Generation of laser radiation of difference frequency in a rectangular millimeter waveguide

Results are reported for an experimental investigation of the generation of the difference frequency of partially mode-locked neodymium laser radiation in a nonlinear lithium niobate (LiNbO/sub 3/) crystal placed inside a rectangular waveguide. Difference-frequency radiation was detected at a wavelength of about 8 mm and also in the wavelength range from 2 to 4.5 mm the measurements indicate that the radiation was maximum in the 70-GHz range and that the spectral density of the radiation was independent of crystal length. Focusing with a cylindrical lens is found to increase the conversion efficiency of the laser radiation approximately twofold in comparison with the case of a homogeneous distribution of the laser beam across the waveguide. The experimental results are compared with theoretical calculations of difference-frequency generation in a rectangular waveguide.