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FocusParameterMonitor FPM
11.4.3
Setting the gain
Historically, the optical amplification is set in the LDS. With the FPM, the exposure time of the CCD sensor is
set here.
The exposure time depends on the power density of the laser. The power density changes with the laser
power and the beam diameter. When measuring a caustic, the beam diameter and thus also the power density
change along the direction of propagation of the laser.
The set amplification therefore depends on:
laser power
beam diameter
position in the beam (along the direction of propagation)
The level of the signal should be around 3000 counts. This will give enough leeway so that the signal does
not immediately overshoot or underdrive when changing the parameters mentioned.
From 4095 counts the signal is overdriven, under 2000 counts the signal is underdriven.
To check the level control, use the detailed view of the level control.
Open the level detail view:
The following window opens:
Fig. 11.2:
Detail view of a layer with scaling
In the detailed view the color scale of the displayed level can be seen on the left-hand side. The scaling is
always scaled to the highest level. The upper end of the scale should be 3000 counts.
If the modulation is not in the desired range, adjust the gain and repeat the level measurement.
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Revision 06 EN 02/2023
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