[Laser] ARRL "coherent"

[TWOSIG at aol.com]

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.
 
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.)
 
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.  
 
 
 
 
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.  
 
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.
 
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.
 
 
James
N5GUI