Submillimeter radiation intensity diistribution near the output aperture of a wide hollow dielectric waveguide

Experimental data are presented on the intensity distribution of submillimeter radiation near the output apertures of a waveguide-type HCN laser and a wide hollow dielectric waveguide excited by the laser. A central dip in the intensity distribution is observed at a certain distance from the output aperture, which is attributed to diffraction at the output end of the waveguide. The distance at which the transition of waveguide propagation to Gaussian propagation occurs and the shape of spatial intensity distribution in the near zone are shown to be rather sensitive to the radiation incidence angle with respect to the waveguide. Experimental results are in good agreement with calculations based on the scalar diffraction theory.