Coherent tunable FIR source

The operation of a new type of tunable, coherent, ( FIR) source has been demonstrated [1J. The device which may be characterized alternatively as either a grating coupled oscillator (GCO) or a Smith-Purcell free-electron laser (SP-FEL) uses the beam in a scanfling electron microscope (SEM) and a diffraction grating mounted in the e-beam focal region of the SEM to produce the radiation. The device, although new , has many antecedents. Sources which use diraction gratings driven by electron beams to produce both incoherent and coherent radiation have a long history {3—9J. A brief review of the relationship of the present work to earlier devices has been presented elsewhere [2}. The uniqueness of the present source is a consequence of the very bright electron beam in the SEM. Because it is bright. the beam supports collective modes of oscillation that are destabilized by the distributed feedback on the grating and coherent Smith-Purcell radiation is generated. In the present note, recent data will be summa.rized and used as a frame work for discussing theoretical limits to device performance.