STOCHASTIC-INSTABILITY IN NONLINEAR ANISOTROPIC FIBER COUPLERS

Transmission characteristics of the nonlinear coherent coupler composed of anisotropic fibers are investigated. First, two approaches, one being the system mode approach, the other being the channel normal mode approach, are compared and proved to produce identical results as far as the weakly guiding and weakly coupling structures are concerned, as in the linear case. Second, it is found that the “critical state” at which the power is equally divided into two channels at any long coupler length 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. Third, the existence of stochastic instability in a nonlinear anisotropic fiber coupler is revealed even when the modes of the individual cores are excited, different from the conclusion reported in the early literature at 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. © 1990 IEEE