The frequency of a 633 nm He-Ne laser is stabilised on a hyperfine-structure-component of natural iodine vapour. The third harmonic component of an applied modulation frequency is detected and a servosystem is used to lock the laser frequency to the zero-crossing of the third harmonic component. The stability and the reproducibility of the laser are determined by beat frequency experiment.

A technique for frequency stabilization of an internal mirror He-Ne laser using phase sensitive detection without cavity length modulation is suggested. The orthogonally plane polarized modes are separated and then convered into two photoelectric signals using two photodetectors. The photoelectric signals are switched alternately so as to generate a square wave, whose amplitude is proportional to the intensity difference between the two orthogonal polarizations. A lock-in amplifier is used to detect this square wave, with the switching frequency as reference. The phase detected signal is used for thermal stabilization of the laser. The frequency stability of 5 × 10⁻⁹ was obtained with an integration time of 1 s.