Near/Far field Alignment
This might help explain Near/ Far field Alginment for those interested. This was given to me a while back.
As you probably know, the "far field" of a laser beam is
unambiguously defined as the intensity and phase of the beam when it
is propagated to an infinite distance from the laser.
That infinite distance can be conveniently accessed simply by placing
a positive focal length lens into the beam and viewing the resulting
irradiance at the rear focal plane of the lens. Unfortunately, the
resulting physical scale of the pattern is not convenient for viewing
without special purpose hardware.
I remedy that simply by projecting the far field on the wall of my
laboratory with a second, short focal length lens. The far field
pattern can them be easily viewed, and photographed with an ordinary camera.
As shown in the attached figure, I used a 200 mm focal length lens to
produce the far field pattern, and then projected that pattern on the
wall with a 25 mm focal length lens. The projection distance in my
setup was 15 feet (4.57 meters). Since that distance is so much
larger than the 25 mm focal length, it is essentially at infinity as
far as that lens is concerned. Thus, the 25 mm lens was axially
positioned with respect to the 200 mm lens to put their focal points
in coincidence. Physically, this is done by determining the distance
between the facing surfaces of the two ThorLabs LMR lens mounts that
corresponds to the focal points being in coincidence, and then
spacing the lenses to achieve that distance between the LMR surfaces.
The size of the far field pattern is then simply (200/25) times the
divergence of the laser times 4.57 meters, or 36.6 mm per mRad. A
target pattern was made up at that scale and was marked "Far Field of Beam".
The "near field" of a laser beam is less well defined. It is often
considered simply as the intensity and phase of the beam as it exits
the laser aperture, but for more precise work, a better place to
define it is at the waist of the beam, wherever that may lie. For
precise determination of the quality of a laser beam, it is important
to determine exactly where the waist of the beam is, and for real
beams, the waist locations may differ in the "X" and "Y"
directions. That is the case, for instance, with the red laser that
I currently have for sale.
Here, I don't worry about any of that. To visualize the near field,
I simply remove the 25 mm lens from the setup shown in the
figure. Then the 200 mm lens is essentially projecting the laser
beam that exists at a plane near its front focal plane onto the
wall. Thus, I am defining the "near field" to be the beam that
exists at the plane that is coincident with the image of the wall as
formed by the 200 mm lens. Working this out, the "near field" plane
turns out to be 211.2 mm in front of the lens. The magnification of
that plane on the wall is then about 22.7, so a target pattern was
made up at a scale of 22.7 mm per mm of beam diameter and was marked
"Near Field of Beam".
Now, the original purpose of setting up this system was not primarily
to characterize the parameters of a laser beam, but was instead to
make it easy to align several laser beams (think red, green, and
blue) to a single axis. To align the beams, it is necessary to bring
them together in 4 degrees of freedom. In other words, one must
align them both in the near field and in the far field. An easy
method of switching back and forth between near and far fields was
needed, and this is what I came up with. One need only to remove and
replace the 25 mm lens to switch between viewing planes, and this is
easily done once the system is initially aligned with both lenses in
place, and with the post holder for the 25 mm lens fixed in position.
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