[Laser] Re: Laser digest, Vol 1 #227 - 7 msgs

[laser@mailman.qth.net]

Years ago (~20), I had access to a really nice spectrophotometer, and used 
it to make some measurements of LEDs and Lasers.  LEDs had quite a 
broad emission spectrum, dozens to hundreds of nm wide, while LEDs
had essentially a zero width spectrum (Well, it was so good it was beyond
the measurement capabilities of the spectrophotometer, and that
spectrophotometer was one of the best ones available.).  So, there's a world
of difference between an LED and  a laser.  I suppose you could consider
it similar to a spark gap transmitter and a quartz crystal controlled transmitter [1].

[1] I wonder if the comparison between a LED and a laser with a spark gap
transmitter and a quartz crystal controlled transmitter has anything to do 
with the ARRLs requirement on optical communications?

In any case, laser diodes have gotten almost as good as HeNes (if not better).
The early semiconductor lasers had some real problems with frequency
hopping and multi-modes.  However, the current generation has improved.
I can remember when semiconductor lasers wouldn't support holography,
nor could you see the laser speckle due to the laser jumping frequencies so
rapidly.  However, now, semiconductor lasers are stable over long time periods.
So, I would have to answer that semiconductor lasers are stable, which would
imply that their light output is very coherent.

As for how coherent modern semiconductor lasers are, I think that can be
determined by viewing the laser speckle, which is due to optical interference.

Dave
WA4QAL

-----Original Message-----
From: "John Matz" <Jematz@megsinet.net>
To: <laser@mailman.qth.net>
Subject: Re: [Laser] Laser comm and ARRL?
Date: Wed, 18 Feb 2004 23:16:51 -0600
Reply-To: laser@mailman.qth.net

Hi guys,

I have been following this thread for a while, and I'd just like to toss in
my two cents.

First, I'm all for allowing LED contacts too.  It's the electronic detection
that should determine a true communication link.

Laser pens and many other lasers are only a few milliwatts output.  They
have a beam divergence (sometimes made by the transmit optics) of about a
milliradian (0.06 degree beamwidth).  Lasers have 1 to 10 nm of spectral
linewidth.  At 650 nm optical wavelength, that's a Q of about 100.  HeNe gas
lasers are about 1 nm, solid state lasers are about 10 nm, as I recall.

LEDs on the other hand have a linewidth of 50 to 70 nm, for a Q of about 10
... they are a bit wider, not "coherent" by the rules ... but how coherent
is a semiconductor laser ... maybe 5 times narrower?  Just how coherent is
coherent?  Neither is super narrow.  Also LEDs are available that can run IR
and/or have much more output power ... like 1 watt.

For what I've seen, for NLOS contacts to be possible, the approach is an
array of 1 watt LEDs running tens of watts optical power out.  The beamwidth
is wider too, matching the receive beamwidths of 0.5 to 1 degree typical,
making cloudbounce practical.  Note this power is spread over many inches of
source and disperses 10 times faster (safer?).  If LEDs are not allowed, we
must use lasers at 1/1000 the output, and pretty much stick with LOS
contacts and super narrow beams.

A compromise I may try soon is to broaden the TX beamwidth to 10 mr (0.5
degree) to match my RX at 10 mr beamwidth.  The range should be just over a
kilometer then.  With these "broad" beams, I can make contest style contacts
with little difficulty in alignment and just enough range to be legal.
How's that for "complying" with the rules?

John Matz KB9II