A wavelength monitoring device for monitoring a beam of light is disclosed having a beam splitter, with opposing first and second spaced apart faces, for receiving optical radiation from the beam of light to be monitored. In operation the first face reflects a first portion of the optical radiation to a first photodiode. The second face includes a grating for reflecting a second portion of the optical radiation to the first photodiode. The grating also reflects a third portion of optical radiation to a second photodiode. The light received by the second photodiode corresponds proportionally to optical power of the incident beam of light. The first photodiode is for detecting a wavelength characteristic of the composite beam and is located so as to receive the first portion and the second portion of optical radiation after the first portion and the second portion of optical radiation have optically interfered to form a composite beam. The first face and the grating are oriented and spaced from one another so that the first and second portions of the optical radiation optically interfere with one another along a path toward the first photodiode.