A laser diode 10 has its frequency locked to the frequency of an absorption line of rubidium water or oxides of nitrogen in cell 20. The level of absorption, and hence the frequency relationship of the laser light with respect to the absorption line is detected by photodetector 34. The absolute frequency of the laser light is guaranteed by adjusting the temperature of the diode before switch-on until the frequency of the light is initially above or below, and then within the bandwidth of a group of desired transition lines, subsequently maintaining the temperature at a constant level, and adjusting the drive current of the diode until the frequency of laser light achieves coincidence with an adjacent transition line. This ensures that the diode frequency is locked to the same absorption line each occasion the diode is switched on. The change in drive current between coincidence with the two adjacent absorption lines is monitored as a further check that the correct transition line has been selected. An error signal for a current servo, which drives the diode, is generated from the photodetector output. An oscillating magnetic field, causing cyclic Zeeman splitting of the absorption lines, and hence cyclic modulation of their frequency, causes a corresponding modulation in the photodetector output. Successive half cycles of the photodetector output are integrated 58 and subtracted from each other in order to generate the error signal; the sign of the error signal thus being indicative of the direction of frequency drift in the laser light. IMAGE
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