THORLABS FSAC, Руководство пользователя

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Interferometric Autocorrelator Chapter 5: Theory and Interpretation
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TT121442-D02
Chapter 5 Theory and Interpretation
5.1. Working Principle
Figure 11 shows the optical layout of the FSAC. The input beam pulse is split and copies are reflected from a delay
mirror and a reference mirror and then recombined at the beamsplitter. The recombined beam is focused onto the
detector. When the delay and reference path lengths are exactly equal so that the optical path difference (OPD) is
zero, the pulses overlap in time and add constructively to produce the maximum possible signal on the detector.
When the OPD exceeds the full width of the pulse so that the pulses no longer overlap in the combined beam, there
is no interference and the photodiode signal is at a minimum.
Figure 11 FSAC Optical Layout
As photocurrent in the FSAC is generated by a two-photon process, the output signal is proportional to the square
of the irradiance incident on the detector, which in turn is proportional to the square of the total electric field
amplitude. And since the speed of the photodiode is several orders of magnitude longer than pulse width, it must
be time integrated. This is written as
| | d ,
where is the complex electric field of the pulse and is the time delay between the two pulses arriving at the
detector 1 . The integrand may be expanded to give
1
Rulliere, Claude.
Femtosecond laser pulses
. Springer Business Media, Incorporated, 2005.