Gas-lens effect and cavity design of some frequency-stabilized He-Ne lasers

The results of Le Floch (1982) concerning the variations of the beam radius at the curved mirror of a hemispherical cavity containing a thin lens in its middle are generalized. Distributed lenses in a cavity (induced lenslike effects of the absorber and gain mediums) are considered, and the spot size variations on the output mirror, where the diffraction losses are supposedly gathered, are determined. This effect is observed when the spacing between the end of the discharge tube (limiting aperture) and the output mirror is negligible or when a diaphragm with a proper diameter is put on the output mirror. It is shown that proper choice of the cavity parameters (length, radii of the mirror curvatures, and free spacings) makes it possible to cancel the beam spot size variations caused by the rapidly varying gas-lens effect of the absorbing medium. Guidelines for designing cavities and determining their optimum lengths are given.