MesaPhotonics VideoFROG FROGscan, User Manual

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Chapter 3. Program Panels
Page 24 FROGSCAN USER’S MANUAL 09/07/2018 Vs 003
3.4 Data Acquisition Panel
Most of the program settings can be set in this Panel.
Time Calibration – The servo calibration constant in femtoseconds per step; each step is ~ 1 fs or 150
nm.
Grid Size – Determines the size of the FROG Trace sent to the retrieval algorithm. For example, when a
64 x 64 array is selected, the region inside the square on the raw video display is resampled to a 64 x 64
pixel array before the pulse characteristics are extracted from the FROG Trace. Larger grid sizes require
more computational time, which causes the program to run more slowly. A good compromise between
speed and resolution is the 64 x 64 array, although for the high-speed computers available today, real-
time performance can be achieved on a 256 x 256 grid. If the program updates the pulse slowly on large
grid sizes, reduce the frame rate.
Scan Mode – Determines the servo settling time. The FAST setting produces faster updates but causes
more FROG Trace instability (oscillation time). High Fidelity reduces the FROG Trace instability to a
minimum.
Sawtooth – This setting uses a sawtooth waveform on the delay servo rather than a triangle wave that is
the default. Using a sawtooth prevents the FROG trace from shifting when the servo changes direction.
The sawtooth wave may increase bearing wear a small amount. Using it does not void the warranty.
Program Mode – Displays the mode of the program: 1) Data acquisition, 2) Demo (synthetic data with
noise added), and 3) Read File (reads a raw FROG Trace file saved by the program.
The other Program Mode display shows the file name as a file is being read.
Using Calibration File – Loads spectrometer response calibrations performed at the factory on request.
The data is appended in Appendix E, where applicable.
VIDEOFROGscan must be closed and then
restarted to load calibration files.
Servo Fault – Indicates a servo error. Flashing indicates that VideoFROGscan has recovered, a steady
illumination requires the program to be exited.
The Control Menu contains the following features:
Time Offset Correction – Centers the FROG Trace along the time axis to remove any effects of pulse
front tilt.
FROG Trace Tilt Correction – Removes any tilt in the trace caused by spatial chirp. Either the wavelength
offset (WO) correction or the spatial chirp (SC) correction (removes both wavelength tilt and wavelength
offset) can be activated. Both cannot be activated simultaneously.
Wavelength Offset Correction – Centers the FROG Trace in wavelength.
Median Filter – Removes speckle in the measured, resized FROG Trace. A 3 x 3 window is moved across
the resized FROG Trace one point at a time. At each point, the center point of the 3 x 3 window is
replaced by the median value, rather than the average. The median is better at removing speckle noise
than the average. Also, the median filter tends to preserve high frequencies better than a low-pass filter.
The median filter should be used when there is sparse, but noticeable speckle.
Background Filter – An adaptive rank filter that removes the background from the FROG Trace.
Level and Threshold sliders determine how the rank filter is applied to the FROG Trace. By adjusting
these sliders, all but the most severe backgrounds can be removed from FROG Traces. The Level
adjustment sets the rank of the absolute value in the 3 x 3 window moved across the FROG trace. The
Threshold sets the maximum signal value for filter application. Typically, the best position for the Level
slider is on the "Low" side (smaller magnitudes) while the best position for the Threshold sliders is on the
Mid to High side. The Background filter should be used when there is background that the background
subtraction cannot remove and the retrieved pulse "wiggles."
Edge Background – Averages four spectra on the edge of the FROG Trace (two on each side) and
subtracts their average from the rest of the spectra. This mode almost always works best.