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search  A real time interferometric autocorrelator is an
electronic tool used to examine the autocorrelation of, among
other things, optical beam intensity and spectral components
through examination of variable beam path differences. See
Optical autocorrelation.   In an interferometric autocorrelator,
the input beam is split into a fixed path beam and a variable
path beam using a standard beamsplitter. The fixed path beam
travels a known and constant distance, whereas the variable path
beam has its path length changed via rotating mirrors or other
path changing mechanisms. At the end of the two paths, the beams
are ideally parallel, but slightly separated, and using a
correctly positioned lens, the two beams are crossed inside a
second harmonic generating (SHG) crystal. The autocorrelation
term of the output is then passed into a photomultiplying tube
(PMT) and measured.   Considering the input beam as a single
pulse with envelope E\left( t \right ),