THORLABS FSAC, Руководство пользователя

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Interferometric Autocorrelator Chapter 4: Setup and Operation
Rev B, May 1, 2017
Page 11
For pulse width measurements, post-processing of the interferometric trace may be used to attain the intensity
autocorrelation. In this case, it is not necessary to detect the second harmonic frequency, and significant attenuation
of the fundamental frequency may be tolerated. See Chapter 5 for more details.
4.4.3. Saturation
If the 2-photon signal is too high, the top of the autocorrelation trace may appear distorted as shown in Figure 8.
This is independent of the voltage output of the photodiode, so it may be observed even for low output signal levels.
If this is found to be the case, then the average laser power of the beam entering the FSAC must be attenuated (by
using a neutral density filter or a Fresnel reflection from an uncoated window, for example). For an 800 nm, Ø1 mm
(
1/
) beam, saturation occurs when the product of the average and peak power of the laser is around
10 W
.
Figure 8 When the 2-photon signal becomes too high, the top of the waveform may be flattened,
resulting in an erroneous measurement.
4.4.4. Excessive Chirp
When a pulse is highly dispersed, the interferometric autocorrelation may appear as in Figure 13, where the tails of
the trace lie outside of the coherence length of the pulse and are equivalent to an intensity autocorrelation, and the
center of the trace coherently interferes, creating a spike. In this case, using the interferometric autocorrelation
alone will underestimate the pulse width. However, post-processing may still be used to find the intensity
autocorrelation trace from which the pulse width may be more accurately estimated. See Chapter 5 for more details.