Suppression of self-pulsing instabilities in free-electron lasers using delayed optical feedback

We show that the macropulse instability affecting storage ring free-electron laser (FEL) oscillators can be suppressed using a delayed optical feedback. The principle, known as coherent photon seeding, consists in reinjecting a very small part of the laser output in the laser cavity in order to create a new deterministic solution. The feedback is shown to be efficient over a large range of the detuning parameter of the FEL cavity, even with very small fractions of reinjected power (<10^-8 here from inside to inside the cavity). The experiments have been performed on the UVSOR-II storage ring free-electron laser.