Coupled mode theory: Limitations and applicability

Abstract

Coupled mode theory (CMT) is commonly employed in the analysis of optical waveguide structures involving gratings. As an approximate theory, there are expected limitations to the applicability of this technique. The limitations put forward in the literature are mostly intuitive as the common derivation of the coupled mode equations is of a heuristic nature. It relies upon a slowly varying approximation, leading to the expectation that CMT fails in structures displaying rapidly varying characteristics. Here, CMT is tested in the geometry of a coupler incorporating a grating where rapid and large variations of the perturbed modes occur. Its applicability in this situation is confirmed.