An optical wavelength demultiplexer has at least two optical fibers and a two-dimensional array of photodetectors arranged in as many rows as the number of optical fibers and as many columns as the number of demultiplex channels. Any one of said optical fibers and a central line of one of said rows which corresponds to said any one of the optical fibers are spaced equal distances from the optical axis of a focusing element such as a collimator lens or a concave diffraction grating. A light ray emitted from any one of the optical fibers is demultiplexed by a planar or concave diffraction grating into light rays, which are applied to a corresponding row of photodetectors to produce output signals in the respective demultiplex channels. The planar diffraction grating and the collimator lens, or the concave diffraction grating is shared by the at least two optical fibers which provide input capabilities.