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Several divalent and trivalent rare-earth ions incorporated in various host crystals have been found to exhibit stimulated emission in the near infrared. A report is presented describing some of the basic characteristics of these materials: absorption and fluorescence spectra, energy level diagrams, optical maser wavelengths and operating temperatures, and thresholds for stimulated emission. Recent observations on the CaWO4:Nd3+ optical maser in continuous operation are also described.
L. F. Johnson (Mon,) studied this question.
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