Editing Optical autocorrelation (section)

{{Use American English|date = March 2019}}
{{Short description|Autocorrelation functions realized in optics}}
[[Image:Kinds of optical autocorrelation.svg|thumb|upright=1.3|Classification of the different kinds of optical autocorrelation.]]
In [[optics]], various [[autocorrelation]] functions can be experimentally realized. The field autocorrelation may be used to calculate the spectrum of a source of light, while the intensity autocorrelation and the interferometric autocorrelation are commonly used to ''estimate'' the duration of [[ultrashort pulse]]s produced by [[modelocking|modelocked]] [[laser]]s.   The laser pulse duration cannot be easily measured by [[optoelectronic]] methods, since the response time of [[photodiode]]s and [[oscilloscope]]s are at best of the order of 200 [[femtosecond]]s, yet laser pulses can be made as short as a few [[femtosecond]]s.

In the following examples, the autocorrelation signal is generated by the nonlinear process of [[second-harmonic generation]] (SHG). Other techniques based on [[two-photon absorption]] may also be used in autocorrelation measurements,<ref>Roth, J. M., Murphy, T. E. & Xu, C. ''Ultrasensitive and high-dynamic-range two-photon absorption in a GaAs photomultiplier tube'', Opt. Lett. 27, 2076–2078 (2002).</ref> as well as higher-order nonlinear optical processes such as third-harmonic generation, in which case the mathematical expressions of the signal will be slightly modified, but the basic interpretation of an autocorrelation trace remains the same.  A detailed discussion on interferometric autocorrelation is given in several well-known textbooks.<ref>J. C. Diels and W. Rudolph, ''Ultrashort Laser Pulse Phenomena'', 2nd Ed. (Academic, 2006).</ref><ref>[[W. Demtröder]], ''Laserspektroskopie: Grundlagen und Techniken'', 5th Ed. (Springer, 2007).</ref>