PURPOSE: To suppress deterioration of performance associated with variation in the length of a resonator by varying the temperature of an etalon based on output parameters supervised such that the peak in the discharge frequency curve of the etalon tracks frequency drift of a laser in a selected vertical mode. CONSTITUTION: Output from a photodetector 52 is amplified through an amplifier 54 and converted through an A/D converter 56 into a digital signal. The digital value is delivered to a main processor 58 processing all operational parameters of a laser. A measured value of power is delivered to an internal etalon tracking program 60 stored in an EPROM connected with the main processor 58. Output from the tracking program instructs a temperature to be set for the etalon in order to maximize the performance.
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