Selective Excitation of the High-Order Modes in the Waveguide Quasioptical Resonator

The method of a spatial Fourier filtration for selective excitation of separate higher types of oscillations with a high degree of discrimination of undesirable modes is proposed and realized. By the matrix method it is carried out a numerical study of characteristics of the lowest transverse modes for the metal resonator of the submillimeter laser from the geometrical sizes of mirrors and parameters of the amplitude-stepped filter in the form of absorbing elements disposed in nodal lines of a field for selected modes in the region of their spatial frequencies on the nonuniform mirror of SDM. Our investigations showed that in the metal resonator for excitation of a higher type of oscillation with circular polarization TE₀₂ the Fresnel number N should make 1.5 - 1.6, and for excitation of a higher type of oscillation TE₀₃ is N> 2. The additional losses introduced by absorbing elements in the selected mode, do not exceed 0.5 - 3 % at change of their relative width within the limits of radiuses 0.04 - 0.14 for the flat mirror of SDM. Thus the difference of energy losses for selected TE₀₂ and TE₀₃ modes and nearest on losses discriminated modes is given not less than 20 %.