THORLABS BC207UV, Operating Manual

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BC207 Series Chapter 6: Operation Instructions
Page 119
MTN026962-D02
6.3.7.7 M² Troubleshooting
Below are examples of typical problems that may occur and appropriate recommendations for
resolving them.
q
The beam does not hit the sensor at all positions of the stage.
Perform a proper Beam Alignment .
q
A timeout error occurs during a measurement.
A timeout always occurs when the Beam Profiler camera does not get a valid image. This may
be caused by:
· Saturation of the camera. Since the beam is focused the intensity is the higher the smaller
the beam width. This may lead to a saturation of the camera. To resolve, attenuate the
laser beam with additional ND filters or decrease the laser output power. Alternatively use
a lens with larger focal length if possible; this will increase the diameter of the focal spot
and in this way lower the focal power density.
·
A Normal Scan is applied and the Fine Scan shall add data points. The exposure time has
then to change from long exposure times to small values. This can take more time than
the timeout allows. Set the timeout to a greater value to solve this problem (
M² Settings
).
·
The beam power is too low for the sensor. Remove ND filters or increase output power of
the laser.
· The beam size is too small and no ellipse could be calculated (if Clip Level Ellipse is se-
lected as beam width). Use a lens with a longer focal length to increase focal beam size.
· The beam does not hit the CMOS sensor. Align the beam, see section Beam Align-
ment .
q
Parasitic reflections walk over the CMOS sensor during the measurement.
This may happen, if the beam is not perfectly aligned and/or the filters are not parallel
to the CMOS chip. The laser beam is reflected from the CMOS surface back to a filter
and again back to the CMOS sensor. The position of such a double reflection changes
with stage movement.
The detection of these reflections can be improved by using the logarithmic color scale
that accentuates smaller intensities.
The scale can be switched from linear to logarithmic by clicking on the color scale.
Then perform a proper Beam Alignment .
q
The M² value differs substantially from the expected value.
For example, the beam has a nearly Gaussian intensity distribution and M² values are larger
than 1.1:
· Check set wavelength (see M² Settings )
· Check Clip Level of the Calculation Area :
4
If the selected Clip Level is too high, the beam might be cut off and the measured beam
area might be smaller than the actual beam extend. This results in a too small beam
waist and a too small M²
(even below 1).
4
If the selected Clip Level is too low, the Calculation Area and the measurement captures
the beam plus noise around the beam. The measured beam width is then larger than
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