Abstract

We have previously demonstrated that a feedback ring in an unstable resonator can be useful in providing narrow bandwidth radiation across the full available aperture of the resonator. In this configuration the light traveling in the forward direction in the ring passes through frequency-narrowing etalons while it is collimated and is then coupled back into the main resonator with a lens to match the divergence requirements. It is subsequently coupled out of the cavity with a scraper mirror, forming the output beam. The reverse wave in the ring passes through the etalons while converging or diverging. It is apertured at the focus of the lens, restricting it to the Fresnel core of the cavity. It thus does not appear in the output radiation, although it can affect the extraction efficiency by saturating the gain in the core. We examine the effect that the reverse wave has on the extraction efficiency and linewidth for cavity geometries involving symmetric and asymmetric rings and ones in which the light is coupled into and out of the ring by either a beam splitter or a polarizer. Use of a polarizer combined with a 90° rotator in the ring and a quarterwave plate in the main cavity allows complete separation of the forward and backward modes of the ring. Results indicate that when the modes are completely decoupled by the polarizing elements the extraction efficiency is relatively low but increases if the quarterwave plate is removed, allowing the forward and backward modes to mix on alternate cavity passes. We also compare the effect of a backward wave suppressing mirror on the extraction efficiency.

© 1988 Optical Society of America