Kerr-induced distortions can be squeezed in the imaginary part of the
optical field by designing the transmission line so that it presents a
real-valued nonlinear transfer function . This condition is roughly
met in nondispersion-managed links with 50% precompensation. This
nonlinear noise squeezing enables dramatic reduction of Kerr-induced
impairments for modulation formats encoded in the real part of the
optical field (e.g. BPSK) .
Furthermore, Kerr-induced distortions can be, in principle, canceled for
links with real-valued nonlinear transfer function when the signal
tributaries in the orthogonal polarizations are complex-conjugated
(i.e., Ey=Ex*) . This specific configuration of the polarization
tributaries is referred to as Twin Waves.
The principle of nonlinear noise cancellation is illustrated in this
example by considering the transmission of 50-Gb/s single-polarization
(SP) QPSK and 50-Gb/s Twin Waves QPSK over 2000 km of uncompensated spans.
Digital frequency pre- and postcompensation is performed at the
transmitter and receiver respectively (see
In VPItransmissionMaker Twin Wave signals are handled as a special kind of arbitrary
4D modulation format, whose bit-to-symbol mapping can be specified by the user via a
text file (see
In order to focus on the impact of Kerr nonlinearities, the EDFA-induced optical
noise has been switched off in the simulation. The constellations of SP DPQSK
without electronic precompensation and with 50% precompensation can be observed in
for 0 dBm input power. For comparison, the constellation of the twin-wave signal
(50% precompensation) is reported in
for the same input power. Kerr-induced distortion is considerably reduced but
not completely suppressed because of random polarization effects.
Keywords: Polarization Division Multiplexing (PDM), Quadrature Phase-Shift Keying (QPSK), QPSK, PDM QPSK, coded modulation, 4D modulation, BER, Kerr, Twin Waves
Similar demonstrations are available in VPItransmissionMaker Optical Systems and on the VPIphotonics Forum.
 X. Liu et al. '406.6Gb/s PDM-BPDK Superchannel Transmission over 12.800-km TWRS Fiber via Nonlinear Noise Squeezing', OFC 2012, paper PDP5B
 X. Liu et al. 'Phase-conjugated twin waves for communication beyond the Kerr nonlinearity limit', Nature photonics, May 2013 DOI: 10.1038/NPHOTON.2013.109