Quasioptical Three-Mirror Echelette Traveling-Wave Resonator with Frequency Tuning: Diffraction Theory and Experiment

We develop the theory of a three-mirror echelette traveling-wave resonator with frequency tuning, in which one of the mirrors is an echelette reflective diffraction grating. The theory makes it possible to allow for the diffraction loss related to finite sizes of the mirrors, the loss due to the existence of a mirror lobe at the grating (coupling loss), and the ohmic losses. The possibility of significant rarefaction in the spectrum of the resonator eigenfrequencies is demonstrated. The structure of mode fields allows one to ensure efficient input of the radiation to the resonator. An experiment performed in a frequency range near 140 GHz confirmed the theoretical calculations.