User guide

LBA-USB User Guide Document No: 11294 Page 99
4.11.2 Knife Edge Method
Beam widths are computed using special algorithms that simulate knife-edge
techniques. The method employed in LBA-USB borrows from two sources:
 ISO 11146 Lasers and laser-related equipment—Test methods for laser beam
parameters— Beam Widths, divergence angle and beam propagation factor. This
document is being revised and split into three parts:
• 11146-1, Part 1: Stigmatic and simple astigmatic beams
• 11146-2, Part 2: General astigmatic beams
• 11146-3, Part 3: Alternative test methods and geometrical laser beam
classification and propagation
 IEEE Journal of Quantum Electronics, Vol. 27, No 4, April 1991 Choice of Clip
Levels for Beam Width Measurements Using Knife-Edge Techniques by Siegman,
Sasnett and Johnston.
LBA-USB offers the operator two methods for computing Knife Edge beam
widths. The 90/10 method presets the Clip% values to 90% and 10%
respectively, and the Multiplier to 1.561. These are the recommended values
based upon the above Siegman, et al. paper, and are very compatible with CCD
camera noise figures. These values are perfectly correct for computing an
equivalent second moment width for TEM00 beams, and are a good
approximation for many beams of mixed modes.
The second Knife Edge selection will allow you to program your own Clip% and
Multiplier values. This option permits configurations for beams requiring special
settings, which could open up all kinds of trouble since the options are so broad.
When Elliptical results are disabled, the computed beam widths will be aligned
with a pair of simulated knife-edges cutting one in each of the X and Y
directions. Hence, the displayed beam widths will be indicated in the results
window as X and Y. If the laser beam is not radially symmetric but does contain
two axes of symmetry, rotate the beam such that the beam's axes align with the
X and Y axes of the display.
When Elliptical results are enabled, the computed beam widths will be aligned
with a pair of simulated knife-edges cutting one in each of the Major and Minor
axial directions. Hence, the displayed beam widths will be indicated in the results
window as Major and Minor. The implication is that the displayed values
represent the major and minor widths of an elliptically shaped laser beam.