Frequency-resolved optical gating (FROG) is a technique used in the field of ultrafast optics to measure the temporal characteristics of short pulses of light, such as those generated by lasers. The primary goal of FROG is to retrieve both the intensity and phase information of a pulse's electric field as a function of time. The technique works by exploiting the nonlinear optical interaction between an incoming pulse and a gate pulse in a nonlinear medium.
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Measure the spectrum of a nonlinear optical gate signal as its relative delay is varied. A retrieval algorithm seeks a complex pulse consistent with the resulting two-dimensional intensity record and the particular gate law. Different gate laws have different ambiguities and response limitations; a small fitting residual alone does not guarantee stable pulses or correct calibration.