FREQUENCY STABILIZATION OF HE-NE LASERS

This method is an improvement on the one used earlier by Bennet et al. A TEM00 mode laser is used with an adjacent axial mode spacing of 640 MHz so that two axial modes which are polarised mutually orthogonally are sustained. The beam is passed through a rotating Polaroid to a photosensor. The modulated part of the photosensor response is extracted as an error signal by phase-sensitive rectification and converted into a suitable current which is then passed through a coil wound around the laser tube. The thermal control of the cavity length by the feedback current ensures the frequency stabilisation of the two modes. The degree of stability achieved, inferred from fluctuations of the error signal voltage, is found to be about 2 parts in 108.