A new design approach to large input/output-number multimode interference couplers and its application to low-crosstalk WDM routers

A new design approach for N x N multimode interference (MMI) couplers with large N is described which yields a device design with extremely low imbalance and insertion loss. Analytical expressions for the phase errors in MMI devices are derived from basic principles; these expressions enable significant phase-error reduction to be obtained by optimizing the index-contrast. This optimization in conjunction with the proper placement of the access waveguides allows significant improvement in the device performance. An application of the design approach is illustrated through the design of an 8 x 8 MMI-based, phased-array demultiplexer with low crosstalk.