Autocorrelator

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. 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 ( t ) {displaystyle Eleft(t ight)} , the constant fixed path distance as D F {displaystyle D_{F}} , and the variable path distance as a function of time D V ( t ) {displaystyle D_{V}left(t ight)} , the input to the SHG can be viewed as This comes from c {displaystyle c} being the speed of light and D / c {displaystyle D/c} being the time for the beam to travel the given path. In general, SHG produces output proportional to the square of the input, which in this case is