Aligna
®
4D User Manual
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fluctuates, the clipped amount of light will vary, which leads to an apparent position fluctuation.
This will be (wrongly) compensated for by the servos due to a position correction.
"AutoGain" forces an automatic setting of the PSD gain values, so the measured intensity
values are in a valid range (e.g. 1 Volt ... 5 Volts). (Normally the target intensity is in the middle
of "min Intens" and "max Intens")
"AutoZeroA" forces an automatic PSD offset setting of angle detector "A", so the actual beam
angle position is defined to be the "0,0"-Position.
"AutoZeroB", same for detector B.
"Auto All" is the same as clicking "AutoGain", "AutoZeroA", and "AutoZeroB"
"reset" sets all four PSD offset values to "0". ( thus the mid of the detectors are displayed
as "0,0,0,0")
(The gain values are not influenced by "reset".)
"save" writes a text file, including the actually set PSD gain and offset values (as well as some
other parameters). This can be inserted into the "User Script" file, which defines the device-
specific parameters. They are used as "Default Values" (when "Default" key is held while
booting. Watch LCD while booting)
Explanation: The PSD offset values define the (4D) precise target pointing for the stabilized
beam. There is no need to align the test beam exactly to the PSD centers (of PSD A and PSD
B). The PSD offsets have to be saved to stabilize exactly to the wanted 4D position, even after
selecting "Default values".
The PSD gain values also depend on the individual optical setup (beam sampler) and laser
property (intensity, polarization,...). If the PSD gain values are saved in the "User Script" the
system will start (even after selecting "Default values") with the correct fitting PSD gain values
without the need of setting these parameters.
Note: Of course the test beam spots should hit the detectors without clipping distinct parts of
the spot (otherwise the measured position is incorrect). The PSD offset values are given in %
of the detector area. "+/- 50%" is the total detector area in X and Y direction. If an X=0 / Y=0
position is achieved only with an offset value of 40%, that means the detector is hit very close
(10%) to its border, and parts of the test beam will not hit the detector (especially for the posi-
tion detector B, which is normally not focussed on).
With small beam diameters (compared to the detector size of, e.g., std. 9x9 mm), the offset
values should be smaller than +/- 30%, with large beam diameters (6 mm at a 9x9 mm detec-
tor, e.g.) the offset values should be within +/- 10%, which requires a better mechanical pre-
alignment, for hitting the detectors better in its mid region.
"LowPass" switches on and off a low passing of the position measurement: Especially in the
case of low repetition rate pulsed lasers, but also in the case of noisy cw lasers there are
pointing fluctuations, which cannot be compensated for:
Shot-to-shot fluctuations cannot be compensated by principle, because the laser pulse is
much shorter than every mechanical servo reaction time, by many orders of magnitude. Thus,
the servos can only adjust the mirror positions for SUBSEQUENT laser pulses, never for the
actual one. Thus, the shot-to-shot noise will remain by principle.
Even in cw systems, there may be fluctuations or vibrations, faster than the servo bandwidth,
so it cannot be compensated for.
In this case, the motors will work continuously, trying to compensate for the noise, which is not
possible. Therefore, it is better to filter out these fast contributions by the Low Pass Filter,
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