help: calculate laser beem diameter

[Leonard Migliore]

In article <31F84C...@ibm.net>, Guy Vardi <yva...@ibm.net> wrote:

> Hello all!
>
> How can I calculate the diameter of a laser beam. I need to find out
> what will be the size of the beam after it pass a known length.

This is a can of worms. In theory, if you know the beam size at any
location, along with the distance to the waist and the divergence, you can
calculate the diameter at any other location. For a circulary symmetric
beam,

D**2 = D0**2 + Theta**2 * Z**2

(ASCII is real great for equation typesetting)

Where D is the diameter at a distance Z from the waist, D0 is the beam
diameter at the waist and Theta is the divergence. The problem is in
actually measuring these values. I babble for about 16 pages in my book
about this problem. Laser beams are very fuzzy, and the size is often
dependent on the measuring instrument. Another issue is, why do you want
to know the size? If it's for calculating focal spots, you'll probably
want the 1/e**2 diameter. If it's to let the beam through without getting
diffraction, you'll want twice or three times that.

--
Leonard Migliore
Laser Kinetics, Inc.

[Guy Vardi]

Hello all!

How can I calculate the diameter of a laser beam. I need to find out
what will be the size of the beam after it pass a known length.

With many thanks,
Guy vardi

[Don Stauffer]

I assume you are not passing it through any further lenses. Thus, it is
a "simple" case of gaussian beam propogation. You need to know the size
and position of the waist if the beam is being "focused" beyond the lens
if there is a lens after the laser. If there are no lenses, just the
laser, you need to know the waist diameter of the laser. In this case,
if the laser is of high "quality", the angle of spread is basically the
diffraction limit for that waist diameter and wavelength. Look for
references on "gaussian optics" or "gaussian propogation."

Don Stauffer in Minneapolis
stau...@htc.honeywell.com