[Laser] Tx lens diameter.

[Andrew T. Flowers, K0SM]

Richard,

The gain essentially comes from increasing the power density at the 
receiver.  Going from a 10 mrad beamwidth to a 1mrad beamwidth should 
gain you 20dB, assuming that the receive optics smaller than the beam 
diameter at the receive point. (Somebody check that--I just got up from 
a nap).   I'm not convinced that this is the way to go for cloudbounce. 
 I tight beam will jump around quite a bit as it ducks in and out of 
ripples in the clouds.  Maybe you want to diverge your beam so that it 
illuminates a patch of cloud instead.  As for signal strength, That 
doesn't seem to be an issue with the little 2mw pointers I have here.  I 
can hear the spot fine with a 4" lens on a my PGP front end.  I built a 
beam expander out of PVC pipe that allows for adjustment of the 
divergence, if you want to play around with that:

http://mail.rochester.edu/~af006m/laser.html

Keep in mind that you'll need to test your expander using a very distant 
target.  A perfectly collimated beam should have a diameter of 6" right 
in fron the of the transmitter, and 6" a mile away.  I used a white 
factory building 3/4mi away and a 20x spotting scope to make my 
adjustments.  Keep in mind that aiming something like this on a LOS path 
will be pretty much impossible.  

I hope that helps!
Andy K0SM/2

Richard Burrows wrote:

>Hi
>Does anybody have thoughts on the merits or otherwise of using  large diameter Tx lens for laser cloud bounce.My initial belief being that above say 1inch dia. for a collimator lens if used with a laser diodeno advantage woul be gained.However If you consider the lens as an antenna then using a 6inch dia. lens should increase the intensity (ant. gain). 
>True or not?
>When using a white led with 6inch lens I get very intense spot, but leds do  have a wider exit angle than a laser diode.So I'm not really convnced I'll get anymore intensity, unless I use several lasers at the focal point.
>
>I note that the beam expander lens used are larger in dia. than collimator lens placed  before them.
>
>Richard - G8BYI
>_______________________________________________
>Laser mailing list
>Laser at mailman.qth.net
>http://mailman.qth.net/mailman/listinfo/laser
>
>