Optical multiplexer / de-multiplexer with regions of altered refractive index

A method of making an optical device to wavelength multiplex/de-multiplex light signals by altering the refractive index of regions within a material is disclosed. A substrate is formed from a material having a refractive index that can be altered by a process. At least one region within the substrate is subjected to the process, thereby altering the refractive index of the substrate within that region. An optical component of the multiplexer/de-multiplexer is formed by or includes the altered region. Also disclosed is an optical multiplexer/de-multiplexer device that includes an optical component that includes a region within a substrate, in which the region has an altered refractive index.