Abstract: This invention relates to the control of the wavelength of the emission from a semi-conductor laser 1. The laser driver 5 is connected in a feedback loop comprising a beam-splitter 6, a photodiode 8 and an amplifier 19, which loop controls the driver 5 to maintain the intensity of the laser emission constant. The emission is also sampled by a beam-splitter 7 and a photodiode 9, which is made frequency-dependent by a filter 10 interposed in the path of the beam. The outputs of the matched photodiodes 8 and 9 are fed via amplifiers 11 and 13 to the positive and negative inputs of a subtractor amplifier 12, the output of which is therefore dependent only upon the emission wavelength. This output is fed as negative feedback to an amplifier 15 for controlling an operating parameter (such as the temperature) of the laser 1 to maintain the wavelength constant.