[Laser] AW: coherence and scintillation

[Dieter Palme]

Hi Ives,
I dont aggree, the coherence length of a laser, i.e. solid state laser,
is much shorter then the light path of your communication. Therefore the
coherence is not a sourche of fluctuation. A small pointer laser is for
some millimeter coherent, a very long gas laser for some kilometer. 
The fluctuation in free air communication over a longer range causes in
the disturbance in the air, diffraction by changing refraction index by
temperature, specific gravity and so on.
73 Dieter dl7udp

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> Gesendet: Freitag, 24. August 2007 10:02
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> Betreff: Laser Digest, Vol 38, Issue 10
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> Today's Topics:
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>    1. Re: 21 mile daytime laser contact durin ARRL 10GHz & Up
>       contest (F1AVYopto at aol.com)
>    2. Re: 21 mile daytime laser contact durin ARRL 10GHz & Up
>       contest (Kerry Banke)
> 
> 
> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Thu, 23 Aug 2007 16:13:46 EDT
> From: F1AVYopto at aol.com
> Subject: Re: [Laser] 21 mile daytime laser contact durin ARRL 10GHz &
> 	Up	contest
> To: laser at mailman.qth.net
> Message-ID: <c1d.1fda303d.33ff447a at aol.com>
> Content-Type: text/plain; charset="US-ASCII"
> 
> Kerry
> A star is a punctual source but it is  also a not coherent and not 
> monochromatic source.
> With a laser the  scintillation is a very complex problem.
> It is not only an erratic  convergence modulation with angular micro 
> displacements associated with beam  global intensity 
> fluctuations. Constructive and deconstructive phase  
> combinations give beats that occur 
> into the photo detector sensitive layer  because micro light 
> speed modulation due 
> to air density fluctuations.
> This  beating effect gives a very deep granulation noise.
> I suppose your multilayer  dielectric filter is fitted in a 
> conical beam just 
> in front of your photo  detector.
> This mounting gives selective rings from the edge to the 
> centre  with a 
> strong bandwidth enhancement.
> With the monochromatic laser light it is  possible the filter give a 
> diffusing ring that reduces the scintillation by a  certain 
> averaging on the 
> convergent area.
> May be the spot motions are  reduced?
> May be the laser light coherence is partially lost after the 
> filter  because 
> its obliquity?
> Normally a filter has no effect on the scintillation  by I 
> had also noticed 
> this effect and I cannot explain why?
> 73 Yves  F1AVY
> 
> 
> 
> 
> 
>    
> 
> 
> ------------------------------
> 
> Message: 2
> Date: Thu, 23 Aug 2007 17:08:01 -0700 (PDT)
> From: Kerry Banke <kbanke at sbcglobal.net>
> Subject: Re: [Laser] 21 mile daytime laser contact durin ARRL 10GHz &
> 	Up	contest
> To: Free Space LASER Communications <laser at mailman.qth.net>
> Message-ID: <68015.38612.qm at web83204.mail.mud.yahoo.com>
> Content-Type: text/plain; charset=iso-8859-1
> 
> Yves - This is very intereresting to me that you have also 
> noticed reduced scintillation when using a filter. It may 
> very well be due to  one of the other reasons you state. It 
> could be interesting to try some form of translucent material 
> in place of the filter to see if maybe it is due an averaging effect.
>    - Kerry N6IZW - 
> 
> F1AVYopto at aol.com wrote:
>   Kerry
> A star is a punctual source but it is also a not coherent and not 
> monochromatic source.
> With a laser the scintillation is a very complex problem.
> It is not only an erratic convergence modulation with angular micro 
> displacements associated with beam global intensity 
> fluctuations. Constructive and deconstructive phase 
> combinations give beats that occur 
> into the photo detector sensitive layer because micro light 
> speed modulation due 
> to air density fluctuations.
> This beating effect gives a very deep granulation noise.
> I suppose your multilayer dielectric filter is fitted in a 
> conical beam just 
> in front of your photo detector.
> This mounting gives selective rings from the edge to the 
> centre with a 
> strong bandwidth enhancement.
> With the monochromatic laser light it is possible the filter give a 
> diffusing ring that reduces the scintillation by a certain 
> averaging on the 
> convergent area.
> May be the spot motions are reduced?
> May be the laser light coherence is partially lost after the 
> filter because 
> its obliquity?
> Normally a filter has no effect on the scintillation by I had 
> also noticed 
> this effect and I cannot explain why?
> 73 Yves F1AVY
> 
> 
> 
> 
> 
> 
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> End of Laser Digest, Vol 38, Issue 10
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