Generation of Naturally Down-Chirped Few-Cycle Pulse from Free-Electron Laser Oscillator and Its Pulse Compression

A Free Electron Laser (FEL) oscillator operating in the superradiance regime generates naturally down-chirped pulses with a few-cycle pulse duration. In the Kyoto University FEL facility, we observed naturally down-chirped 4.2 cycle FEL pulses with ringing sub-pulse structures at the wavelength of 10.7 μm. We performed 1st trial to compress the down-chirped pulse by inserting a thick germanium rod that has positive group velocity dispersion. As a result, the FEL pulse with a peak wavelength of 8.7 μm having a 5.0-cycle pulse duration (146 fs) was compressed down to a 3.7-cycle pulse duration (106 fs). It was also found that spectral broadening took place in the germanium rod and the FEL spectral width was broadened from 4 to 7.7 THz.