OPHIR BeamSquared, Руководство пользователя

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and Z Step values. The user may modify these values, if desired,
before starting the run. Additional information on adjusting the
Automated Stepping values is described in section 4.1.1.
Four Cuts Method 4.1.3
The second mode operates off of a novel, but non-ISO method. This
method requires special setup conditions but in return can perform M
2
runs faster than other modes. The basis for this approach was
introduced by Thomas Johnston.
1
This method teaches that making
four well-chosen samples of the caustic provides an accurate set of
results in much less time than the standard ISO method. Extensive
testing by Ophir-Spiricon has shown that under most conditions the
results correlate well with the ISO method.
BeamSquared has modified the method by taking 6 cuts. The
nomenclature of Four-Cuts is maintained to preserve the link to Mr.
Johnston and his original thesis. 6 cuts are used to account for the
different X and Y axial results and to incorporate the direct divergence
measurement at the lens focal length. Mr. Johnston persuades that the
best place for taking the first 3 samples is between .5 and 2 Rayleigh
lengths from the waist. Experimentation has shown that making these
measurements in the far field region works just as well if not better.
If the beam is highly astigmatic, the four-cuts method has been known
to fail. In this case, the X and Y waist locations are far apart and it is
difficult to find the 3 starting cut points that satisfy the initial
conditions.
As part of the Configuration Wizard shown in section 4.2.1, the user must establish the first three Z cut
points:
 Z1 should be selected before the waist.
 Z2 and Z3 should be selected after the waist relatively close together.
 Z2 should have a smaller beam width than Z1, and Z3 should have a larger beam width than Z1 as
shown in the graphic in the Wizard (shown to the right).
These conditions must apply to both the X and Y beam width plots. This is why only low astigmatic beams
work with this method.
The Z locations for the first 3 cuts are entered into the configuration dialogs. These positions are used to
predict the location of the X and Y waist locations, cuts Z4 and Z5. Measurements are performed at these
locations and a final sample is taken at Z6, the lens focal length. A least squares hyperbolic fit to the data is
performed and the final M
2
results computed.
1
Published in
Beam Propagation (M
2
) Measurement Made as Easy as It Gets: The Four-Cuts Method
published in the
journal
APPLIED OPTICS
/ Vol.37, No.21 / 20 July 1998.