Chapter 7. Retrieving Pulses
Page 50
FROGSCAN USER’S MANUAL
09/07/2018 Vs 003
Chapter 7. Retrieving Pulses
Once the system is aligned and FROG Traces are appearing on the raw video display, pulses will be
automatically retrieved by the VideoFROGscan software. Through the use of controls on the Panel, the
measured FROGscan Trace can be adjusted and optimized. For best operation, the FROG Trace must
be centered in the Live FROG image displayed in the Summary Panel.
7.1 Optimizing Pulse Symmetry
For the best retrievals, the FROG Trace should be centered and symmetric about the t=0 delay displayed
in the live FROG Trace, Summary Panel.
If the trace is off-center along the wavelength axis, a linear phase will appear in the spectral phase. This
can be quite bothersome when trying to null the phase of a pulse compressor, for example.
If the FROG Trace is not centered along the time axis, the pulse and gate will separate in time causing a
loss of accuracy of the retrieved pulse. For the most accurate reconstruction, the pulse and gate should
overlap perfectly.
In addition, non-symmetric live FROG Trace may cause spectral chirp in the pulse. Large amounts of
spatial chirp are often observed in ultrafast optical parametric amplifiers.
If you are having problems with the FROG Trace symmetry, adjust the Overlap Mirror until the FROG
Trace is symmetrical. This may result in reduced signal intensity, but smaller signals are better than
inaccurate results.
7.2 Centering the FROG Trace in time
To center the FROG Trace in time, set the scan center by adjusting the servo center using Time Control
on the main panel or use the automatic FROG Trace re-centering under the Data Acquisition Panel. The
time center is set correctly when all of the traces overlap in the autocorrelation graph on the summary
page.
7.3 Adjusting the Grid Size
Increase the Grid Size in the Date Acquisition Panel if the pulse is so chirped that the Time Window must
be increased in order to capture the entire temporal part, and the spectral portion of the FROG Trace is
being clipped. Selecting the Grid Size menu item under in the Data Acquisition Panel gives access to the
grid size radio buttons.
NOTE: increasing the grid size increases both the amount of time required for data acquisition and the
amount of time required for convergence. While typical frame times for a 64 x 64 grid size is about 0.5
seconds for the spectrometer, increasing the grid size to 256 x 256 increases the frame to
about 2 seconds (0.5Hz.).
7.4 Setting the Background Level
The spectrometer has a large offset that can cause problems with retrievals unless it is removed. Indeed,
background has a very deleterious effect on the retrieval of FROG Traces. If too much background is
present, retrievals will appear noisy. However, if too much background is removed, the pulses will not be
measured correctly and unphysical time-bandwidth products can appear. Examples of FROGscan Traces
with
a
) too much background,
b
) too much clipping and
c
) good background are shown in the following
Figure 30, using a diode LASER beam as an example.
Summary of Contents for VideoFROG FROGscan
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