[Laser] Atmosphere turbulence bubbles

[TWOSIG at aol.com]

Chris Long spoke of light beams collimated by large fresnel lens having an  
advantage over small, presumably laser, beams when dealing with what he termed  
"turbulence bubbles".  
 
As I understand, the smaller the light beam, the smaller the bubble that is  
likely to disrupt the communication path, and the more frequently the 
disruption  will occur.    I think that puts a lower practical limit on the  beam size 
and total beam power that can be used to establish a communications  channel 
over a given distance with a minimum data rate.  Otherwise, for a  light 
communication channel, you could use a large and powerful transmission  beam with a 
small aperture receiver, or a moderate beam with less power to  a moderate 
aperture receiver, or in theory a small transmit beam with low power  and a 
large aperture receiver.  This disruption by the atmosphere will make  a small 
beam diameter  channel un-reliable, even with increased  power.  ( And adding 
power to a small beam, just makes it more of a hazard.  )   More than just beam 
size, the effect that Chris was discussing  seems to put a laser at  even more 
of a disadvantage because its coherence  leads to diffraction interference 
that further disrupts the signal.
 
I discussed the idea with my brother, and he suggested two things.   The 
first was that there may also be an effect due to the polarization of a  laser 
beam.  I am not sure what the polarization parameters of a gas laser  or rod 
laser, but laser diodes have (I am told) polarization that is  similar to the 
elliptical beam shape of the emitter.  Why this would  be important, I do not 
understand.  ( After spending the weekend with two  grandsons, age 1 and 3, I am 
sure that if he did explain it to me, I have  forgotten.  Wonderful things, 
grandchildren.  They remind me that it  is not so important if I forget technical 
stuff. )
 
Anyway, my brother's comments about polarization lead to the second  
suggestion.  That is to build a test rig to determine the effects of  polarization and 
spatial separation  on an atmospheric optical  communications channel.  Start 
with a laser diode emitter over a range that  has channel disruption.  Vary 
its power and polarity to  gather baseline data.  Then add a second laser diode 
emitter that has  similar disruption characteristics for power and polarity.  
Run the two of  them at various separations, power settings, and 
polarizations to see what the  effects are.  Maybe spacing two lasers 20 centimeters 
apart, with 90  degree polarization differences will improve the channel 
reliability.  Then  get creative and use four laser diodes.  My brother suggested the 
four  equally spaced in a line.  I think on the corners of a square would be  
better.  If somebody was really curious about the coherence effects, they  could 
get a four way beam splitter so the four beams could come from a  common 
source..........( Did I mention that my brother is a EE that used to be  the test 
engineer on an electronics productions line, and that I used to test  ICBM and 
satellite rocket motors for USAF. )
 
Ok, so I dream up a lot of elaborate testing.  Maybe the idea of  multiple 
beams from a cluster of emitters is not worth the effort and cost  compared to a 
more powerful laser and a beam expander to get the hazard down  while 
compensating for atmospheric turbulence.
 
If somebody likes the idea, let us know how the testing turns out.
 
James
N5GUI