[Laser] Laser comm and ARRL?

Thu, 19 Feb 2004 11:09:24 -0500 (GMT-05:00)

No, laser diodes are constructed with a single chip.  This chip is 
designed with special mechanical properties (polished/cleaved
ends) so as to form an optical cavity which permits light to bounce
back and forth within it (with a bit leaking out on each bounce 
from one of the ends).  The doping levels are adjusted so that
above a critical current level, an unstable population inversion of 
atoms exists (e.g., there are more excited atoms than unexcited 
atoms).  Then, when a single photon starts bouncing back and
forth, as it passes each excited atom, it causes another photon
to be released.  This process keeps repeating, with the decayed
atoms being re-excited by the current through the device.  If the
current is too low (e.g., below the lasing threshold), then not
enough excited atoms exist to allow the lasing action (e.g., enough
light leaks out so that the remaining bouncing photons don't
encounter enough excited atoms to keep things going).

The reason why a single LED, or even a bank of them, won't produce
the same light as a laser is that, with the LED, each photon of light
gets created randomly, and moves off in a random direction.  With a
laser, each new photon is emitted at the same frequency and travels
in the same direction as the photon that stimulated the emission of
the new photon, so the two photons (and, every other photon that
gets created by these two) are locked at the same frequency and 
travel in the same direction.  Thus, the output frequency of all of the
photons from a laser is exactly the same.  Plus, the direction of 
emission is the same, or, at least, it should be for an ideal laser.  
One of the problems is that multiple modes may exist in the optical
cavity, and multiple sets of photons can be bouncing around over
different paths (multi-mode).  Or, the mechanical dimensions of the 
optical cavity can change (e.g., with temperature), and this can affect 
the frequency of light that the cavity lases at (mode hopping).

With an LED, the frequencies of the photons, while close, are not 
the same, nor is the phase of the photons, or is the direction they 
are emitted in, since each is produced entirely separately from the 
others.

Or, at least, that's how I understand it.

Dave
WA4QAL
-----Original Message-----
From: [email protected]
Date: Wed, 18 Feb 2004 20:26:05 EST
Subject: Re: [Laser] Laser comm and ARRL?
To: [email protected]
Reply-To: [email protected]

I agree with Art, KY1K, that the ARRL rule that prohibits the use of LEDs for 
a light frequency contact seems unfair.  The frequency (color) of the light 
generated by a red LED seems to be equal to that produced by a simiconductor 
laser.  As I understand the manufacture of laser diodes, it is a row of LEDs 
spaced so that at the trigger current the light from one LED causes the neighbors 
to fire.  (If I am wrong, somebody please explain it better.)  A single LED, 
or a bank of them, may have phase noise, but I haven't seen much amateur 
equipment that could measure it.  Does anyone have equipment that will work better 
with coherent light?

The idea of communicating with light seems to be suffering more from the fact 
that it is not practical to regulate who is doing it.  An amateur license is 
a grant from the government for the privilege of operating a transmission 
device.  (Some would like to make it a crime to receive radio signals without 
permission, but that is another political debate.)  Light is something that comes 
and goes without much regard for legislation.  The sun comes up, I can use as 
I see fit, and am technically capable of, whatever sunbeams that happen to 
come my way.  The sun goes down, I can look at the moon and stars, again within 
my technical limits.

What then can the ARRL set into its rules that will separate licensed 
amateurs from tinkerers?  They are in the business of promoting amateur radio, not 
light communication.

I have always thought that part of the reasoning for some of the light 
related rules, was that the "technology" of electronic communication could not 
compete with the capability of sunlight, an eight inch mirror and human eye to 
provide a communication link of 183 miles in the 1890s.  When a laser experimenter 
breaks that Heliograph record with a voice contact, I will celebrate the 
technological achievement.

I think that getting the ARRL to change those rules would be a lot easier if 
you could show them that if they encourage light experimentation, then the 
membership of  ARRL would grow by ten percent.  

I think that it is easier to follow their rules.....Or just ignore them and 
do your own thing.

Good luck.

James
N5GUI