THORLABS M2MS, Operating Manual

Page: 17 / 167

BP209 Series 5 Operating Principle
Rev: 8.1, 14-Feb-2022 © 2022 Thorlabs Page 15
5 Operating Principle
The BP209 Series uses the principle of scanning slits mounted on the perimeter of a rotating
drum. This drum is equipped with a position encoder, which delivers exact information on the
actual drum position to the analyzing software. Two silt pairs (5 µm slit width for normal mea-
surement and 25 µm slit width for knife-edge measurement) are mounted to the drum.
The slit pairs are oriented orthogonally at 45° and -45° with respect to the rotation axis.
This scanning axis is tilted at 45° so that the scanning directions of the slits appear as 0° (hori-
zontal) and 90° (vertical), respectively. These scanning directions are marked as X and Y on
the front of the instrument.
Using the rotation mount these scanning axes can be tilted to within ± 60° in order to adapt it to
the major and minor axes of an elliptical beam.
Beside the two slit pairs, the drum has an aperture with a neutral density (ND) filter. This aper-
ture is used to take integral power measurements during every revolution.
Note that this power measurement readout is the result of a separate integral beam power
measurement (through the ND filter), and not a result of mathematical integration across the
scanned beam profile.