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Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX
olarization-Multiplexed System Outage due to Nonlinearity-
Induced Depolarization
Marcus Winter, Dimitar Kroushkov, and Klaus Petermann
ECOC MMX / Th.10.E.3
P
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 2
inter-channel nonlinear polarization effects
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 3
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
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 4
SOP evolutionback-to-back (fully polarized)
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 5
SOP evolution(without amplifier noise)
significant nonlinear depolarizationrapid (symbol-to-symbol) fluctuations of the SOP
can we model this and can this become a problem?
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 6
cross-polarization modulation (XPolM)► basics
► statistical models
► XPolM and polarization multiplex
► open questions
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 7
XPolM basics
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 8
XPolM is closely related to XPM
nonlinear variation of therefractive index refractive index difference
proportional to sum of interfering channels‘powers
(polarization-dependent)Stokes vectors
results in the modulation of signalphase polarization
(phase difference)
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 9
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 ► Karlsson and Sunnerud, JLT, pp. 4127+, 2006
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 10
Poincaré sphereprobe channelDWDM interferersStokes vector sumnonlinear rotation
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 11
statistical models
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 12
(interferer) Stokes vectors are not constant
► 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
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 13
► carousel model (Bononi et al., JLT, pp. 1903+, 2003)
► pump and probe rotate when both carry a mark two-channel system, no PMD►
► 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►
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 14
SOP distribution resembles diffusion
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 15
DWDM power/channel threshold for mean probe DOP=0.97
► resonant dispersion map, 10 × 10 Gbps OOK interferers► @ 50 GHz spacing
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 16
depolarization of probe vs. number of 3 dBm interferers
► difficult to simulate, expensive to measure► saturates at about 20
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 17
XPolM and polarization multiplex
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 18
► selective upgrade: 10G NRZ » 100G PolDM RZ-QPSK ► fits into 50 GHz grid
a typical PolDM system
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 19
polarization DEMUX must be aligned to PolDM subchannels(visualization in Jones space)
► 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:
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 20
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)
XPolM causes symbol-to-symbol fluctuations around mean SOP
► cannot be (fully) compensated (again like XPM)
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 21
interleaving RZ-shaped symbols minimizes crosstalk generation
time
field
am
plitu
de a
t y-R
x
aligned subchannels
interleaved subchannels
► crosstalk is never zero because pulses at Rx are no longer RZ (accumulated GVD, PMD, noise, filtering)► synchonized sampling necessary at Rx
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 22
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
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 23
the statistical ensemble (mean DOP = 0.975)
► DOPs and ROSNRs spread over large range ► for DOPs < 0.98 (0.97), ROSNR penalties become significant
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 24
open problems
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 25
outage probability ► progress made when defining outage via DOP
► (see proceedings paper, figure shows 10-5 probability) ► outage via ROSNR much more interesting / relevant
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 26
influence of CMA (polarization demux) window length ► SOP correlation over several symbols possible ► systems with little or no inline GVD compensation ► correlation can be used to reduce crosstalk ► fast algorithms needed
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 27
dispersion map / interferer modulation format ► especially uncompensated spans ► GVD pulse distortion no longer negligible ► PSK formats no longer ideal
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 28
regimes of dominance ► SPM vs. XPM vs. XPolM ► depends on many factors
► (dispersion map, modulation format, PMD, GVD, …) ► first results by Bononi
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 29
summary
Winter et al.: XPolM in PolDM Systems, Th.10.E.3, ECOC MMX 30
► XPolM in DWDM systems causes depolarization
► diffusion model correctly predicts simulated behavior► in many situations
► depolarization creates detrimental PolDM crosstalk
slides available at http://www.marcuswinter.de/research/ECOC2010
► there are still many open questions about XPolM