Abstract: The output frequency of a semiconductor laser beam is stabilized using a grating or another optical device which has an interaction with the beam whose parameters are sensitive to the beam frequency. The grating (130) generates positive and negative first order diffracted beams (132, 134), whose angle of diffraction is sensitive to the beam frequency, and is detected by split photodetectors AB, CD respectively. In the event of a change in beam frequency the outputs from the photodetectors AB, CD will vary to provide an error signal for the laser frequency. This error signal is insensitive to variations in angular alignment between the beam and the diffraction grating. This is because variations in frequency and variations in angular alignment cause different changes in the outputs of the photodetectors AB, CD.