Frequency-Resolved Optical Gating:
It wasn't until the 1990s that the pulse measurement problem was solved (by Rick Trebino—the founder of Swamp Optics—and several of his post-docs). Interestingly, the solution was simple: all that was required was to spectrally resolve the light pulse generated in an autocorrelator. The trick is in the pulse-retrieval mathematics, and Trebino invented a robust pulse-retrieval algorithm to retrieve the pulse from the spectrally resolved autocorrelation. He called the technique Frequency-Resolved Optical Gating (FROG). FROG succeeds in measuring the complete intensity and phase vs. time and frequency.
PDF summary of FROG
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