User guide

Page 108 Document No: 11294 LBA-USB User Guide
4.21.1 The Focal Length Method
This method is based upon the beam width of a focused beam’s spot size and
the focal length of the focusing optic. Divergence results will be computed in
the X and Y aligned axes of the beam if Elliptical results are disabled, or for
Major and Minor axes beam orientations if Elliptical results are enabled.
The Focal Length divergence method provides a means for finding the far-field
beam divergence at any point in the beam propagation path. As shown below,
the calculation performed by LBA-USB is quite simple, however the optical setup
must be done with great care. The user to suit his particular application must
provide the optic. The focusing optic must be large enough to accommodate the
input beam, without introducing diffraction effects. You can use either refracting
or reflecting focusing optics, but in either case, you must place your camera’s
detector at the exact focal length of the optical element. The Divergence result is
based upon the focused spot size as described in the equation below:
divergence
W
f
f
=
⎛
⎝
⎜
⎞
⎠
⎟
−
tan
1
Where: W
f
= The width of the focused spot at distance f from the
optic.
f = The focal length of the imaging optic at the
wavelength of the laser.
If you are not already versed in the theory behind the Focal Length method,
we recommend the following reference document:
Laser Far-Field Beam-Profile Measurements by the Focal Plane Technique , by
G.W. Day and C.F. Stubenrauch, NBS Technical Note 1001, March 1978. This
publication is no longer in print. A copy can be obtained from the Spiricon Sales
or Service department.
4.21.2 The Far-Field Method
This method is based upon the actual measured increase in laser beam width as
it expands in the far-field region. Before using this method, be sure that your
measurements will be made in the beam’s far-field region, and the size of the
beam does not grow larger than your camera’s ability to contain it. Divergence
results will be computed in the X and Y aligned axes of the beam if Elliptical
results are disabled, or for Major and Minor axes beam orientations if Elliptical
results are enabled.
Note: We strongly recommend that you do not use the Elliptical mode when using the
Far-Field method, but rather rotate your camera to bring the axes of the laser into
X and Y alignment.