[NLRS] Light wave communications

Sat Mar 16 12:15:05 EDT 2013

Hey Doug and the group,

That was a great posting, Doug.  Lots of interesting information there.  I
have also done a bunch of reading in the last couple of years on light
communications.  I've also looked into the use of laser diodes for
illumination during nighttime coyote hunting, using a digital camera
connected to a rifle scope to see the beam, since the camera can "see"
infrared (and can be used for aiming a comm link too.)

One thing that has spooked me a little is the possibility to literally "put
your eye out, kid".  These more powerful lasers can do serious eye damage
and it would be easy to sweep your eye or even accidentally catch a
powerful reflection, especially from a laser that's not in the visible
spectrum.  So just a word of caution on that.  But I do hope that hams can
do more and more with this technology, because it's now so readily
available and makes homebrewing equipment very simple and inexpensive.

73,

David  W9HQ

On Sun, Mar 10, 2013 at 3:59 PM, Doug Reed <n0nas at amsat.org> wrote:

>
>
> Hi gang.
>
> I've sent a number of emails in the last year with information about
> light wave (optical) communications. In the past we'd have been
> talking about using lasers for communications, and the ARRL contest
> rules used to be written that way. The current rules specifically
> mention LEDs and still require electronic amplification on the receive
> side.
>
> "1.12. Above 300 GHz, contacts are permitted for contest credit only
> between licensed amateurs using mono-chromatic signal sources (for
> example, laser and LED) and employing at least one stage of electronic
> detection on receive.  Laser usage is restricted to ANSI Z136 Class I,
> II, IIa, and IIIa (i.e.; output power is less than 5 mW)."
>
> Since most local and regional governmental organizations prohibit
> shooting lasers into the air, LED light sources are a much "safer"
> bet. In addition, they are easy to expand and collimate for best
> reception at a distance. The 2005 amateur radio optical communication
> record in Australia was based on 1 watt red LED diodes and page-size
> Fresnel lenses and reached over 100 miles.
> <http://www.modulatedlight.org/Modulated_Light_DX/MODULATED_LIGHT_DX.html>
>
> The large beam diameter also reduces the amount of signal loss due to
> scintillation in the atmosphere. The web pages by Clint KA7OEI are the
> best US source of information on amateur optical comms that I've
> found....
> <http://www.modulatedlight.org/optical_comms/optical_index.html>
>
> Many more links on this web page: <
> http://www.aladal.net/toast/comlinks.html>
>
> Within the last few months, some German hams tested a fast-scan ATV
> video link over a 50 mile path using 20mw IR lasers and photo
> detectors. They couldn't use visible red light because they were
> shooting along a frequently used air corridor between the two
> sites....
> <http://www.darc.de/index.php?id=24358>
>
> And while line-of-sight is a good idea and was required for the
> longest distance records, it isn't a requirement! Just like we do rain
> bounce and snow bounce at 10GHz and 24GHz, for light comms, we can use
> cloud bounce and atmospheric scattering to extend range beyond your
> local obstructions. If you look around a bit you will find
> documentation of laser light bounce and scatter tests that were run
> over 15 years ago.
> <http://www.k3pgp.org/laserscatter.htm>
>
> I would tend to say that cloud bounce is going to be short range if
> you think about low-altitude storm clouds. But if you think of those
> high wispy clouds you get on a clear summer day, those should be good
> for some pretty long distance....
>
> "Scientists at the International Telephone and Telegraph Federation
> laboratories in New Jersey have tried using clouds as the reflecting
> medium. They found that the typical cumulus clouds can scatter the
> beam to a receiver more than 150 km. from the transmitter."
>     From "LASERS, Tools of Modern Technology", 1968, Page 82.
>
> A better choice might be atmospheric forward scatter from dust and
> other particulate matter in the air when you are shooting near the
> horizon. If nothing else, there are ice particles in the troposphere
> 30 miles up.... The K3PGP link mentions tests he ran out to 26 miles
> using digital modulation of a 5mw laser light beam in 1997.
>
> Regarding the choice of LED diodes, if you look on Ebay or in any
> catalog, you will find that high-power LEDs are available in just
> about any color you could want. Most LED comm work has been done using
> red LEDs near the same 650nm wavelength generated by most laser light
> pens. I'm sure you've all read stories about people getting arrested
> for shining a laser pointer at car drivers or aircraft pilots. How
> would you like to be the one shining a very bright 8" diameter beam of
> red light into the air from our favorite Mounds Park hill above the St
> Paul downtown airport? How long do you think it would be before the
> police arrived? Other than for some short range testing, I think I'd
> like to avoid visible light if I can.
>
> If you avoid visible light, your next choices are infrared: 850nm,
> 940nm, and 1550nm. 940nm is the middle of an absorbtion peak so we
> don't want to use it. 850nm is the most commonly used wavelength for
