[Laser] Super bright LEDs (was comms at 100 + miles in Australia)

[John Matz]

Just for the information, I believe that gas lasers (HeNe) are about 1-2 nm
spectral width at around 630 nm center wavelength.  Laser diodes are about
10 nm spectral width at 630 nm.  LEDs are about 50 nm spectral width at 630
nm.  I think these numbers are about right.

Yes, the ARRL requires a "coherent" source ... but how coherent?  LEDs are Q
= 15 to 20, but laser diodes are only 50 to 100.  Gas lasers are Q = 500,
but even they are not really narrow.  Some have tried to do superheterodyne
receivers ... and the IF has to be GHz's wide ... and the gas lasers have to
be tuned to the proper freqs and/or locked ... and they still drift badly.
I would certainly support the use of monochromatic LEDs for sources.  It
would allow some NLOS QSOs too.

The Ronja system uses super-bright red LEDs that I believe are used in auto
and truck tail lights.  These are listed in the BOMs on their website
www.twibright.com I think. They are relatively inexpensive.  They can be
modulated at 10 MBPS, but the video bandwidth has to be 10 MHz.  The
sensitivity suffers with 30 or 40 dB more noise, and the range is therefore
only 1 km.

John Matz KB9II
----- Original Message -----
From: <aflowers at frontiernet.net>
To: "Free Space LASER Communications" <laser at mailman.qth.net>; "Steve
Bailey" <stevebailey at hypernet.com>
Sent: Tuesday, March 08, 2005 8:48 PM
Subject: [Laser] Super bright LEDs (was comms at 100 + miles in Australia)


>
> > Personally, I can't imagine needing the ARRL to have fun.
>
> Me neither.  This sounds like a really fun project.  Say, where does one
> get some of these super-bright LEDs?  I poked around Ebay and saw a few,
> but I'm not sure what the going rate for them really is.  I saw a "red"
> one and a several "white" ones.  A single wavelength would probably be
> better since I can apply some of my filters to it, but I'm not sure
> what the spectral content of these things really looks like.  I havea
> bunch of fresnels that should be good for this.
>
> Andy
> K0SM/2
>
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