Observation of single-mode operation in a free-electron laser. Technical report, 1 October 1985-30 December 1986

Time-structure and frequency-spectrum measurements of the UC Santa Barbara free-electron-laser oscillator show that for periods shorter than 5 microsec. The laser operates in a stable single-cavity mode having a strikingly small fractional frequency bandwidth of 10/sup 8/. This constitutes the first demonstration of high-temporal-coherence operation of a free-electron laser. For periods longer than 5 microsec, the laser-frequency changes in unexpected quantized steps, which may be explained in terms of a homogeneously broadened gain profile coupled to a small monotonic drift in electron-beam energy.