A frequency-stabilization system of a helium–neon laser with an internal-mirror plasma tube is described. The temperature of the plasma tube immersed in water flow is regulated by controlling the temperature of the water by a temperature regulator. The frequency shift is detected with a Fabry-Perot etalon and phototube. The output variation of the phototube is fed as an error signal to the regulator. The error estimation is also discussed. The relative uncertainty of the frequency stabilization is less than ±3 × 10−9 over a period of longer than 24 h.
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