Induced and Spontaneous Interpulse Phase Locking in a Free-Electron Laser

We present the first experimental demonstration of phase locking between successive optical micro-pulses 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.