Abstract
We investigate theoretically nonlinear transmission in space-division multiplexed
(SDM) systems using multimode fibers exhibiting rapidly varying birefringence. A primary
objective is to generalize the Manakov equations, well known in the case of single-mode
fibers. We first investigate the case where linear coupling among spatial modes of the
fiber is weak and derive new Manakov equations after averaging over random birefringence
fluctuations. Such an averaging reduces the number of intermodal nonlinear terms
drastically since all four-wave-mixing terms vanish. Cross-phase modulation terms still
affect multimode transmission but their effectiveness is reduced. We verify the accuracy
of new Manakov equations by simulating the transmission of multiple 114-Gb/s bit streams
in the PDM-QPSK format over different modes of a multimode fiber and comparing the
numerical results with those obtained by solving the full stochastic equations. The
agreement is excellent in all cases studied. A major benefit of the new Manakov
equations is that they typically reduce the computation time by more than a factor of
10. Our results show that birefringence fluctuations improve system performance by
reducing the impact of fiber nonlinearities. The extent of improvement depends on the
fiber design and how many spatial modes are used for SDM transmission. We also consider
the case where all spatial modes experience strong random linear coupling modeled using
a random matrix. We derive new Manakov equations in this regime and show that the impact
of some nonlinear effects can be reduced relatively to single-modes fibers. Finally, we
extend our analysis to multicore fibers and show that the Manakov equations obtained in
the strong- and weak-coupling regimes can still be used depending on the extent of
coupling among fiber cores.
© 2012 IEEE
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