DOPED-CRYSTAL AND GAS LASERS

This article is confined to a brief review of the present state of laser theory and of resonant cavities, together with a discussion of the present situation in gas and doped-crystal lasers. For the most part, the developments of the last 2-3 years are reported following on the article by Geusic and Scovil in the 1964 issue of this publication. An outline of the general features of Lamb's theory is followed by a discussion of axial modes, relaxation oscillation and of mode locking. Parametric effects are briefly mentioned. Resonant cavities are dealt with fairly fully, including results for anisotropic cavities and for cavities with coupling holes. Discussion of gas lasers is largely confined to helium-neon, ion and molecular gas systems; a bibliography on other gas and vapour lasers is included. The general features of Raman and Brillouin lasers are mentioned in so far as they provide methods of producing lasers at frequencies additional to those corresponding to direct transitions. After a brief introduction to the properties of doped-crystal laser materials, and to a discussion of loss mechanisms (which play a dominant role in the performance of such systems), the properties of trivalent and divalent rare earth ions in various host crystals are examined; actinide and transition metal ions are also briefly dealt with. The article closes with a section on second-harmonic generation, viewed as a source of additional laser transitions.