[Laser] ARRL "coherent"

[Glenn Thomas]

Hi all!

It can be most difficult to get the ARRL staffers to change anything with respect to rules or policy. They are, without exception highly dedicated, overworked and underpaid folks who try to do the very best for Amateur radio in general.

When you have issues with the way the ARRL is doing things, the best person an ARRL member can contact (you are an ARRL member, right?) is your division director, who's addresses are up front in QST. The directors set ARRL policy and also control the ARRL purse strings, thus ARRL HQ staff pays very close attention to the board. FWIW, I am copying this e-mail to both my director and vice-director.

<RANT>

As a technical matter, the requirement for "coherence" above 300 GHz is flat out stupid. Someone elsewhere in this thread suggested that the line width of a HeNe laser is a nanometer or so. In terms of equivalent bandwidth, it's several GHz wide. Diode lasers are several tens of GHz wide. Bandwidth of 1% or so would not be considered coherent on any other band, so why this one?

While we're at it, in how many laser applications does laser coherence matter much? Most of the applications I can think of, including communications, work because of the ease with which a laser beam can be focused to the point of being diffraction limited. Even if the signal is "coherent" when it starts out, coherence is almost always lost within the first few meters!

I agree with several of you, the ARRL rules with respect to communications at frequencies over 300 GHz are very poorly written and need to be reconsidered. The reason for the current rules that was given when they first came out was a desire to prohibit Field Day QSOs at night via flashlights at 100 meters! Hey, if flashlights (or a shipboard Aldis lamp - same thing really) can work in an emergency situation, why not on Field Day? ...or any other contest? You'd think the "CW forever" crowd would support this!

</RANT>

73 de Glenn WB6W

At 12:33 AM 3/10/05 -0500, TWOSIG at aol.com wrote:
>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
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