It was established experimentally that the use of unstable resonators with large equivalent Fresnel numbers made it possible to generate a single transverse mode in a homogeneous active medium. The angular divergence of this mode was close to the diffraction limit. A neodymium-glass laser with a divergence of 2 · 10 −5 rad along one coordinate was developed.
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Anan’ev et al. (1972) studied this question.