Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems
Cross-Polarization Modulation in Polarization-Multiplexed Systems
M. Winter, D. Kroushkov, and K. PetermannIEEE Summer Topicals
July 2010
Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems2
typical DWDM system with a nonlinearity probe
► CW probe is unaffected by linear effects / SPM ► other channels are 10 Gbps OOK in 50 GHz grid
Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems3
SOP evolutionTx output (fully polarized)
Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems
SOP evolution(without amplifier noise)
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significant nonlinear depolarizationrapid (symbol-to-symbol) fluctuations of the SOP
what is going on and is this a problem?
Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems5
cross-polarization modulation (XPolM)► basics
► statistical models
► XPolM and polarization multiplex
► experiments
Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems
XPolM basics
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Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems7
XPolM is closely related to XPM
nonlinear variation of thebirefringence
(index difference between x and y)refractive index
proportional to sum of interfering channels‘Stokes vectors powers
results in the modulation of signalpolarization
(phase difference between x and y)phase
Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems
nonlinear polarization effects known since at least 1969 ► e.g. Kerr shutter (Duguay and Hansen, APL, pp. 192+, 1969)
XPolM first described in its „current version“ in 1995 ► Stokes space Manakov equation ► collision of two solitons ► Mollenauer et al., Optics Letters, pp. 2060+, 1995
many-channel formulation in 2006 ► Menyuk and Marks, JLT, pp. 2806+, 2006
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Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems9
Poincaré sphereprobe channelDWDM interferersStokes vector sumnonlinear rotation
Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems
statistical models
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Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems
(interferer) Stokes vectors are not constant
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► length (intensity) varies due to walk-off► (interferer and probe group velocity differs)
► direction (SOP) varies due to PMD ► (interferer and probe birefringence differs)
► both effects are random
various models have been proposed to describe this behavior
Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems
► carousel model (Bononi et al., JLT, pp. 1903+, 2003)
► pump and probe rotate when both carry a mark two-channel system, no PMD►
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► diffusion model (Winter et al., JLT, pp. 3739+, 2009)
► SOPs evolve as random walk ensemble mean values only►
► Karlsson‘s statistical model (JLT, pp. 4127+, 2006)
► influence on PMD compensation mostly two-channel system, no PMD dependence►
Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems
SOP distribution resembles diffusion
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Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems
DWDM power/channel threshold for mean probe DOP=0.97
► resonant dispersion map, 10 × 10 Gbps OOK interferers► @ 50 GHz spacing
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depolarization of probe vs. number of 3 dBm interferers
► difficult to simulate, expensive to measure► saturates at about 20
Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems
XPolM and polarization multiplex
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Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems17
► selective upgrade: 10G NRZ » 100G PolDM RZ-QPSK ► fits into 50 GHz grid
a typical PolDM system
Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems
polarization DEMUX must be aligned to PolDM subchannels(visualization in Jones space)
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► otherwise crosstalk occurs from x to y and vice versa
► crosstalk increases with misalignment angle and with► length of field vector
detected field at y-Rx:
Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems
modern coherent receivers can handle subchannel SOP changes with PMD time constants
► DCF abuse with a screwdriver: 280 µrad/ns(Krummrich and Kotten, OFC 2004, FI3)
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XPolM causes symbol-to-symbol fluctuations around mean SOP
► cannot be compensated (again like XPM)
Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems
interleaving RZ-shaped symbols minimizes crosstalk generation
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time
aligned subchannels
interleaved subchannels
► crosstalk is never zero because pulses at Rx are no longer RZ(accumulated GVD, PMD, noise)
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10 × 10G NRZ interferers w/ 100G PolDM-RZ-QPSK probe
► 256 ps/nm RDPS, 10 interferers, SSMF, no PMD ► power/channel threshold is reduced by up to 2 dB
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the statistical ensemble (mean DOP = 0.975)
► DOPs and ROSNRs spread over large range ► for DOPs < 0.98 (0.97), ROSNR penalties become significant
Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems
Xie showed how PolDM interferers can cause negligible XPolM compared to single-polarization (PTL, pp. 274+, 2009)
► requires RZ pulse shape and subchannel interleaving
► neighboring half-symbol slots have orthogonal polarization states
► probe SOP oscillates but rotation does not accumulate
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Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems
experiments
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► onset of nonlinear penalties at much lower powers ► (near) saturation of penalties for large channel spacing
(van den Borne et al., ECOC, 2004, Mo 4.5.5)
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► saturation of penalties for large number of interferers(Renaudier et al., PTL, pp. 1816+, 2009)
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► benefit of PolDM vs. OOK interferers(Bertran-Pardo et al., OFC, 2008, OTuM5)
Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems
summary
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Marcus Winter: XPolM in Polarization-Multiplex Transmission Systems29
► XPolM in DWDM systems causes depolarization
► diffusion model correctly predicts simulated behavior
► depolarization creates detrimental PolDM crosstalk
► can be reduced by interleaving PolDM subchannels
slides available at http://www.marcuswinter.de/publications/ST2010
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