Formulae for the Design of Polarization-Insensitive Multimode Interference Couplers

We present simple analytical formulae to explicitly show the effects of various waveguide parameters in the design of multimode interference (MMI) couplers with a general strip waveguide structure. In particular, we find that polarization-insensitive operation can be achieved by using a suitable width of the waveguide core to balance the modal birefringence in the slab waveguide which is the intermediate structure in the process of fabricating the strip waveguide. This finding suggests an efficient and practical approach to realizing polarization-insensitive MMI couplers based on reliable measurement data.