cross-polarization modulation in polarization-multiplexed systems

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


SOP evolutionTx output (fully polarized)


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?


cross-polarization modulation (XPolM)► basics

► statistical models

► XPolM and polarization multiplex

► experiments


XPolM basics

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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


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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Poincaré sphereprobe channelDWDM interferersStokes vector sumnonlinear rotation


statistical models

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(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


► 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►


SOP distribution resembles diffusion

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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


XPolM and polarization multiplex

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► selective upgrade: 10G NRZ » 100G PolDM RZ-QPSK ► fits into 50 GHz grid

a typical PolDM system


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:


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)


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)


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


the statistical ensemble (mean DOP = 0.975)

► DOPs and ROSNRs spread over large range ► for DOPs < 0.98 (0.97), ROSNR penalties become significant


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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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)


► saturation of penalties for large number of interferers(Renaudier et al., PTL, pp. 1816+, 2009)


► benefit of PolDM vs. OOK interferers(Bertran-Pardo et al., OFC, 2008, OTuM5)


summary

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► 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

cross-polarization modulation in polarization-multiplexed systems

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