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.
An optical Kerr gate rotates a probe's polarization in proportion to a delayed replica's intensity in the weak-gate regime. Resolving the gated spectrum gives a pulse-characterization record with a third-order gate law, distinct from a second-harmonic gate.
The self-gated second-harmonic spectrogram has the displayed ideal form. Its delay marginal is proportional to the intensity autocorrelation by the Parseval identity. Absolute time and wave phase are undetermined; and also give the same record. Finite crystal acceptance and spatial walk-off modify the ideal gate.
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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.