Abstract

The use of multimode interference (MMI) rib waveguides as couplers/recombiners in passive and active photonic applications has become well established ^1,2 and leads to a number of advantages over conventional Y-junction and directional couplers, such as lower losses and shorter propagation lengths. The propagation characteristics of high-wall (deeply etched ribs with strong lateral optical confinement) MMI couplers are well described using the paraxial approximation for the propagation constants of the waveguide, resulting in analytical expressions for the self-imaging length Δ and power splitting points³. For many applications stripe- loaded (a shallow etched rib) MMI devices are preferable. For example, Krauss et al⁴ suggested, within the context of ring laser integration, a shallow etch would improve overall laser performance as the active layers could remain unexposed and side-wall scattering losses be reduced. However, implications for self-imaging and power splitting performance have not yet been fully investigated. To date modelling of stripe-loaded MMI waveguides has involved either Finite Element⁵ or Finite Difference⁶ methods. Unlike analytical methods, these are both time consuming and dependent on the discetization of the mesh, making certain geometric combinations and the modelling of higher-order modes difficult. Here, we present an efficient, semi-analytical approach based on the Spectral Index (SI) method⁷. The SI method has already been shown to give accurate and fast solutions for single rib waveguides and directional couplers⁷.

© 1995 Optical Society of America