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.
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.
In general, SHG produces output proportional to the square of the input, which in this case is The first two terms are based only on the fixed and variable paths respectively, but the third term is based on the difference between them, as is evident in The PMT used is assumed to be much slower than the envelope function
, so it effectively integrates the incoming signal Since both the fixed path and variable path terms are not dependent on each other, they would constitute a background "noise" in examination of the autocorrelation term and would ideally be removed first.
Assuming enough space is given in the component setup, the PMT could be fitted with a slit to decrease the effect the divergent fixed and variable beams have on the autocorrelation measurement, without losing much of the autocorrelation term.