A COMPARISON OF BEAM-PROPAGATION AND COUPLED-MODE METHODS - APPLICATION TO OPTICAL-FIBER COUPLERS

We analyze the instantaneous power in the local normal modes of an opt ical fiber coupler with both various three-dimensional paraxial electr ic field propagation methods and coupled-mode theory. Our calculations show that large improvements in speed result if a low-order, split-op erator, finite-difference propagation algorithm is substituted for the standard split-step fast Fourier transform technique. Further, as fie ld evolution procedures incorporate higher-order modes, the calculated power transfer ratios agree far better with experiment than the corre sponding coupled-mode predictions. The coupled-mode results however, c an be improved by taking into consideration the exact shape of the ref ractive index profile and higher-order modes, while the approach is si mple to program, and yields qualitatively correct predictions for perf ormance trends.