A device and method for multiplexing or demultiplexing M optical signals, each having a different wavelength λm. An MMI waveguide has a length such that each optical signal that propagates into the MMI waveguide at an i:th access waveguide, where i≦N, and N is greater than or equal to 2, produces N self-images inside the MMI waveguide at a respective distance Im from the access waveguides. Each of M wavelength selective reflectors is arranged near a respective distance Im, wherein the m:th reflector reflects the optical signal carried by the m:th wavelength, where 2≦m≦M, while being substantially transparent to all other wavelengths. Each of M phase adjustors is arranged relative to a corresponding reflector, wherein the m:th phase adjustor adjusts the phase of the self-images of the optical signal carried by the m:th wavelength to create a single self-image at a selected output access waveguide when reflected by the m:th reflector.