Abstract: An optical device for optically multiplexing or demultiplexing light of different predetermined wavelengths is provided, the optical device comprising at least one first waveguide (11) and at least one second waveguide (12) formed on a substrate (10), wherein the at least one first waveguide and the at least one second waveguide intersect at an intersection, comprising a diffraction grating structure (13) formed at the intersection. There exists a first wavelength or wavelength band travelling within the first waveguide (11) exciting the grating structure and being diffracted an angle corresponding to an outcoupling direction and there exists a second wavelength or wavelength band, different from the first wavelength or wavelength band, travelling within the second waveguide (12) exciting the grating structure and being diffracted at an angle corresponding to the same outcoupling direction.