Possibility for mode-locked operation of a femtosecond UV storage ring free-electron laser using a low-loss Fabry–Perot resonator

A storage ring free-electron laser (SR-FEL) is inherently a self-mode-locked optical system. The gain broadening due to electron energy spread affects the small signal gain in order to determine the output coupling. Here, the dependence of the small signal gain, the optimum output coupling, and pulse duration on both electron energy spread and loss of a Fabry–Perot resonator in UV SR-FEL were investigated. It was shown that the output coupling strongly affects the mode-locked pulse duration and the present picosecond pulses can be shortened to femtosecond ones using a proper low-loss resonator.

► A proper Fabry–Perot cavity is proposed to allow multi-mode operation as a UV storage ring self-mode-locked FEL resonator.
► The gain broadening due to energy spread slightly affects the number of the axial modes and the optimum output coupling of the resonator.
► The absorption loss strongly influences the number of modes due to the induced periodic loss. When cavity loss decreases based on the inherent mode-locking effect of SR-FEL, the picosecond optical pulses in practice can be shortened to femtoseconds.