Development of frequency-resolved optical gating for measurement of correlation between time and frequency of chirped FEL

A femtosecond infrared-chirped free-electron laser (FEL) is an effective tool of dissociating molecules without the intramolecular vibrational redistribution. The ultrashort FEL pulse with the broadband spectrum is achieved operating the long-pulse electron beam from an energy recovry linac at Japan Atomic Energy Agency. Until now, the broadband spectrum of ultrashort pulse was measured to be ωσ/ω₀ = 14% with the central wavelength of 23 μm and the pulse duration of 320 fs at FWHM by an autocorrelation of fringe-resolved second harmonic generation. However the information of pulse shape and variation of frequency depending on the time during the pulse were not obtained. Since it is essential to know both information in the pulse for the dissociation of the molecule, we will measure them by frequency-resolved optical gating (FROG).