Side-band modes in open optical resonators of pulse-periodic free electron lasers

In pulse-periodic free electron lasers in which the axes of the pump (electron beam) and an optical resonator with certain geometric parameters do not coincide, the hyperfine mode structure of the radiation consists not only of the classical super-modes (comb systems of longitudinal modes), but also of additional modes at side-band frequencies. The physical basis for generation of such modes is described with examples of optical resonators of the Novosibirsk free electron laser (NovoFEL).