from 3-km SMF-DCF-DSF link Output pulse Input pulse ( without phase correction) already compressed several hundred times Applied phase Output pulse ( with quadratic & • No remaining distortion! cubic correction) Chang, Sardesai, and Weiner, Opt. Lett. 23 , 283 (1998) PURDUE UNIVERSIT Y UL T RAFAST O PT IC S & O PT IC AL FIBER C O MMUNIC AT IO NS L A BO RAT O RY A.M. We ine r (O FC 2008)
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