Masers and lasers with two-dimensional distributed feedback

A two-dimensional distributed feedback is an effective method of producing ultra-high power spatially-coherent radiation from an active media, that is spatially extended along two coordinates, including relativistic electron beams with sheet and annular geometry. The present paper describes progress in the investigations of planar and coaxial FEMs based on the novel feedback mechanism. The effective transverse (azimuthal) mode selection has been demonstrated under the transverse size of about 20-25 wavelengths and narrow-frequency multi-MW microwave pulses have been generated in the Ka- and W-bands.