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.

FROG-apparatus schematic. Spectrally resolving the nonlinear-optically generated pulse in an autocorrelator makes the difference.

Measured FROG trace (top left) of a 4.5fs pulse by Balthuska, Psheniknikov, and Wiersma.   Note that FROG also yields the pulse spectrum (green) and spectral phase (violet).  It requires no assumptions about the pulse shape.  And the "retrieved trace" (top right) confirms the measurement's accuracy, as well as the pulse train's stability.

PDF summary of FROG

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