[Laser] beam expander

[Andrew T. Flowers, K0SM]

Angel Vilaseca wrote:

>Hi,
>I am afraid I don't fully understand. The narrowest beam will indeed be
>more affected by
>scattering, but if you are using a beam *expander*, shouldn't you
>actually see less flutter?
>
Angel,
The beam expander makes the beam wider, but doing so also causes the 
beam to diverge less.  For example, your laser pen might have a beam 
diameter of 1mm at 1 meter away and a diameter of 1 meter at 1 km 
distance ( = 1 mrad divergance).  The beam expander uses lenses to widen 
the beam so that it might have a diameter of 2cm at 1m, but only 20cm at 
1km (=0.1 mrad divergance).  This results in greater power density at 
the receiver *if* you can aim the beam!

What David is saying is that disturbed air currents cause the light beam 
to bend thus move the beam off of the receiver at long distances.  With 
more divergant beam, this would be less of a problem since a small 
amount of refraction should still allow part of the "wide" beam to 
strike the receiver.

Anyone would like, I can draw up a small picture in JPG format.  Just 
let me know.  I've discoved that talking optics is a pain when you don't 
have a way to draw things!  Here is a short expaination of a beam expander:
http://www.mellesgriot.com/glossary/wordlist/glossarydetails.asp?wID=105

Andy
K0SM