Abstract
We present results of second harmonic generation (SHG) frequency-resolved optical gating (FROG) measurements on the mid-IR free-electron laser (FEL) at the Stanford Picosecond FEL Center. These are the first SHG FROG measurements performed in the mid-IR or on an FEL. The observed pulses have an optical wavelength near 5 micrometers , and the field profiles reconstructed from the FROG trace exhibit narrow-line absorption and free-induction decay caused by atmospheric water vapor. The SHG FROG signal is easier to isolate than for the polarization gate geometry; hence the SHG traces are not corrupted by a residual background. The experiment used only 10% of the full laser power, and the spectrum and autocorrelation were quickly calculated from the FROG trace, demonstrating the feasibility of using SHG FROG as a real-time diagnostic for the FEL facility.
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