[Laser] Lasers vs LEDs & ARRL contests

[Dave]

> Date: Fri, 18 Jul 2008 19:59:52 EDT
> From: W0ZQ at aol.com
> Subject: [Laser] Lasers vs LEDs & ARRL contests
> To: laser at mailman.qth.net
> Message-ID: <c3a.3c264751.35b28878 at aol.com>
> Content-Type: text/plain; charset="US-ASCII"
> 
> Hello to all Laser/light enthusiast.   

Hello.

> As some of you may  know, I am the ARRL's Dakota Division VHF/UHF Contest 
> Committee (VUAC)  representative.  

It's quite nice to be hearing that there are ARRL representatives
soliciting opinions.

> I am seeking your comments.  One of the items 
> that  has been under discussion by your VUAC is the ARRL General VHF Contest 
> rule 1.12  that states "1.12. Above 300 GHz, contacts are permitted for contest 
> credit only  between licensed amateurs using coherent radiation on 
> transmission (for example,  laser) and employing at least one stage of electronic 
> detection on  receive."  This rule requires that the contact be made by licensed 
> amateurs  and that the detection requires at least one stage of electronics, that 
> much  seems fairly clear despite whether you agree or disagree with that part 
> of the  rule.   What is less clear is the requirement that the source use  
> "coherent radiation" and "(for example, laser)".  How coherent is coherent  ?
> 
> What I would like your comments on is this.
> 
> 1.   Given  this rule as it is now, does it include or exclude the use of 
> narrow band  LEDs like the Luxor.  I am no optics expert but it seems to me that 
> some of  these newer power LEDs are very close in coherence to really poor  
> lasers.   Does the current rule have enough latitude to include narrow  band LED 
> emitters ?

I'd like to see the rule modified.  As others have pointed out,
coherence has little meaning once the signal has left the transmitter,
due to atmospheric/lens/source thermally induced distortion.
Additionally, it restricts the transmitting device unnecessarily (e.g.,
lasers only).

Another consideration, which may be especially important in view of the
current security situation post-9/11, is whether we really want to be
seen as encouraging shining lasers in free space paths?  There have been
several incidents of lasers being used to dazzle aircraft pilots, and
the situation has gotten so bad in Australia that they're in the process
of banning/restricting lasers.  Yes, this is a touchy subject, but one
which deserves some consideration, especially as lasers have become
easier to get and more powerful.  Obviously, experienced optical
communications people know how to handle lasers safely [3], but 
beginners first getting into this aspect of the hobby may not have
the same safety awareness.

[3] "Don't stare into the laser with the remaining good eye!"  :*)  [/3]

Personally, I have no objection to the use of a "narrow band" optical
source, whether that source is produced via a laser (solid state, gas,
or even optically pumped/dye/etc.), a LED, or even a narrow band optical
filter on a broadband source.

Maybe the appropriate thing to do is to consider the purposes of the
rule.  One is to avoid the "two kids with a flashlight" situation.
I could make the claim that we probably shouldn't even be limiting these
two kids, since it may be a path to get young people interested in
amateur radio (whether that "radio" is in the optical or RF bands).
But, it probably is necessary to level the playing field, and to
encourage some development of the state of the art.

Another purpose of the rule is to avoid contaminating the whole of the
optical spectrum with a broadband source.  By restricting the emission
to be of a "narrow bandwidth", however we choose to define this, it
allows the possibility of space multiplexed communications (e.g.,
communications using different wavelengths/colours of light).  This
may be especially important for multiple groups reflecting their
light beams off of a high altitude reflective surface (e.g., balloon?
water tower?  Cloud?).

> 2.  IF we could rewrite the rule, what should  the rule say ?  Do we want to 
> be much more specific and tighten the rule so  it states "Laser only", or do 
> we want to open the rule up to specifically allow  the use of newer 
> technologies like power LEDs ?    In either case  what should the rule say ?

I think the rule has to keep the requirement that the communications
only be between licensed amateurs.  There's the matter of that little
requirement in Part 97 of the FCC rules that states that amateur radio
operators may only communicate with other amateur radio operators
(except in some very special circumstances) [1].

[1] This doesn't ban an amateur from talking to a non-amateur over a
light based path; but it does seem to require that the amateur revert
to a non-amateur during that particular communication, which would seem
to provide justification for not allowing such contacts for credit in an
amateur radio contest.  Ok, so I'm picking at nits here.  :-)  [/1]

I'd like to see the coherence requirement dropped, since it's
meaningless once the signal has left the transmitter.  Plus, since
it's an instantaneous measurement anyway, I think it would be better
replaced by a rule that requires the source to be narrow bandwidth
over a period of time.  For example, a laser could be producing coherent
radiation, but could be mode jumping all over the place, and producing
a broad bandwidth signal.  I think the rule would make more sense as
to limit the bandwidth emitted by the transmitter over a particular
period of time (e.g., to eliminate mode jumping and/or spurious optical
frequency emissions).

The question arises, though, of how to measure the bandwidth of an
optical source (in order to avoid producing a specification which can't
be enforced).  The expensive option would be to use an optical
spectrophotometer.  The cheap option may be to use either narrow band
optical filters to measure the output, or to use a prism or diffraction
grating to measure the output frequency spread.

> It seems that on the one hand we don't want to allow a blinking white light  
> bulb, but if not that where is the line ?   

The problem gets a bit more difficult when you consider what you do and
don't want to allow, and perhaps why.  On the one hand, going back to
the "two kids with flashlights" scenario, you probably don't want to
allow that.  On the other hand, what about someone who builds a large
area array using fluorescent tubes?  What about if the phosphor used
in the tubes is, rather than the traditional white mix, a particular
colour [2].  Do we want to discourage someone from trying some
experimentation using such a system?  How do we keep the playing field
level?

[2] For example, you can now get CFL bulbs which are red, blue, green,
yellow, or even UV ("black").

Another idea is that maybe there should be a requirement to limit
the beam width of the emitted signal.  That might go a lot farther
for eliminating "light pollution" than specifying coherence as a
factor.  But, what would be a good beam-width requirement for an
optical source?  How about scanned sources?  Would we want to recommend
any way of measuring the beam width of an optical source?

> Again, your  comments and perspective are most welcomed.

It's encouraging to see someone actively seeking thoughts on this
matter.

> 73, Jon
> W0ZQ
> Dakota  VUAC Rep

Dave
WA4QAL