Observation of Single-Mode Operation in a Free-Electron Laser

Time-structure and frequency-spectrum measurements of our free-electron-laser oscillator show that for periods shorter than 5 \ensuremath{\mu}s the laser operates in a stable single-cavity mode having a strikingly small fractional frequency bandwidth of ${10}^{\ensuremath{-}8}$. This constitutes the first demonstration of high-temporal-coherence operation of a free-electron laser. For periods longer than 5 \ensuremath{\mu}s 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.