Small-aperture open resonators: the maximum possible miniaturization

The quasi-optic beam field has been represented as an angular spectrum of plane waves and, on this basis, an expression has been obtained that determines the minimum achievable size of the open resonator reflectors, maintaining the lower oscillation modes with small energy losses. The found ratio has been experimentally tested while analyzing the field distribution in a semi-symmetric open resonator by using the probe method. It has been demonstrated that it is impossible to obtain a diameter of the field spot on the reflectors in an open resonator that is smaller than the wavelength