Tunable Far-Infrared Radiation Generated from the Difference Frequency between Two Ruby Lasers

Far-infrared radiation generated from the difference frequency between two temperature-tuned ruby lasers operated on the $R_1$ and $R_2$ transitions has been observed. This radiation is continuously tunable over the frequency range 20-38 ${\mathrm{cm}}^{-1\mathrm{}}$. 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.