A study of nonlinear threshold and outage probability in coherent optical systems

Abstract

We investigate, for most popular modulation formats, the nonlinear threshold (NLT) versus per-channel symbol rate at a constant bandwidth efficiency for a long-haul coherent optical transmission over a 20x100km single-mode fiber link, both dispersion-managed (DM) and dispersion-uncompensated (DU). The NLT is analyzed by decoupling the nonlinear Kerr effect in its main manifestations, showing how the dominant nonlinearity changes with symbol rate and modulation format. We show that the NLT normalized to the symbol rate reveals the symbol rate (hence the granularity) achieving the longest distance. We present a formula yielding the system reach based on knowledge of NLT measurements on a much shorter link, providing reach simulations to verify its accuracy in both DM and DU links. We also investigate the interaction of polarization dependent loss (PDL) with Kerr nonlinearity by decoupling single- and cross-channel nonlinear distortions in both DM and DU homogeneous 100G polarization division multiplexing (PDM)-quadrature phase shift keying (QPSK) systems and in a DM hybrid 100G PDM-QPSK/10G on-off keying system. Thanks to the decoupling method, we provide reasons for the interaction using basic comprehensive examples. Both average Q-factor and its distribution are investigated in both DM and DU links, showing that realistic PDL values affect the outage probability (OP) notwithstanding a negligible average penalty. Finally, we present a novel adaptive Stratified-Sampling (SS) algorithm to efficiently assess the PDL-induced OP in presence of fiber nonlinearity and polarization mode dispersion.