THORLABS M2MS, Operating Manual

Page: 76 / 167

BP209 Series 7 Operation Instructions
Page 74
21424-D02
7.1.5 Knife Edge Mode
With the 5 µm slits, a beam diameter down to 20 µm can be measured in the standard slit
scanning mode. For beam diameters below 20 µm, the BP209 models offer the so called Knife
Edge Mode in combination with the 25 µm slit pair. As the beam diameter is smaller than the
slit width, during the scan different portions of the beam pass the slit and are incident on the
photo diode. This way, the measured power increases from zero to the total beam power, re-
mains shortly at this level and subsequently drops to zero again.
Since the drum position is known exactly, the original beam profile in the X and Y direction can
be reconstructed with a good resolution.
The knife edge mode requires a proper beam alignment - the beam should hit the sensor as
close to the center as possible. Further, the Beam Profiler must be set to its maximum resolu-
tion, i.e., minimum scan speed and maximum bandwidth.