The basic optical configuration of the autocorrelator is similar to that of an interferometer. An incoming pulse train is split into two beams of equal intensity. An adjustable optical delay is inserted into one of the arms. The two beams are then recombined within a non-linear material (semiconductor) for two photon absorption. The incident pulses directly generate a non-linear two-photon absorption photocurrent in the semiconductor, and the detection of this photocurrent as a function of interferometer optical delay between the interacting pulses yields the pulse autocorrelation function. The two-photon absorption process is polarization- independent and non-phase-matched, simplifying alignment. The two beams propagate in a collinear fashion (interferometric configuration). This configuration results in an autocorrelation signal that is on top of a constant background. This background is produced by photon absorption photocurrent resulting from the portions of the scan during which the pulses are not over lapped.