[Laser] Re: Laser Digest, Vol 11, Issue 6

[wa4qal at ix.netcom.com]

> Message: 10
> Date: Thu, 10 Mar 2005 18:17:26 +0100
> From: Karel Kulhavy <clock at atrey.karlin.mff.cuni.cz>
> Subject: Re: [Laser] Re: Laser Digest, Vol 11, Issue 5
> To: wa4qal at ix.netcom.com,	Free Space LASER Communications
> 	<laser at mailman.qth.net>
> Message-ID: <20050310171726.GB24823 at atrey.karlin.mff.cuni.cz>
> Content-Type: text/plain; charset=us-ascii
> 
>>> [1] The light emitting effect from diodes was discovered in 1906, by some of
>>> the early experimenters of crystal detectors.  It was noted that when a 
>>> current flowed through carborundum (Silicon Carbide), a faint bluish-yellow
> 
> Will it work if I try it with a current source and silicon carbide
> sandpaper? Will the grains glow blue? :)

I've been trying this, yet I haven't been able to get any
current to flow through the Silicon Carbide grit that I have.
I'm not sure if that's due to the stuff being doped wrong, or
whether I just haven't found the right place yet, or whether
I need to work on my contacts.  If I remember my solid state 
theory correctly, it's necessary for the bulk of the material to 
be doped as a P type, and one of the metal electrodes forms 
the N contact as a Schottky junction.  Or, something like that.  
In any case, give it a try and see if it works.

If you want to see some of the references, try here:

http://www.rpi.edu/~schubert/More%20reprints/1907%20Round%20%28Electrical%20World%29%20A%20note%20on%20carborundum.pdf

or do a web-search on "H. J. Round", and/or Carborundum LEDs.

>>> One possible reason for the effect may have been to rule out things such
>>> as large incandescent light bulbs, which would seem to give an unfair
>>> advantage to people using them.  Additionally, there were things such as
> 
> I think they give fair advantage.

Perhaps.  But, one of the reasons for the existance of the amateur
radio service is to promote technical advancement.  Brute forcing 
optical communications isn't really promoting technical advancement,
at least unless you found a way to modulate them at audio frequencies.

> You could use a welding apparatus for an arc and a large fresnel lens
> and some kind of optomechanical modulator and I think your balls would
> pop out, not from the light, but from the achievable distance ;-)

Perhaps.  For that matter, you could remove the shielding from a nuclear
reactor and blast out gamma rays by the megaWatt.  But, I sure 
wouldn't want to be standing anywhere near the receiver (After all,
gamma rays and X-rays are electromagnetic radiation, similar to light.).
 
>>> solar based signalling systems dating many decades (centuries?) into the
>>> past.  Such systems, while being interesting, have little relation to "radio".
>
> And they worked exceptionally well. They were being in routine use for
> ranges like 20kms by army.

Yes, they certainly did.  
 
> I just don't see why the fact that something has been usefully used
> before disqualifies it in eye of ARRL for contensts.

You have to remember that the reason for the existance of the ARRL is
to promote amateur radio.  While other types of communications do 
exist, the ARRL isn't interested in promoting them.  Thus, they don't 
hold contests for broadcast band DXing, nor do they hold marathon
races to see who can deliver a piece of paper the fastest, nor do they
see who can reflect sunlight the farthest.  Those methods, while they're
certainly communications, are not amateur radio, and thus the ARRL 
has tried to distinguish what is and what is not amateur radio in the
contests it sponsors.

>>> I briefly played with an optical system back in the 1970s using a fluorescent 
>>> tube (although I never made any contacts).  The fluorescent tube was a
>>> good souce back then for producing a lot of light (40 Watts or more)
>>> with a somewhat reasonable frequency response (100 Hz or more, primarily
>>> limited by the response of the existing phosphors that were commercially
>>> available, but that could probably be improved with higher speed 
> 
> I have read somewhere that the red phosphor is very fast. How far up do
> you think it would be possible to go with a fluorescent?

It's quite easy to get phosphors that respond in the sub-millisecond time
period, so 1 KHz should be easy.  Some commonly used CRT phosphors 
respond down into the 10s of microsecond range, so 10 KHz should be
fairly easy.  There are also some exotic phosphors which respond down 
into the nanosecond range, so this would permit MHz range signals.

Here's a paper with a list of commonly used CRT phosphors:

http://libai.math.ncu.edu.tw/~shann/bcc16/new/hardware-bible/wrh17.htm

> You could just take a pack of florescents, light them up with some
> not-so-strange circuit and impose FM modulated signal in hundreds of
> kilohertz. You would get a radio, LEGALLY!, covering the whole
> neighbouring area!

Yes.  You could even have multiple stations superimposed on that signal.
And, as an added bonus, it would double as a night light for the 
neighbourhood.  Maybe make it an aircraft navigation/hazard avoidance
light on a tall tower?  Suspend it on a tethered balloon, and you could cover 
a lot more distance.

> The users would just use something like Ronja RX with a positioner. If
> they wanted to change station, instead of MHz there would be GPS
> coordinates.  And it would be like changing satellites with a dish
> system.

Yes.  Just beware of fog.

> You could also modulate it digitally with manchester or whatever and
> get a whole packet of DRM (digital radio mondiale).

Yes.  Use it as a community internet high-speed downlink for digital 
data.  I'm kind of surprised that someone hasn't tried this yet.
 
> "Yes, we are doing amateur radio. Radio mondiale. Digital radio
> mondiale. And we do it on optics because we don't want to pay fees for
> HAM registration. Tune in, here is our playlist ;-) "

It would be broadcasting (which is illegal for the amateur radio
service, at least, in this country).  But, I think it would be very 
interesting.

> Cl<

Dave
WA4QAL