Optimisation of a metal waveguide cavity for a submillimetre laser

Theoretical and experimental investigations were made of the transverse modes of a submillimetre laser with a cavity in the form of an oversize circular metal waveguide. It was found that, for uniform and nonuniform reflectors, the mode formed by the lowest TE_0m guided waves had the highest output power and could be selected by a suitable choice of the length of the free-space distance between the waveguide and a reflector. A study was made of the increase in the power and in the degree of polarisation of the output radiation when the outcoupling aperture was displaced from the cavity axis. The experimental results agreed with a numerical calculation carried out in accordance with the proposed procedure.