| 摘要 |
A laser diode has its frequency locked to the frequency of an absorption line of rubidium (water and oxides of nitrogen are also disclosed). The level of absorption, and hence the frequency relationship of the laser light with respect to the absorption line is detected by a photodetector. The absolute frequency of the laser light is guaranteed by initially adjusting the temperature of the diode until the frequency of the light is brought into coincidence with a first absorption line, 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. Additionally, 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 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.
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