An optical diffuser (1) including a fiber-optical piece comprising: a first face (2) and a second face (3); a plurality of substantially parallel optical fibers (4) each extending along a respective optical axis (13) from an input end of the optical fiber exposed at the first face of the fiber-optical piece to an output end of the optical fiber exposed at the second face of the fiber-optical piece; an input optical diffraction grating (10A) positioned adjacent to the first face of the fiber-optical piece to extend over at least a part thereof such that light diffracted by the input optical diffraction grating is directed into optical fibers of the plurality of optical fibers in a direction oblique to said optical axes.