3-DIMENSIONAL COUPLED MODE METHOD FOR SIMULATION OF COUPLER AND FILTER STRUCTURES

A numerical coupled mode method, including cross power terms and employing the exact two-dimensional fields in the waveguide cross section, is presented as an alternative to propagation methods. It avoids the limitations of the standard beam propagation method and is a fast method suited for simulation of couplers and filters which have no abrupt structural changes. The method is tested for couplers with weakly and strongly confined waveguides and for buried vertical couplers including feeding branches. The results show that full treatment of two-dimensional cross section is of special importance for design and simulation of waveguide devices for which the effective index approximation is no longer valid. The method enables fast simulation of the coupled intensity for a whole set of couplers as given by the mask pattern. Therefore, direct comparison of simulated results and measurement is possible. In addition, design rules for symmetrical couplers with feeding branches which have arbitrary power division are derived. The validity of the theory is tested by comparing the theoretical results to the results of a symmetrical coupler working as a 1.3 mum/1.55 mum demultiplexer and to an asymmetrical meander type coupler both of which were fabricated on GaInAsP/InP.