A single-shot frequency-resolved optical gating (FROG) was fabricated and used to characterize pulses from a femtosecond, chirped-pulse amplification Ti:Sapphire laser developed at KAIST. The self-diffraction (SD) and second harmonic generation (SHG) FROG schemes were used to obtain the FROG trace (the spectrogram of the FROG signal) from which the intensity and phase profiles of a selected pulse was reconstructed using FROG algorithm. An interferometric autocorrelator and a single-shot intensity autocorrelator were used to measure the pulses from the Ti:Sapphire oscillator and the amplified pulses at the last stage, respectively, as the primary steps for better understanding of the FROG. The laser system could be optimized in real time by monitoring FROG trace on a charged-coupled device (CCD). The measured pulse duration, when the laser condition that produces the shortest pulse was achieved, was 29 fs, giving 2 TW output power with energy of 60 mJ.
The quantative parameters regarding the phase, such as quadratic chirping parameter and group delay dispersion (GDD), were measured by polynomially fitting the phase of reconstructed field from the SD FROG trace. The GDD per unit length of a fused silica block measured by FROG was consistent to the value calculated with the optical parameters of the fused silica. The measured GDD when the pulse was not sufficiently compressed was, also, consistent with the expected GDD of the compressor composed of two gratings. This operations show that the FROG is a powerful diagnostics to analyze the phase information of ultrashort pulses.
Self-consistency check was done using the concept of FROG marginal to find any possible systematic error. The inconsistency, however, was observed between calculated FROG marginal and measured one, which resulted in the inconsistency between measured pulse spectrum and retrieved one. The re-calibration of FROG trace was possible by comparing the calculated FROG marginal with measured one even if some systematic errors existed in the present FROG setup. The phase-mismatch in the nonlinear medium generated by broad-band spectrum seems to be one of the reasons for systematic errors in both FROG's, and it was found that the spectral responsivity of the CCD distorted the phase information of SHG FROG significantly. Additional experiments are in preparation to overcome these errors and to improve this FROG setup.
