[Laser] misc light comm

[laser@mailman.qth.net]

Hmmm.  Lots of good idea sharing about the ARRL rules and how to encourage 
more activity communicating with light.  

Can't say I understand any better how a laser diode really works, but my MCW 
555 & laserpointer still works when I connect the battery.  Reading one of the 
descriptions of photons bouncing between partial mirrors, I wondered why LDs 
radiate in both directions ( Can't they use a full mirror on one
side?  Wouldn't that put more light in the desired direction? )  I also 
wondered if squeezing the semiconductor chip would change the color of the output 
light, and could that be used to frequency modulate the beam? If you did that, 
how would you demodulate it at the receive end?

To encourage more activity in light communications, I thought that there 
should be competition between school classes to build and demonstrate light 
communications devices.  Considering age and skills, flashlight bulb or mirror 
devices for elementary students, LEDs for middle school students, and low power 
(laserpointer) laser diode for high school students.  Separate catagories for CW, 
digital, and voice systems.  Lots of potiential to incourage light 
communications and interest in science.  (OK, I confess.  I used to teach Math and 
Science.  Things that are fun can be educational.)

The comparison of LEDs and LDs to spark gap and quartz crystal transmitters 
got me to thinking that the spark gap compares better to an incandecent bulb 
where most of the energy goes off in heat instead of
visible light (wasted broadband signal).  The LED would be more like an LC 
controled oscillator (coil and capacitor) that changes frequency with 
temperature, humidity, vibration, vastly superior to the spark gap, but not suitable for 
all applications.  The laser diode then compare to the crystal controled 
oscillator, not perfect, but much more stable than an LC circuit under most 
conditions.

A really good comment was that making a light band contact ( turning back to 
ARRL contests) is not as simple as pulling a laser pointer out of your pocket 
and sending CW.  Maybe there is a goal  here to shoot for:  Making a laser 
contact between stations should be as easy as picking up a pair of binoculars and 
scanning the horizon for another station.  It certainly should be no more 
complicated that taking a small telescope out of a case, set up on its tripod, 
and viewing the moon, a planet, or stars.  A Rover, I should be able to park on 
the top of a hill, set up a telescope with attached laser transmitter and a 
receiver sensor in the eyepiece, then scan the surrounding hills (In some parts 
of Kansas, that is tough).   The task would be easier with a compass and map 
of other popular rover sites.  I can make my station easier to "find" if I put 
up a reflector.  It may never be as easy as taking my Handie-Talkie out of my 
pocket and scanning for an active local repeater.  Still, there are some 
similarities - if I can hear someone, but not talk to them, I need to improve my 
signal - preferably by getting more of my signal in the direction I want it 
(change the antenna from a rubber duck to a 1/4 wave or 5/8, maybe go to a beam).  
I should be able to tell that my signal will be better because I will be able 
to hear them better.  As a last resort, I should raise my output power.  If I 
can't hear anybody when I know they should be there, then I need a better 
antenna, or better receiver.

The comparison of today's light communication technology with the 1890's 
Heliograph shows some areas for improvement.  The beam width of a heliograph is a 
half of a degree, not quite 9 milliradians.  The light source for the beam 
moves across the sky at more than 4 milliradians per minute.  In 120 seconds, or 
less, the equipement has to be adjusted to track the sun.  The receiver for a 
heliograph is the human eye, possibly aided by a small telescope.  The 
"receiver" has to operate on signals which are in full daylight.  The heliograph was 
designed to be transported on a pack horse.  It was built for and used by the 
military.

My Ramsey LBC6 sends a beam more than 3 milliradians  - it more than covers a 
stop sign that is 800 feet away and 2.5 feet across.  I can stand in my yard 
and point the LBC6 transmitter at the stop sign.  My hand is steady enough to 
hold the beam on the receiving unit held in front of the sign.  If the wind is 
not blowing, a camera tripod is steady enough to hold the beam completely on 
a 3 mR target.  

The field of view of the receiving units are limited by "noise" and how you 
mount them.  In daylight and without a shade on the photo-transistor, the 
receiver is "full quiet" on the signal.  Just after sunset, with plenty of light to 
drive without headlights, the unshielded phototransistor was able to detect 
signal when pointed in the opposite direction.  I haven't tested the units at 
maximum range, but I suspect the rated field of view is more than 20 degrees 
from centerline.  

Range: specs say 1/4 mile.  With lenses to gather more light, miles. How far 
could I see the beam?  Lets just say that if I was lost in an Alaskan 
wilderness, I could use it to signal search planes that I couldn't hear.  ( Of course, 
I could do the same thing with a fire, and keep warm. )  

What does that say to me about where improvements need to be made for 
convenient light communication?  

My opinion is that a laser pointer has plenty of power and beamwidth for a 
transmitter.  Until we start talking to the Moon, a laser pointer can be seen 
far enough.  More power creates safety issues.  Not worth it.  The beam width 
could be more, but it is acceptable.  At ten miles, the beam covers about 200 
feet.  A typical to good mount (or tripod) for an amateur astronomy telescope 
should be able to hold the beam steady in a light wind.  

That leaves the receiver and its optics.  My first gripe there is field of 
view.  If I am transmitting a beam 4mR wide, why does my receiver pick up 
everything for 700?  As a suggestion, if the transmit beam is 4mR, then the receiver 
should be 40mR from the half power point across to the other, and the 
"finder" scope (or just an open gunsight) should have a 100 mR field of view, which 
is about what you get with a set of 7X sport binoculars.

My real gripe is sensitivity.  With an optical tube of 100 mm aperture, and 
the field of view described above, I should be able to receive the voice 
modulated signals of any laser beam I can see with the naked eye.

Is this really any more difficult than building a 10 inch Dobsonian 
telescope?  It shouldn't take any longer to unpack, setup and point a light comm device 
than it does to unpack a large amateur telescope, set it up and point it at 
Mars.

James
N5GUI