User guide

Page 54 Document No: 11294 LBA-USB User Guide
The displayed quantitative results consist of the following:
 Total energy
 % in Aperture
 Peak fluence
 Min
 Peak Location, in X and Y
 Centroid Location, in X and Y
 Beam Width, in both X and Y or Major and Minor axes
 Beam Diameter
For a detailed discussion of the above items see Chapter 3 and Chapter 4.
Beam Width Method
The Beam Width Method drop down box determines the technique used to
compute the beam width results. The first two methods (4 Sigma and 90/10
Knife Edge) are computed based on industry standard definitions. The
remaining three choices are user definable, so use care in setting up and
restoring their related options. See section 4.11 for a technical discussion on the
methods described here.
Important: Beam width measurements should never be made without performing an
Ultracal! operation. Failure to perform an Ultracal! almost certainly leads to
inaccurate beam width results.
Note: The Diameter results are always computed as the mean of the two Beam Width
results.
Select one of the five available Beam Width Methods:
 4 Sigma – Also known as the second moment method directly computes second
moment beam widths in the X and Y beam axial directions, or along the
computed orthogonal Major and Minor axes of the beam if the Elliptical
calculations are enabled. The 4 Sigma method should always be used in
conjunction with the LBA-USB Auto Aperture feature. Of the five methods, the 4
Sigma method is the most sensitive to noise. If the camera or beam noise
content is high, consider employing Frame Averaging or Statistical analysis to
hone in on more accurate results. A notable advantage to this method: it is not
influenced by mode content.
 90/10 Knife Edge – This Knife Edge method uses a fixed 90% and 10% of
energy as the moving edge Clip% points. The correction Multiplier is fixed at
1.561. These settings yield highly accurate second moment equivalent beam
widths for beams that are predominantly TEM00 in mode content, and for many
other mixed mode combinations. There are a few modes for which this method
will not be as accurate; neither is it suited well for Top Hat shaped beams. This
method computes beam widths in the X and Y beam axial directions, or along
the computed orthogonal Major and Minor axes of the beam if the Elliptical
calculations are enabled.
 Knife Edge (user definable) – If the laser mode content is not suitable for
measurement using the previous Knife Edge method, then use this method and
program your own Clip and Multiplier factors. For example if the beam is
almost a pure donut shape (TEM01*), use the previous 90 and 10 Clip% setting,
but change the Multiplier to 1.533. For a circular Top Hat, use a Multiplier of
1.455. For a square Top Hat use a Multiplier of 1.444. This method computes