In a three-guide coupler with multimode central waveguide, more than t
wo modes of the entire structure participate in the coupling between t
he outer waveguides. Using a three-mode approximation we found simple
conditions for complete power transfer between the outer waveguides: t
he device length has to match certain multiples of the conventionally
defined coupling length. The specific form of the relevant modes allow
s us to design a magneto-optic isolator or circulator with significant
ly reduced device length (as compared to the conventional nonreciproca
l coupler). The performance of the proposed devices is simulated by pr
opagating-mode calculations. Estimates for admissible fabrication tole
rances for the layer thicknesses are presented. (C) 1997 Society of Ph
oto-Optical Instrumentation Engineers.