Open resonators with apertures in reflectors

A theoretical analysis is made of resonators with two mirrors having apertures of different dimensions and shape. The problem is reduced to integral equations of quasioptics. The solution of these equations, including two-dimensional systems (when the shape of the aperture differs from the reflector shape), is obtained applying the perturbation theory. The resultant simple expressions for the principal characteristics of the oscillation modes are sufficiently accurate in a wide range of resonator geometries of practical interest. Further progress is reported of the application of the perturbation method to open resonators.