[Laser] Atmospheric turbulence bubbles

[Chris L]

   Quite so about the modulated optical comms beam size,

   but  in  general,  the  bigger  the  aperture  at BOTH transmitter AND
   receiver,  the  less  the  effect  of  atmospheric  turbulence and the
   greater will be the system's optical gain.

   The  effect  of  atmospheric  turbulence  was  discussed, shown in CRT
   screen grabs and analysed in the 1983 BSTJ article to which I referred
   in  an  earlier  posting.  In  my experience, that turbulence is worst
   under the following conditions:

   *hot,  days of high humidity, particlarly windy days (Melbourne summer
   nights during heatwaves).
   *at low elevations over a sea path (fresh water seems less so...)
   *with the beam close to the ground along its whole length.

   The 104 mile path in Tasmania between Mounts Barrow and Wellington was
   quite  an  elevated  path, with both ends above the 1200 metre contour
   and  few  high  hills  between.  We  had a mild to cool night with the
   mountaintop  temp  about 6 to 10 degrees Celsius. The curvature of the
   earth  accounted  for (I think) a 600 metre bulge in the earth between
   stations...  however the fast flutter fade was not as horrendous as we
   expected  from  earlier  tests  over  a 30 km path near sea level. The
   problem  evidently rapidly reduces with altitude - so you Californians
   have far better paths than we do, with your 14000 foot mountain ranges
   and your Death Valley...!

   Optical  aperture  should  be  maximised  at  BOTH  ends  of the link,
   transmit  AND receive.  My  refs  give  the received power transferred
   between two collimating lens systems at great distance as:

   (Source  radiance  x  transmit  aperture area x receive aperture area)
   divided by (square of distance)

   The  areas of each aperture actually MULTIPLY to provide final optical
   gain  and  transfer  of power - and the turbulence disruptions I named
   'bubbles'  (for  want of a better expression) would probably better be
   called  waves  or zones. They are very atmospheric condition dependent
   and  path  dependent.  In  small apertures, the fading rate can run as
   high  as 500 Hz in bad conditions, but lowers in frequency and reduces
   in  fade depth as the aperture is increased. See the BSTJ article 1983
   for practical examples and proof... I find that these atmospheric turb
   disruptions are  a  problem  here in sunny Australia for any path over
   about  6  km,  particularly  across  metropolitan (built-up) areas. If
   you're  shooting  your beams over chimneys while the house occupant is
   operating an open fire, you get a worst case scenario!

   Refer also:

   R  K Long and D R Wortendyke, ''Laser and Applications'' W Chang, ed.,
   Ohio State Univ Press, Columbus, 1963.

   S Gardner, 1964 IEEE International Conv. Records, New York. IEEE N.Y.


   We attribute our success with optical DX --- ENTIRELY --- to our usage
   of  lenses  of  extremely  large  aperture  for  both transmission AND
   reception,  and  the  usage of sources of high intensity and radiance.
   Luxeons  are very nearly the ideal source... a Godsend of 21st century
   science!  (Pardon  my  enthusiasm  - and Lumileds can leave the payola
   under my door, thanks ... AHEM!)

   All the best again,

   Chris Long.