[Laser] ARRL "coherent"

[Paul B. Webster VK2BZC]

On Thu, 2005-03-10 at 09:46 -0500, wa4qal at ix.netcom.com wrote:

> 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 is no doubt, the rationale behind the rules.  It is no longer
permissible to use spark transmitters to send radio communications
because they do not make effective use of bandwidth (by a long shot).
Development of *radio* communications is (nowadays) all about making
effective use of bandwidth, there is no purpose in "developing" systems
which do the exact opposite.

  All the sources discussed other than lasers, LEDs and monochromatic
gas discharge tubes, are the optical equivalent of spark transmitters -
historically fascinating but irrelevant to developing efficient use of
the frequency domain.

  A similar example is as has been mentioned, the development of
microwave communications.  Klystrons and Gunn diodes are relatively
coherent, but are not conducive to band sharing, and "state of the art"
has moved to systems with specified carrier frequencies using
superheterodyne control.  Obviously such techniques are orders of
magnitude (literally) more difficult at optical frequencies, yet they
are the (only) future of optical communications.

Now whereas it may seem that optical communications is dominated by
spatial domain multiplexing, i.e., you aim the beam where you want it,
and aim the receiver where you want it to "see", or select which optical
fibre you wish, this does not define the whole spectrum(!) of
situations.  Frequency domain multiplexing *will* become important
(already is, in fact) in maximising the use of single fibres, and would
be prerequisite to any serious (or commercial) use of "scatter" or
"broadcast" communications.

-- 
  Cheers,
    Paul B.