Recent developments in the measurement of the intensity and phase of ultrashort pulses using frequency-resolved optical gating

Recently, the authors developed a simple and general technique, which they call Frequency-Resolved Optical Gating (FROG), that succeeds in measuring the full intensity and phase evolution of an individual, arbitrary ultrashort pulse. In brief, FROG involves measuring the pulse spectrogram, i.e., the spectrum of the signal pulse in an auto- or cross-correlation using any instantaneous nonlinearity, yielding signal intensity vs. delay and frequency. This trace can then be shown to fully and uniquely characterize the pulse-the pulse extraction problem is equivalent to two-dimensional phase retrieval, a solved problem from image science. In the work with lower-intensity pulses the authors utilize (multi-shot) self-diffraction (SD) and second-harmonic-generation (SHG) FROG arrangements, for which all previously published algorithms are unreliable. As a result, they have made significant improvements to these algorithms for these arrangements, including the use of least-squares optimization, generalized projections, Weiner-filtering, and a variety of other techniques. The authors report significantly improved performance, especially for SHG FROG. They have also performed an in-depth study, using SHG FROG, of the operation of a Kerr-lens-mode-locked Ti:sapphire oscillator