A new type of unstable resonator has been developed that permits multiple-selected-line operation. Selected-line operation is obtained with a diffraction grating that is oriented at an angle that is different from the Littrow angle. Secondary feedback mirrors at the appropriate angles provide an independent threeelement cavity for each wavelength selected. The off-Littrow configuration is less efficient than a singleline cavity at the Littrow angle because there are secondary zero-order beams at each wavelength as well as a primary zero-order beam at all selected wavelengths. In the limit of a high gain this loss in efficiency is shown to be significantly reduced. Two types of multiple-selected-line unstable resonators are considered: an edge-coupled design, which produces an annular beam with a 45° output coupling mirror, and a continuously coupled design, which uses the primary zero-order beam as output. Experiments have been carried out with a multiple-selected-line unstable resonator on a cw HF laser. Selected-line operation was obtained on the P₂ (5) and P₁ (6) HF transitions in a continuously coupled design.

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