Formation of low time-bandwidth product, single-sided exponential optical pulses in free-electron laser oscillators

The detailed shape of picosecond optical pulses from a free-electron laser (FEL) oscillator has been studied for various cavity detunings. For large v alues of the cavity detuning the optical pulse develops an exponential lead ing edge, with a time constant proportional to the applied cavity detuning and the quality factor of the resonator. This behavior has been observed at two separate FELs that have completely different resonator layouts and ele ctron beam characteristics, and using different methods of optical pulse le ngth measurement. The optical pulses have a full width at half maximum time -bandwidth product Delta t(FWHM)Delta f(FWHM) of 0.2-0.3. The results prese nted here can be used to predict the optical pulse length and corresponding minimum spectral width that can be generated in a FEL pumped by short elec tron bunches. This is important for the design of new infrared free-electro n laser user facilities, which need to make a balanced choice between short pulses for high temporal resolution and narrow bandwidth for linear and no nlinear spectroscopy.