Abstract
Unstable resonators are an effective source of low-divergence high-power laser radiation and have been used successfully with many different types of laser. However, the large divergence associated with radiation on one of its passes inside the unstable resonator makes it difficult to obtain narrowband radiation from these devices. Although successful single-line operation of unstable resonators in the IR has been reported,1-2 effective extension of these cavities to narrower linewidths or shorter wavelengths is limited. We describe here an alternative geometry for a negative-branch unstable resonator that avoids the restrictions on linewidth imposed by the large divergence of the beam within the cavity and allows operation at arbitrarily narrow linewidths. We use an asymmetric ring in the feedback section of the resonator so that only collimated light passes through the frequency narrowing optics (prisms, gratings, or etalons). Collimated light from the main cavity enters the ring through a beam splitter after passing through a hole in the scraper out- coupler. After passing through the frequency narrowing elements the beam is focused so that its subsequent expansion matches the requirements of the cavity. The beam is then returned to the main cavity with the beam splitter, expands to fill the available aperture, and is recollimated by the output mirror for another round trip. An aperture is used at the focus of the lens to block the coun- terpropagating beam in the ring, ensuring that the feedback comes only from the beam that passes through the frequency-narrowing optics while collimated.
© 1988 Optical Society of America
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