[Laser] Tx lens diameter.

Thu Jun 17 11:08:37 EDT 2004

John,

Unless you are planning on talking with yourself (ie, TX and RX in the 
same place), there is no guarantee that the spot size and field of view 
will be the same.   I'm not sure that a larger dot is easier to align 
from the RX point of view either--as long as all of the light strikes 
the detector you will get the same amount of signal.  It won't matter if 
it is coming from a point or a patch.  The possible advantage of 
illuminating a patch of sky is that a point would have a tendacy to 
dodge in a and out of crevasses in the cloud.  I'm still playing with 
that here too.

Andy K0SM

John Matz wrote:

>Hi
>The laser pointers I use have a "screw-in-screw-out lens for focussing the
>beam.  I can adjust it to 1 mrad or 10 m rad beamwidth with a turn of the
>lens with an appropriate flatblade screwdriver.  My receiver has a field of
>view of 10 mrad.  It would make sense for cloudbounce to make your tx spot
>the same size as your rx fov ... yes?  Easier to align ... all tx signal
>gets into rx ... as long as you don't have local sky illumination problems
>where a smaller rx fov would pick up less local light interference.
>Filters?
>John Matz KB9II
>
>----- Original Message -----
>From: <TWOSIG at aol.com>
>To: <laser at mailman.qth.net>
>Sent: Wednesday, June 16, 2004 5:28 PM
>Subject: Re: [Laser] Tx lens diameter.
>
>
>>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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