SIMULTANEOUS OPERATION OF A FREE-ELECTRON LASER ON 2 HARMONICALLY RELATED WAVELENGTHS

The interacion of light at the fundamental and the third harmonic freq uencies in a free-electron laser (FEL) oscillator is explored using th e 1-D finite-pulse-mode code HFELP. The code, which assumes that only the TEM00 transverse mode is present at both harmonic frequencies, tra cks the temporally-finite pulse electric field amplitudes of the funda mental and the third harmonic as they interact with an rf-linac-genera ted electron micropulse inside a wiggler. The evolution of the pulse p rofiles is tracked over many passes through the wiggler and optical re sonator, for various initial conditions including frequency-dependent mirror reflectivities. Results include pulse-dependent third-harmonic coherent-spontaneous emission (CSE) with, and without, multiple-pass i nterference effects; the effects of fundamental sidebands on third-har monic CSE: and lasing competition between the fundamental and third ha rmonic in overlapping spatial regions of the electron micropulse.