Induced and spontaneous interpulse phase locking in a free-electron laser

Abstract

We present the first experimental demonstration of phase locking between successive optical micropulses in a free-electron laser in which the electron pulses are separated by a fraction of the optical-cavity round-trip time. A Fox-Smith interferometer arrangement has been used to induce a high degree of interpulse phase coherence. Surprisingly, a substantial amount of coherence has also been observed without the phase-locking arrangement. A tentative explanation of this phenomenon is given.