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A helium-neon optical maser has been operated successfully with concave mirrors external to the gas-discharge tube. The latter is sealed at both ends with optically flat windows at the Brewster angle to the beam axis. This arrangement simplifies the tube construction and enhances its versatility. The alignment of concave mirrors is considerably less critical than that of plane mirrors. Measured beat frequencies between several pairs of concave resonator modes agree closely with computed values.
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Rigrod et al. (1962) studied this question.
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