Re: how to deliver and focus

In article <1127205953.010350.201820@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
"Marco" <marco@xxxxxxxxxxx> wrote:

> I have to deliver a CO2 laser beam (100W, 4mm diameter, 4mrad
> full-angle divergence) on two targets getting the same spot size. The
> fisrt target is located 1 meter far from the source, while the second
> is 20m away! On each target I have a single lense for focusing the
> beam. In between there are also other target, but what count here are
> the the first one and the last one. I supposed to place a 5x beam
> expander just after the source to enlarge (better for focusing) the
> beam size and to have a lower divergence, but 0.08mrad of divergence
> still force me to have a too big beam size on the 20m target, even
> bigger than my focusing ZnSe lense. I was thinking to displace to each
> other the 2 lenses of the beam expander, in order to have a behaviour
> like a low positive lement instead of an afocal element, to compensate
> the divergence with this pre-focusing. My question is, are there other
> better solutions? Doing this way could I reach a good spot size, i.e.
> lower that 200um?
> All your help is really welcome!
> Thanks, Marco
>

Displacing the expander lenses will work quite well. As I calculate it,
a 5X expander can be made to produce a beam size of 20.2 mm at both 1
meter and 20 meters from the original beam waist at the laser. You just
have to set the expander to put the expanded waist (18mm) 10 meters away
from the laser. The focused spot will be 200 microns if you use a 254 mm
focal length focusing lens; much smaller spots may be produced if you
use shorter ones.
.

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