The beam expander reduces the divergence.
Since the spot diameter is the divergence times the focal length.
In the beam expander, the ratio of the input beam divergence to the output beam divergence is
the ratio of the output beam diameter to the input beam diameter.
If the output beam diameter is 10 times the diameter of the input beam, then the output divergence
will be 1/10th that of the input divergence.
For the same focal length, the spot size will then be 1/10th what it would have been using the
original beam.
I once built a megawatt UV laser with an output beam diameter of about an inch.
We used a 12 inch astronomical telescope as a beam expander to limit the divergence, so we
could try to hit the laser reflector that Apollo 11 left on the moon. Counting the photons we
received about 3 seconds later, we got better than 80% confidence that they were our
reflected photons.
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Get a free science project every week! "http://scitoys.com/newsletter.html"On Fri, Mar 30, 2012 at 1:54 PM, John Griessen <john@industromatic.com> wrote:
On 03/30/2012 11:19 AM, Nathan McCorkle wrote:Yes, but you think of the focal length as negligibly short.
does the smaller focal length allow less
overall divergence to occur? As in, if the focal length were longer,
the rays would have a longer time to diverge?
You take care of whatever divergence has happened by unmagnifying it
to a small spot with a short focal length lens system.
For the original question, You choose a fatter beam to get the near in
effects where divergence varies less, see http://en.wikipedia.org/wiki/File:GaussianBeamWaist.svg
You use the fat beam in the part of the sketch called Zr
"where the origin of the z-axis is defined, without loss of generality, to coincide with the beam waist, and where
http://upload.wikimedia.org/wikipedia/en/math/e/5/8/e58a707d1ccf4ed6cefac660633038a8.png
is called the Rayleigh range." http://en.wikipedia.org/wiki/Gaussian_beam
Then, for some light source, it might make sense to expand it to a collimated beam of a certain width,
then focus down to a spot. The advantage would be that over the different distances,
(as in the distances a laser cutter mirror moves to),
the spot size would be changing less because it is in the Rayleigh range.
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