An optical multiplexer that adjusts the wavelength response and compensates for temperature effects by using rotatable mirror. The wavelength response of the device is adjusted by aligning the mirror at a correct angle with respect to the surface terminating the optical waveguide grating. The temperature dependence of the index of refraction of the material comprising the waveguides is compensated for by rotating a reflecting surface of the mirror, the rotation based on differential thermal expansion. Some exemplary embodiments may comprise a slab waveguide on a substrate (the slab waveguide having a first and second arcuate end surfaces) attached to a submount, a mirror assembly rigidly attached to the submount (the mirror assembly comprising a first and second materials having different coefficients of thermal expansion), and an optical waveguide grating (upon the substrate attached to the submount) optically coupled between the second arcuate surface and the mirror assembly. A portion of the mirror assembly between the reflector surface and where the mirror assembly is rigidly attached to the submount deforms as a function of temperature to change an angle between the optical waveguide grating and the reflecting surface.