[NLRS] Light wave communications

[Doug Reed]

Hi Dave.

It is because of the dangers of lasers that there is so much hassle
about using them "in the wild." Visible red lasers are not commonly
used in light wave communications any more. The majority of new work
and new records are being made with high-power LEDs, typically in the
1 to 3 watt range, although higher power versions are available.

There are actually a number of advantages to using LED light over
laser light. The primary advantage is that the light from a LED is
already spreading and it is easy to collimate into a large diameter
beam with a simple one or two lens system. A large diameter beam is
less effected by scintillation and other optical effects over distance
so it gets through where a laser would not. This seems counter
intuitive but that is what happens. Scintillation is what causes stars
to twinkle. "Stars twinkle, planets don't."
<http://www.modulatedlight.org/optical_comms/optical_index.html>

It is possible to buy beam expanders for a laser but they are
expensive and when you're done you are still working only with a <5mw
beam. And if the police come looking, it is a laser you are shining
into the air and they have ordinances about that.... By the time you
expand a 5mw laser beam to 1" or 2" diameter, it will be safe to look
into, but I don't think I ever saw any ordinances that take that into
effect. In most areas of the country, shining a laser into the air can
get you arrested. Doing the same with an LED flashlight will not.

I've got some visible red 1W LEDs that I bought to play with. In the
long run I'll use 850nm infrared LEDs since the beam will NOT be
visible and will draw less attention. It makes the initial setup and
aiming slightly more difficult but aiming the IR light beam can't be
any different than aiming a 10GHz RF signal, and we've managed to do
that OK.... And the IR photo-diodes can be purchased with built-in IR
filters so they are less bothered by light pollution too.

As for viewing the IR light, you hit it on the head about using some
sort of digital camera. The CCD sensor in the camera usually has good
response from visible light down into the IR spectrum but the
manufacturer adds a filter before the sensor to filter out the IR and
pass only visible light. You would probably still see something close,
like a TV remote control pointed at the camera, but it may not be very
sensitive. There are a number of web sites that give suggestions and
instructions for modifying a digital camera to remove the visible
light filter and replace it with a IR-pass filter.

73, Doug Reed, N0NAS.