Sep 19 2014
Autocorrelator is a tool used for measuring the autocorrelation of spectral components and optical beam intensity by investigating the variable difference is an optical path of the beam. Autocorrelation refers to the correlation of a time series data with its own past and future values.
Optical autocorrelators may be used to measure the duration of ultrashort pulses produced by modelocked lasers within femtosecond or picosecond durations. They make use of autocorrelation signal generated by the non-linear process such as second harmonic generation.
Their are two main types if autocorrelators: intensity autocorrelators and interferometric autocorrelators. The fundamental principle of autocorrelators is that copies of incoming pulses were created by a beam splitter and superimposed on a nonlinear medium where they tend to overlap.
Interferometric Autocorrelators
Interferometric autocorrelators include a Michelson interferometer having a variable arm length difference. The superimposed pulses with the same polarization are collinearly propagated into the nonlinear crystal. The maximum signal is achieved when the pulses from the splitter undergo constructive interference. The interferometric autocorrelation is obtained by determining the average power of the frequency-doubled signal.
The average of different oscillations of the interferometric autocorrelation can be considered for long pulses. The interferometric autocorrelation is susceptible to chirps, which make it possible to obtain more information on pulses.
Intensity Autocorrelators
In intensity autocorrelators, the arm length difference and the timing of the pulses can be varied by the variable delay line. A shorter wavelength output is achieved through sum frequency generation if the arm length difference is made small. The overlap of two pulses in the crystal is reduced if the relative time delay is increased, which in turn results in weak mixing product.
The pulse duration in intensity autocorrelators can be measured by recording the power of the mixing product as a function of the arm length difference.
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