The response of the short pulse free-electron laser oscillator to periodic perturbation of the cavity synchronism

The detailed optical pulse structure of the short pulse free-electron laser depends on the synchronism between optical and electron pulses in the cavity. For certain cavity detuning a regular train of optical pulses are produced and the macroscopic energy of the optical pulses exhibits a limit cycle. We show that the instabilities associated with the limit cycle and chaotic behaviour can be examined by periodically adjusting the cavity desynchronism. Resonances have been observed at the limit cycle, the period doubled, etc. frequencies. Moreover, the optical power close to these resonances is enhanced and the frequency of the optical pulse microstructure in the cavity is locked to the perturbing frequency leading to possible new methods of synchronising external lasers to the free-electron laser.