LIMITATIONS OF THE VARIATIONAL APPROACH IN SOLITON PROPAGATION IN NONLINEAR COUPLERS

Demonstrated are inherent problems in attempts to apply variational methods based on the lagrangian density to analysis of soliton coupling in directional couplers. A number of controversies related to this method are discussed. It is shown that only the stationary asymmetric solutions which are close to the lower bifurcation point can be described with reasonable accuracy. Comparison of the results obtained with assorted trial functions and with exact results are given. The discrepancy in describing the field evolution starting from certain initial conditions is not only quantitative but also qualitative.