High-Power Single-Frequency Lasers Using Thin Metal Film Mode-Selection Filters

Troitskii and Goldina recently showed that a thin metal film can be used inside a He-Ne laser cavity to produce single-frequency output. 1 A thin lossy film will favor oscillation on a mode which has a standingwave minimum at the film position. The simplicity of this technique is very attractive. We have, therefore, investigated both theoretically and experimentally its efficiency and its application to high power continuous wave (CW) lasers. In Section II we develop formulas, both rigorously and using a lumped-circuit approach, which relate the complex refractive index of the metal film to its characteristics as a mode filter. We also show how the complex refractive index of a given metal film can be determined from measurements of the reflectivity and transmissivity of the film. Section I I I describes experiments using this thin-film technique to obtain single-frequency operation of a continuous wave argon ion laser; Section IV describes the results obtained with a neodymiumdoped yttrium aluminum garnet laser. In Section V we discuss these results and the applications of this technique. 1405