Diffraction Loss and Selection of Modes in Maser Resonators with Circular Mirrors

Interferometer-type resonators used for optical masers usually have a number of modal resonances falling under the gain profile of the active medium. Therefore optical maser oscillators generally can and often do oscillate in many modes, each mode having its own characteristic frequency and field pattern. Such a multimode, multifrequency output is undesirable for applications in communications and metrology. Many mode-selection schemes have been devised and tried, but most of them involve added complications and are beset by problems of stability. A simple solution was obtained by Gordon and White, 1 who built a stable single-frequency gas optical maser using a short, thin discharge tube. The resonator length was made short to reduce the number of longitudinal resonances and the mirror curvature was chosen so that only the lowest-order transverse mode had enough gain for oscillation. (The dif917 918 T H E B E L L SYSTEM T E C H N I C A L J O U R N A L , MAY 19fi5 fraction losses of the higher-order modes were all greater than the gain of the tube.) Choosing the appropriate mirror curvatures for mode suppression is preferable to the commonly used method of aperturing the mirrors, because the former method does not restrict the amount of active material that can participate in maser action and therefore is capable of producing greater output power. In order to select a pair of mirrors with the appropriate radii of curvature for single-mode operation, it is important to know accurately the diffraction losses of the modes as functions of the mirror size, spacing and curvature.