[Laser] Tx lens diameter.

[TWOSIG at aol.com]

I think that the short answer to your question is that a large collimating 
lens is not better than a small one.  

The collimating lens takes the light rays comming from the laser diode in a 
broad elliptical cone and bends them into a narrower elliptical cone.  The 
intent is to get the rays to be fairly close to parallel.  If the lens is close to 
the diode, the beam will not have expanded much, so a small lens will capture 
all of the rays.  (This is where most laser pointers put a making disk with 
an even smaller round hole in front to form a round dot.)  If you put the lens 
further away from the diode, it will need to be bigger because the 
uncollimated light has expanded before it gets to the lens.

As a practical matter, you cannot get a perfect beam, and for cloud bounce 
you probably would not want to.  If you shift the lens a little from "perfect" 
the beam will not converge completely to parallel, or it will converge too much 
comming to a smaller patch a short distance from the lens.  Either way at 
long distance the beam will be expanding, but how much it expands is something 
you can control.

I would compare "antenna gain" of a system to the beam expansion, not to the 
diameter of the lens.  Lets say that you have two lasers set up, one with a 
10mm lens and the other with a 100mm lens.

If you set them up with "perfect" collimation they would put a 10mm and 100mm 
dot on a target 1km away.  If you use a light meter with a field of view of 
less than 10mm, then the small dot will show 100 times brighter than the big 
dot.  If the light meter has a field of view of more than 100mm and you include 
the entire dot, they will be equally bright (assuming they came from equal 
lasers).  "Gain" doesn't make much sense from the transmitter perspective, 
because it will depend more on how you measure.

Set the two lasers up so that they each form a 10m on our target.  Now I can 
say that the two transmitters have the same gain.  Change one of them to a 5m 
dot, and I will say that it has 6dB gain over the other one, unless my light 
meter has a field of view of more than 5000mm!


There is a good reason to use a larger lens when the laser diode emits more 
power:  Beam density.  It is a safety thing.  A laser pointer at 0.1mW and 5mm 
diameter is probably safe (somebody please check those numbers), but a 10mW 
laser would need to be 50mm diameter to give the same safety margin.  You may 
intend to expand the beam to 10m per km for cloud bounce, but you don't want a 
hazard anywhere.  Safety First.  Safety Always.

If you are thinking about using a laser assembly that already has a 
collimating lens, I would suggest you use a beam expander, which is a simple telescope. 
 That way you do not need to adjust the collimating lens.

James
N5GUI






In a message dated 6/16/2004 2:26:33 AM Central Standard Time, 
r.burrows at blueyonder.co.uk writes:
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
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