In the 1960s, laser pulses broke the nanosecond barrier and became shorter than even the fastest electronics could measure—and the field of ultrashort-laser-pulse measurement was born. A simple, primitive pulse-measurement technique emerged, called intensity autocorrelation. It yielded only a rough measure of the pulse length (even then, it required an assumption about the pulse shape). And it yielded no information about the pulse phase, or color.
Why measure the phase?
Intensity autocorrelation, in other words, a blurry black-and-white image of the pulse. Worse, for an unstable train of pulses, it yielded a narrow feature, called the "coherent artifact," which was often mistaken for the much longer pulse length As a result, autocorrelations of unstable pulse trains always usually yielded a shorter pulse than was in fact present.
PDF summary of intensity autocorrelation
Invented in the 1980s, interferometric autocorrelation was the second technique used to measure ultrashort laser pulses. Like intensity autocorrelation, it uses a laser pulse to measure itself, but it also interferes the light generated in the nonlinear-optical medium with light generated by the individual beams. It yields some phase information, but not enough to determine the pulse intensity and phase. This tutorial examines the characteristics and shortcomings of this also obsolete method. Basically, it yields information essentially equivalent to the intensity autocorrelation and the spectrum.
PDF summary of interferometric autocorrelation
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