Frequency stabilization of gas lasers

Methods of frequency and wavelength stabilization of gas lasers are reviewed with emphasis on principles of operation and results of stability measurements rather than experimental details or apparatus design. Most of the stabilization work has been done with the He-Ne laser. The best long-term stability is about 1 part in 10⁹. Various wavelength measurements of stabilized He-Ne lasers by interferometric comparison with standard lamps give agreement to about 2 parts in 10⁷. Wavelength shifts due to variations of gas pressure and gain are important factors in limiting the absolute wavelength stability of gas lasers. Optical standards of length and their use in length measurements are considered.