Everything about Frequency-resolved Optical Gating totally explained
In optics,
frequency-resolved optical gating (
FROG) is a derivative of
autocorrelation, but is far superior in its ability to measure
ultrafast optical pulse shapes. Further, it can determine the phase of the pulse.In the most common configuration, FROG is simply a background-free autocorrelator followed by a spectrometer.
It is the two-dimensional nature of the FROG trace that allows the extraction of the actual pulse shape
and phase from the data.
The basics
Frequency-Resolved refers to the fact that the final signal is a spectrum. Before explaining Optical Gating, it helps to recognize that the pulse is really interacting with itself. In most configurations, the pulse is split and recombined, as in an interferometer. However, in this case, the recombination doesn't occur on a beam-splitter, but rather in a nonlinear medium, which allows the two beams to interact with each other. It is this interaction that allows the pulses to "gate" the spectral information of the other pulse. So Optical Gating refers to the fact that the measured spectrum is really from a time-slice of the pulse, and that time-slice is determined by the pulse nonlinear interaction. The gate function depends on the type of nonlinear interaction allowed.
Mathematically the FROG trace is simply a
spectrogram but with an unknown gate function:
» (generally the
rms difference). If this error (termed the "G" error) is sufficiently small, exit the loop.
GOTO 4.
Further Information
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