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Far-infrared radiation generated from the difference frequency between two temperature-tuned ruby lasers operated on the R₁ and R₂ transitions has been observed. This radiation is continuously tunable over the frequency range 20-38 cm^-1. Lithium niobate was used as a mixing crystal. The expected frequencies of the far-infrared radiation were measured using a Fabry-Perot interferometer. The phase-matching conditions were also verified.
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Faries et al. (1971) studied this question.
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