> amateur light comms. 1550nm is the wavelength most often used for
> "free space optical data links" and might be the best wavelength
> overall, if you can find the parts. Most 1550nm equipment on Ebay is
> for fiber optic use. But 1 watt and 3 watt 850nm LEDs are easy to find
> on Ebay at quite reasonable cost. The primary difficulty with
> non-visible light is trying to verify you've adjusted the focus
> correctly....
>
> Another good thing about using 850nm infrared light is that the
> receiver photo detectors can be purchased with a built-in infrared
> filter to block at lot of the ambient visible light interference. If
> you check the catalogs you will see that most detectors are available
> with and without the "F" filter option and the only difference between
> them is the F model is restricted to infrared light and the visible
> spectrum is attenuated. That improves your receiver sensitivity, just
> like putting a bandpass filter ahead of your HF/VHF radio allows it to
> shrug off strong out-of-band signals.
>
> What I'd like to suggest is that if you are interested in playing
> around with optical communications, you plan to use 850nm in the long
> run, although I admit it is a LOT easier to start with common red LEDs
> when first playing around... If you start with a red LED for transmit
> and a visible light photo detector, you can change to a 850nm IR LED
> very quickly and either put a IR filter ahead of the photo detector or
> replace the detector with the "F" version. (Did you know that the
> black portion of a 35mm black & white film negative makes an excellent
> IR filter?) But don't forget that IR light will focus at a different
> distance than the red light so you will have to adjust the diode mount
> when you switch to the IR diode.....
>
> As for the communications mode, perhaps the easiest is CW with on-off
> tone keying of the TX LED. That is effectively what Gary WB0LJC and
> Bob W0AUS and everyone else were doing 20-40 years ago with
> chopper-wheel modulated Helium-Neon lasers..... Most current
> experimenters are using some sort of voice modulation: baseband, PWM,
> sub-carrier FM, or SSB. What I prefer is to stay with a FM mode that
> can use on-off keying of the LED for maximum signal amplitude.
>
> If you are already playing with any of the sound-card software
> programs like FLDIGI, you already have most of a very capable digital
> communications platform running on your computer. You could very
> easily connect it to a LED transmitter and photo-detector receiver to
> do optical communications over a very respectable distance.... Other
> experimenters have shown that even a simple optical system using a 4"
> lens can make Q's longer than 30 miles.
> <http://www.earf.co.uk/nanotrx.htm>
>
> I'd like to consider using PSK31 or a similar digital mode for the
> modulation. We have the whole response band of the photo detector to
> work with, but lets limit ourselves to the range below 25KHz. This
> gives us 20+KHz of bandwidth and would let us have a lot of QSO's
> going on. With software like Multi-PSK or PropNETPSK, your detector
> would pick up any beacon signals within its field-of-view, and the
> software would decode and display them. PropNETPSK even has a robot
> QSO mode where it beacons CQ CQ CQ DE N0NAS N0NAS and will
> automatically work any station that replies. Or if robot mode is off,
> it still transmits beacons on a regular basis and logs any beacons it
> hears.
>
> For a real silly-season idea, how about mounting your light comms
> equipment to an AZ-EL rotor, run the ADSB software on a DVB-T dongle
> to get real-time location info on airliners up to 250 miles away. Plug
> the location into the 10GHz rain scatter program to predict when the
> plane is in position for forward scatter to a distant station. Let the
> software point the rotor and start sending a beacon.... This might
> even work pretty well with sound-card meteor scatter software.... We
> don't have to hit the plane directly, all we need is to hit the
> contrail behind the plane for a few minutes in order to make the Q.
>
> By the way, if you want to try this for 10GHz airplane bounce,
> coupling the ADSB plane beacon and the rain scatter program would
> probably work to predict when a plane would be in the desired path.
> You can find a good site that talks about "Aircraft Enhanced
> Propagation" at this site:
> <http://www.qsl.net/vk3bjm/ads-b.htm>
>
> If you search some of the ADSB software links, you will find that the
> ADSB software can be configured to forward its "catches" on the
> Internet. There are web sites that collect the info like the APRS
> sites and it is possible to get data from remote user's so this could
> be a full-blown path prediction system.... I know there was discussion
> in the past about trying 10GHz plane scatter but the problem was
> knowing when the plane would be in position. With the ADSB receiver
> software, that problem goes away..... Just something to think about...
>
> I was using ADSBscope for display software, with the ADSB# (ADSBsharp)
> receiver software for the R820T DVB-T dongle. Less than $20 worth of
> hardware.... It will work quite a bit better with an outdoor
> antenna.....
> <http://www.sprut.de/electronic/pic/projekte/adsb/adsb_en.html>
>
> 73, Doug Reed, N0NAS.
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