[Laser] Re : "How to" page for laser pointer communications now online

[f1avyopto at aol.com]

 Hello Clint.
Thanks for your very interesting page about the use of laser for optical communication through the air.
I do not want talk again about differences between lasers and LEDs because these points have been very well commented.
However one of the very great lasers advantage to LEDs is their ability to illuminate a very far away rather small target without overflow.
If the optical receiver can concentrate to the photodetector all the laser illuminated area picture, the direct beam energy is nearly conserved to the target and the losses are only due to the atmosphere absorption into this first beam.
If the target has a good albedo like snow for a red laser or a grassy hill for a near infrared laser, the diffused bounced light can be used to built a low rate communication not line of sight link.
>From the target relay to the receiver, the 1/D^2 classical energy decreasing law applies and because the current response of a photodetector, the received power reduces versus a 1/D^4 law.
It is the same law for a point to point laser link but with a bouncing relay it is only from the target to the RX ! 
Because the illuminated target receives all the laser beam, its area integrates very efficiently the laser light decoherence scintillation.
The bounced signal contains then only the laser modulation with a very low extra noise. 
The signal is weak but perfectly stable.
The following example is from a 4 km not line of sight laser path with a low power 800 nm laser (CD R/W laser diode).
The bouncing relay is a low albedo factory chimney :
http://sd-1.archive-host.com/membres/up/22679775843705539/F5PNP.wav
That means also it is possible to use very sensitive FFT programs to extract a very weak signal as JASON (-40 dB) or WOLF-GUI (-45 dB).
By this way we succeeded a 50 km not line of site com via a grassy hill as relay with a only 40 mW IR laser;
The aiming to the target can be do easily with the following system.
It needs a low cost video camera.
The parallax is not a problem because it becomes really negligible beyond a few kilometres.
On the attached example we had used a pencil mark on the screen displaying the aiming camera pictures.
http://sd-1.archive-host.com/membres/images/22679775843705539/laser_aiming_process.jpg
By moving the laser and its rigidly fixed infrared camera from the church tower to the woody hill located around 7 km, both spots stays close by together and very stable.
This system is very powerful.
You can reach any target as far as you want with a very great accuracy because the pencil spot is the point where the laser is present without to see it.
A low cost analogic IR MOS camera has a 0,4 lux sensitivity with its 3 mm native objective.
But if you use it with a 10 cm high quality lens (F8DO laser in appendix) you become able to get clear pictures nearly by full night !
I hope these details can give few ideas to do some exotic experiments :o))
I am sorry for my always so poor English.
73
Yves F1AVY
http://f1avyopto.wifeo.com/


 

-----E-mail d'origine-----
De : C. Turner <turner at ussc.com>
A : laser at mailman.qth.net
Envoyé le : Mardi, 13 Avril 2010 20:48
Sujet : [Laser] "How to" page for laser pointer communications now online

Hello,

Although my main interest has been the use of high-power LEDs for 







optical through-the-air communications over long distances, our 







testing/experimentation has also included the use of lasers - including 







cheap <$3 laser pointers.  Even though we still think that LEDs are 







better for such things, given the ubiquity of laser pointers and their 







relative ease-of-use, there is no reason to ignore them.















Because we already have built highly-sensitive optical receivers - and 







in our LED experiments we have already set them up (that is, pointed 







them) it is fairly easy to throw into the mix some experiments with 







laser pointers - and having already-aimed receivers makes things simpler.















Having done these sorts of experiments/tests for several years means 







that we have done our share of hitting our heads against the wall and 







(hopefully) learning what works - and what doesn't.  This has allowed us 







to develop various techniques and build a few simple pieces of equipment 







that help in the setting up of such links.  We now have it so that we 







"routinely" set up 2-way laser pointer to laser pointer contacts over a 







20+km distance without much difficulty and have, on several occasions, 







set up 1 and 2-way laser-pointer communications over a distance of 







greater than 172km (107 miles) - which, we *think* is the current 







distance record for terrestrial 2-way voice-on-laser communications.















The URL of this page is:  







http://modulatedlight.org/optical_comms/using_laser_pointers.html















As with other pages at the site, this one is often being updated, so 







things will be added/changed from time-to-time, so feel free to send a 







comment (using the link at the bottom of the page) for 







suggestions/changes.  Of course, there are other ways to do things 







besides what has described - and if you are successful in doing so - let 







us know!















73,















Clint







KA7OEI















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