MesaPhotonics VideoFROG FROGscan, User Manual

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Chapter 5. Maximizing the Intensity of the Signal Beam
Page 42 FROGSCAN USER’S MANUAL 09/07/2018 Vs 003
5.4 Troubleshooting
The Signal beam is not visible.
The Signal beam will appear only when the beams overlap in the crystal. Therefore, the device must be
aligned for simultaneous temporal overlap and spatial overlap, which can be very difficult. The following
approaches have been used successfully:
1. Replace the crystal with the 50 µm Pinhole supplied and confirm that both beams are visible behind
the pinhole. Ensure that the Pinhole is in the same vertical plane as the Crystal. Using a 5/64 ball
head driver to adjust the 5/64 hex sockets on the back of the 1
st
Turning Mirror so that the Delay
beam is visible through the Pinhole. Then adjust the Overlap Mirror so that the beam from the fixed
delay is visible through the pinhole. Reinsert the crystal and check whether the Signal beam is now
visible.
2. To make sure that the beams overlap spatially, walk the beams off the SHG crystal using a 5/64 ball
head driver to adjust the 5/64 hex sockets on the back of the 1st Turning Mirror. At the edge of the
crystal, the beams will diffract both because of the edge of the crystal and the glue attaching the
crystal to the substrate. When using a visible laser, the diffraction will be visible on a business card. If
the beams are in the same place, when moving the 1st Turning Mirror, the diffraction on both beams
will appear at the same time, in exactly the same way. Do this for both X and Y (vertical and
horizontal) directions. Repeating this procedure 2-3 times will almost always get the beams
overlapped well enough to see the Signal SHG beam.
3. If the Signal beam is still not visible, loosen, BUT DO NOT REMOVE, the socket head cap screws
that hold the Crystal holder to the base plate, using a 2.5mm ball end hex driver. Then move the
entire crystal holder assembly forward, toward the center of the FROGscan. This places the crystal at
a point where the input beams are large enough to be visually overlapped without the use of a
pinhole. The Last Position Mirror may need to be adjusted in order to get the SHG signal into the
center of the Spectrometer SMA Connector.
Then adjust the overlap mirror so that the beams are overlapped. Now look for a FROG signal by
running the software on a large grid size, e.g. 256x256. Once there is a FROG Trace, adjust the Time
Center to center the FROG Trace. The device is now “timed.”
Slowly and systematically walk the crystal back to the point where the beams are focused while
adjusting the spatial overlap of the beams and the Last Turning Mirror. Tighten the socket head cap
screws in the base of the crystal mount using the 2.5mm ball end hex driver.
In the Data Acquisition Panel of VideoFROGscan, set the grid size to 256 x 256.
Returning to the Summary Panel, set the Time Delay Spacing to about 2X smaller than your
approximate pulse width. Increase the Time Span using the timespan control and look for the FROG
Trace in the live view. You can also use the live view of the spectrometer to view the spectrometer
output as the servo is scanned.
The spectrometer appears to have moved during shipment
The Spectrometer is optimally positioned in the factory. If during shipping the spectrometer has shifted
position it may be necessary to reposition it. This is only for extreme cases.
1. Adjust the physical location of the Spectrometer until there is a signal in the plot of the Setup and
Alignment Panel in VideoFROGscan.
2. Follow the steps described in Section 5.2 “Optimizing the Entry of the Signal Beam into the
Spectrometer.”