The Multimode Interference coupler according to the invention has smoothly continuous inwardly tapered sidewalls (301) which define the width of the multimode region (W1) along the propagation axis of the device. The inward taper causes the average width of the device to be reduced in comparison to known couplers having straight sidewalls (202). Further, invention allows the access waveguides (303, 309) to remain sufficiently spaced, while reducing the overall length of the device, to avoid lithographic gap fill-in and unwanted optical coupling between the waveguides.