[Laser] Tx lens diameter.

[Richard Burrows]

Andy

Thanks for that,food for more thought.

I have attempted last winter cloud bounce using 6 x cd ir lasers each with
their own 1 inch lens.Within my own garden I could regularly send PSK31
between two PC's upto +20dbs above noise floor.I convinced myself that the
signals were coming back from clouds since when hole appeared in clouds the
link  was not there and came back when clouds returned.So I and another
Amateur  G8DRK tried a 12km bounce.Far too confident, nothing ,we then tried
a 3Km bounce and still could not get a link.At that point we gave up.The
equipment used for these tests were the K3PGP front end fed from a 10inch
dia very shiny suacepan lid (yes) with Spectran SW,for the field trips the
Tx was a Pic based CW beacon 560Hz into 6x5mW IR cd lasers each with 1"
lens, it was difficult to align them to single spot so they did cover larger
area.I used tv mono camera to view IR on distant chimney pot
As I said  in the garden no problem even with Rx to Tx spaced many yds away
from each other and shielded.
So was I picking up the out going beam that was illuminating the moist air
rather than the cloud.I do not know for sure but i suspect I was.
So not really giving up just rethinking the solution.

Richard   G8BYI

----- Original Message ----- 
From: "Andrew T. Flowers, K0SM" <aflowers at frontiernet.net>
To: "<font color=red><B>== Free Space LASER Communications ==</font></B>"
<laser at mailman.qth.net>
Sent: Wednesday, June 16, 2004 5:14 PM
Subject: Re: [Laser] Tx lens diameter.


> 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
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