PURPOSE:To obtain the laser of low cost excellent in the reproducibility of frequency absolute values by a method wherein an AC magnetic field of modulation frequency is generated in a laser tube with a coil heater, and the laser tube length is thermally controlled on modulation signals strong in the output generated thereby. CONSTITUTION:The coil heater 2 is wound around an internal mirror laser tube 1. The laser tube generates photo beams of approx. 1/10 of the output formward normally backward. This weak photo beam or part of the output light from forward is introduced t a polarizing plate 3. The polarizing plate 3 adjusts its direction of transmission so as to come incident with the direction of polarization on a specific mode. If the laser tube oscillates only on vertical modes 1-2, the light transmitting through the polarizing plate 3 at this time is only of single vertical mode. Its intensity is converted into an electric signal by a photo detector 4 and inputted to a synchronizing detector 6. This detector 6 detects in phase synchronization only the component of a modulation frquency (f) given by an oscillator 5. A signal detected in synchronization is supplied with a suitable multiplication factor and a suitable time constant by a control signal processor 7 and inputted to a heater-power control circuit 8.
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