[Laser] Re: Laser Digest, Vol 11, Issue 5

[wa4qal at ix.netcom.com]

> Message: 2
> Date: Thu, 10 Mar 2005 00:33:01 EST
> From: TWOSIG at aol.com
> Subject: [Laser] ARRL "coherent"
> To: laser at mailman.qth.net
> Message-ID: <b9.534ec06f.2f61360d at aol.com>
> Content-Type: text/plain; charset="US-ASCII"
> 
> I would like to add a few comments on the issue of the ARRL recognizing  only 
> transmissions from coherent sources.  It comes from one of  the general rules 
> for contests above 50 MHz that they publish.   The particular rule applies 
> only to the frequency band "all above 300 GHz" which  the FCC allocates to 
> licensed amateur radio operators ( and as a practical  matter not exclusively).  
> The rule technically only applies to contests,  not to distance records or to 
> other activities, but it does infer proper  operating guidelines.

One possibility might be to petition them to change the rule.  If enough
amateurs were of the opinion that it should be changed, then there's a
good chance that the contest committee would change it.  Maybe.

Also, note that the optical band is being used by quite a few people
now, including quite a few who are using it for internet linking (Do a
search on RONJA.).
  
> About a year ago, I contacted the ARRL contest organization about the  rule.  
> Based on the experience, observe the following:
>  
> The ARRL is not enthusiastic about answering questions about the  rule.  
> (There is not a lot of activity up there and the staff would rather  spend time on 
> other things.)
>  
> If you are persistent, and I mean really persistent, "I will be polite, but  
> I am really, REALLY tired of being ignored" persistent, you can get them to  
> talk to you.  (And they are nice people after you get through.)
>
> They do not admit to remembering why the rule was written the way it  was.  
> (So I made up my own story of why.  It has several unique  provisions, found in 
> no other rules.  It requires that the contacts be made  between licensed 
> amateurs - presumably because you do not need a license to  legally operate on 
> those frequencies.  It requires the receiver to have at  least one stage of 
> electronic detection - you cannot use biological detectors,  such as seeing a 
> flashing light.  Note that the electronic stage of  detection could drive a light 
> bulb which was then decoded visually.  There  might be other non-electronic 
> means to detect radiation. Then there is  the coherent part of the rule.  Since 
> the text of the rule identifies a  laser as an example of what is acceptable, 
> the rule must have been written after  the laser was popularly known and the 
> term coherent was used because it was a  buzz word that is associated with 
> lasers.  I am sure the intent was to rule  out common light sources such as 
> sunlight, candles, incandescent bulbs, neon  bulbs, and other gas discharge bulbs.  I 
> am not sure if the rule was  written before the development of the light 
> emitting diode, so I am not sure if  the original intent would have excluded it.)

Commercially available light emitting diodes were around since the 1960s [1], 
so it's pretty certain that they were around when the rules were written

[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
light was produced, which was not thermal in origin.  However, the effect
was forgotten since there was no commercially exploitable option at that
time.

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
solar based signalling systems dating many decades (centuries?) into the
past.  Such systems, while being interesting, have little relation to "radio".

> They are not interested in establishing any technical standard that would  
> allow you to measure the coherence of a transmission source.  I translate  
> "coherence" as meaning frequency stability, but others might not, so there 
> is  not > even a definition of the technical terms used.  

Technically, choerence means that all of the wavefronts are synchronous.
This doesn't really imply frequency stability, although it does apply that
all regions of the device (laser, etc.) are producing signals at the same
frequency at the same time in order that the wavefronts are synchronous.
However, the entire device is free to drift in frequency pretty much at will.
Realistically, this means that the device may drift in frequency, and, typically,
most lasers do exhibit some amount of "mode jumping". 

What the coherence requirement does prohibit is devices which produce
the optical energy with the wavefronts not in synchronization.  Such devices
include most thermal sources (incandescent lamps, fires, carbon arc lamps,
nuclear explosions, etc.), as well as most cold lamps (fluorescent, 
electroluminescent, glow discharges, LEDs, etc.).  

It's possible that the requirement for the radiation to be coherent was 
adopted to restrict bulk emitters, such as waving a blanket in front
of a campfire, or flashing the car headlights.  In any case, it effectively
prohibits quite a few light sources which might otherwise be of interest.

One of the other possible reasons for the requirement involves the
duality of light, and the fact that, when you get down to the point
where you're detecting only a few photons in extremely weak signal
detectors, if the source isn't coherent, then you can get (random)
interference patterns in the received light.  Practially, though, due 
to varying atmospheric effects, this won't come into play.

> I predict that there will be technical standards set and the rule will be  
> changed.  I just do not know how soon, or in what way the rule will  change.  
> There are distance records being set at 403 GHz using a Gunn  diode, which is 
> certainly not a laser.  If that equipment gets used in an  ARRL contest, 
> eventually someone will ask that it be accepted (or possibly  rejected) as a 
> "coherent" source.  There is also some talk of breaking up  the one band "all above 
> 300 GHz" into multiple bands.  That is reasonable  and probably due pressure 
> from commercial interests wanting to use the  spectrum without having to put up 
> with those amateurs who aren't using it  anyway.  

It's possible (even probable) that, at one point in the future when the state of
the art has advanced far enough, that the 300 GHz upper limit may be 
pushed up even higher to make room for additional commercial, public
service, military, and/or military bands.  This has happened before, and
will almost certainly happen again.  The 300 GHz limit was almost certainly
set because the technology at the time was incapable of generating signals
at that frequency or above.  But, since 403 GHz Gunn diodes are possible,
the technology is obviously improving.  And, commercial interests are sure
to appear.

> My opinion is that anyone that is doing light communications should  
> completely ignore the rule.  And for the most part, ignore the ARRL  too.  I know that 
> this is laser communications maillist, but if the use of  a laser is the 
> wrong light source, because it is too expensive, too dangerous,  it makes the 
> neighbors want to sue you, the atmosphere disrupts it,...whatever  the reason, use 
> something else that does work.  In the end, what makes a  good light 
> communication system will be used, and what does not work well will  be abandoned.

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 
phosphors), although the light was distributed in all directions, and the
areal size of the source made for focusing the output somewhat difficult 
(parabolic reflector?).
  
> Besides, ARRL contest rules allow you to write rules for specific contests  
> that take precedence over the general rules.  If you want to make a contest  on 
> light frequencies only that does allow non laser sources, do it.  And go  
> have a good time building and testing the equipment.

That might be the best short term solution.

> James
> N5GUI

Dave
WA4QAL