A Stable, Single-Frequency RF-Excited Gas Laser at 6328A

o One of the reasons for t h e interest in lasers is their potentially very large information bandwidth. Realization of this potential requires, among other things, lasers which oscillate in a single frequency of much improved stability. Since a typical cavity dimension is thousands of wavelengths, lasers are inherently multimode devices. For this same reason, the cavity Q is orders of magnitude greater t h a n the Q for a particular transition, and the laser frequency is determined principally by cavity dimensions. As a result, the precise frequency of a laser oscillator is extremely sensitive to cavity microphonics. Several techniques have been used to obtain single-mode operation. J a v a n et al 1 obtained a single axial mode in the first gas laser by reducing gain per pass and maintaining it barely above threshold. Then, of t h e large number of allowed axial modes, oscillation occurred in only t h a t mode closest to the maximum of the atomic transition. As J a v a n pointed out, this required very fine control of the excitation to prevent variations in gain. Three-mirror cavities, proposed by Kleinman and Ivisliuk 2 and executed by Patel and Koglenik, 3 provide one axial mode even at excitation levels well above threshold. T h e third mirror varies t h e frequency dependence of cavity losses and thus aids in the discrimination against all but one mode. However, t h e addition of a mirror compounds the already serious problem of stabilization of the elementary two-mirror cavity.