In this paper, we propose a compact multimode-interference-based wavelength MUX/DEMUX By using the quasi-output state, the device length can be greatly reduced. The device performances are numerically investigated by using the finite-difference beam-propagation method (FD-BPM). Simulation results show that the device performances, in terms of coupling loss and extinction ratio, are enhanced. (C) 2004 Wiley Periodicals, Inc.