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An improved approach for optimization of N X N silica multimode interference (MMI) couplers is presented in this paper. Using guided-mode propagation analysis, we adjust the width and length of the multimode waveguides in the devices to optimize loss and uniformity. We show that the length of the multimode section can be varied in a well-defined range to find optimal device performance. The range is linked to the propagation constant spacing of fundamental and higher order modes of the multimode waveguide. We apply this approach to optimize the design of a 4 X 4 silica MMI coupler, and demonstrate that the device width and length must be adjusted simultaneously to achieve both low loss and good uniformity. It is found that optimal performance can be achieved with multiple length/width combinations.