Stochastic instability in nonlinear anisotropic fiber couplers

Transmission characteristics of a nonlinear coherent coupler composed of anisotropic fibers are investigated. The system mode approach and the channel normal mode approach are compared and shown to produce identical results so long as weakly guiding and weakly coupling structures are involved, as in the linear case. It is found that the critical state at which the power is equally divided into two channels at any long coupler lengths, as predicted in the early literature, does not exist in a conventional nonlinear coherent coupler unless the initial excitation beam is oriented along the coupler geometric axes. The existence of stochastic instability in a nonlinear anisotropic fiber coupler is shown even when the modes of the individual cores are excited. This result is different from the conclusion reported in the early literature with the assumption of negligible intensity-induced power exchange between the axes. The reason for such instability and the way to avoid it are also discussed