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A new design approach for N/spl times/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/spl times/8 MMI-based, phased-array demultiplexer with low crosstalk.