Optical multiplexing/demultiplexing device having a wavelength dispersive element

A method and device for optical multiplexing or demultiplexing of optical signals. The device has a grating for dispersing an input beam into a plurality of sub-beams which are focused by a simple lens and a GRIN lens on a glass block having a plurality of waveguides embedded therein with a predetermined spacing therebetween. In order to correct for an offset in alignment, e.g. due to ambient temperature changes, between the sub-beams and the waveguides of the block, the latter is mounted to a thermally activated actuator for an controlled angular displacement which is selected to compensate for the offset.