Angular Distribution of the Intensity of a Mid-Infrared Femtosecond Radiation Source in a Nonlinear Crystal with a Signal Wave Formed in a Filament in a Gas

The divergence of mid-infrared radiation obtained by difference frequency generation of femtosecond titanium–sapphire laser pulses in a 2-mm-thick nonlinear HgGa2S4 crystal has been measured. At a pump radiation aperture of 2.8 mm, the output radiation divergence is 5.2 and 9.2 mrad at wavelengths of 7.2 and 10.5 μm, respectively. It has been shown experimentally that the angular distribution of the difference frequency radiation intensity differs significantly from the Gaussian one and the divergence of the central maximum is determined as in Fraunhofer diffraction on a circular hole with a radius equal to the radius of the interaction region.