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Results of experiments are presented indicating that low-frequency beats detectable from ruby lasers are beats between transverse modes. Three-dimensional resonator theory is reviewed with respect to its applicability to ruby lasers. Agreement between theory and experiment shows that the modes of an excellent optical quality ruby rod are described by nonconfocal resonator theory. Results of beat-frequency measurements with approximately confocal rods are in good agreement with theory. The variation of transverse mode beats with pump energy and temperature indicates that single-frequency operation is possible at room temperature and up to 20% over threshold if axial frequency selection is used.
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C. Martin Stickley (1964) studied this question.
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