MesaPhotonics VideoFROG FROGscan, User Manual

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Chapter 8. Warnings
09/07/2018 Vs 003 FROGSCAN USER’S MANUAL Page 55
Turn on the time centering by clicking the Time Offset Correction on (under the Control Menu in the Data
Acquisition Panel). Be sure the background subtraction is on.
8.6 FROG Trace Clipped
VideoFROGscan can sense when the FROG Trace is being vignetted or clipped. The clipping can occur
either along the wavelength axis or the time axis. Clipping can, unfortunately, cause problems with both
the stability and/or the accuracy of the retrieval. If the FROG Trace is being clipped temporally, then the
temporal scan range can be increased. However, if this causes the FROG Trace to be clipped along the
wavelength axis, then a larger grid size must be used.
Quick Fix (one or more of the following):
Change the shape of the FROG area to better accommodate the FROG Trace.
Use a larger grid size.
8.7 High Spatial Chirp
Spatial chirp occurs when the color varies across the spatial front of an ultrashort laser pulse, which can
cause an asymmetric FROG Trace. However, FROGscan cannot detect this directly. Only single shot
SHG FROG devices can detect this directly. FROGscan detects it indirectly. Because the spectral content
varies across the spatial profile of the beam, if only a section of the beam is used to generate the FROG
Trace, the retrieved pulse spectrum will show a spectral shift or different spectral characteristics.
However, even in multishot geometries (such as FROGscan) spatial chirp can make the FROG Trace
asymmetric. This occurs because the crystal may not be set at the exact focus, and the interaction at the
crystal can still have spatial and hence spectral variations. Because a spatially chirped pulse is a
pathological FROG Trace, the algorithm can sometimes fluctuate between two pulse lengths.
Quick Fix (one or more of the following):
Correct the asymmetry by adjusting the overlap between two beams at the crystal to make the FROG
Trace symmetric.
Check for optics that can cause spatial chirp.
8.8 Other Warnings:
Retrieval Asymmetry, Time Calibration Warning, Frequency Calibration Warning
VideoFROGscan uses a fuzzy logic AI engine to try to diagnose abnormalities in the FROG trace and the
retrieval. When abnormalities are detected, any one, or all of the above warnings (Including High Spatial
Chirp) can appear. When these warning occur, the user should check to make sure:
1) The measured FROG trace is not much smaller that the retrieved FROG trace
2) The FROG trace error is not too high given the complexity of the pulse (small traces should be no
more than about 1 % and large, complex traces can be several percent)
3) The retrieved FROG trace very nearly exactly matches the measured FROG trace
4) The measured FROG trace or the retrieved FROG trace are symmetric about t=0.
5) The FROG trace sets in a sea of blue (wings are near zero intensity)
If all of the above are true and you are still getting the warnings, you can ignore the warnings. The
software is just doing the best it can.