THORLABS M2MS, Operating Manual

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BP209 Series 10 Application Note
Rev: 8.1, 14-Feb-2022 © 2022 Thorlabs Page 143
with d
min
= minor axis and d
max
= major axes of the approximated beam ellipse.
Orientation denotes the angle
q
between the major ellipse axis and the horizontal X axis.
Range: -90° <
q
90°.
10.4 X-Y-Profile Measurement
Beam Width at Clip Level (xx%)
Beam width is the distance between the two points where the opposite edges of a captured
beam profile are bisected by the X or Y axis and the intensity falls to a certain percentage of the
peak power. This percentage is called clip level.
Preferred clip levels are for instance 50 % (Full Width at Half Maximum) and 13.53% (exactly
1/e²). Since the Beam Software supports a variable clip level, the beam width is always dis-
played with its clip level in brackets.
Note
Please note that "Beam Width" is always the diameter, not the radius of the beam.
10.5 Gaussian Fit Measurement
The beam profiles of coherent light sources, such as lasers and the output of fibers, show a
distribution more or less close to Gaussian. If focusing a Gaussian beam, it converges into a
waist, after which it diverges.
The Gaussian fit uses the method of least squares in order to fit the X and Y cross section
profile of a beam to a Gaussian shape. In other words, the Gaussian fit represents an
approximation of captured measurement data to a Gaussian distribution.
Gaussian Intensity is the intensity distribution of the Gaussian fitted beam profile.
Gaussian Diameter is the width of the Gaussian fitted profile at a 1/e² intensity clip level.