Second-order optical autocorrelation measurements are reported for a far-infrared free-electron laser. Second-harmonic generation in an 840-μm-long CdTe crystal is used to provide the nonlinear autocorrelation signal. At wavelengths of 10.4 and 24.5 μm, FWHM pulse durations of 220 and 500 fs, respectively, are measured. In both cases this corresponds to six cycles of the optical field. The pulse energy ranges up to 25 μJ. Evidence is presented that each pulse evolves into a train of subpulses at large values of the cavity desynchronism.

• Received 7 June 1995

DOI:https://doi.org/10.1103/PhysRevLett.75.1755