[Laser] Scintillation and Adaptive Optics

[Terry Morris W5TDM]

Hi Art & Glenn,

I think you are right about the atmospheric thickness being greater along 
the horizontal path.  I knew that, but it was late and didn't surface to the 
level of consciousness. So yes, Kerry and Lee's link was probably more like 
3-4 atmospheres. As to the water vapor and other atmospheric gases, it is as 
Glenn stated, absorption is very wavelength dependent.  I think Kerry and 
Lee are operating around 920-925 nm where there is a narrow water vapor 
window.

On the subject of adaptive optics, I have been trying to think of some way 
to implement at a lower cost. This morning I thought I might have an idea, 
but on more research decided it would not work. It involved using a beam 
splitter ( microscope type slide ) set at 45 degrees to the incoming beam. 
This would sample something like 4% of the intensity, then direct that to a 
CCD ( web camera) to act as a wavefront sensor. Take the video signal, and 
maybe just invert, then send it to one of TI's DLP chips ( Digital Light 
Processors ). Then I realized that we would need the DLP to be an ALP ( 
Analog Light Processor). TI's DLP mirrors are either on or off, would need 
linear movement. If they were linear, one could place it to reflect a 
corrected wavefront to the detector. If it was an ALP the correction would 
not be real-time, but probably close enough. Oh well, if it was that easy it 
would have already been done.

73
Terry W5TDM


>From: Art <KY1K at verizon.net>
>Reply-To: Free Space LASER Communications <laser at mailman.qth.net>
>To: Free Space LASER Communications <laser at mailman.qth.net>
>Subject: RE: [Laser] Scintillation and Adaptive Optics
>Date: Wed, 22 Aug 2007 13:39:02 -0400
>
>Terry,
>
>Thanks for a very simple, yet elegant description of the problem and it's 
>'remedy'. It is very helpful. I marked your message in my archive, so I can 
>refer to it again if needed.
>
>I have one question however....
>
>You state that 21 miles is 'about' one atmosphere thick. Isn't a linear 21 
>mile path more like 3 atmospheres thick? Above 20K feet, the atmosphere 
>gets pretty thin pretty quickly. I'd say a 21 mile line of sight path 
>through the dense lower atmosphere should be considerably more than 1 
>atmosphere of thickness, shouldn't it?
>
>Since it's also quite a bit cooler at 20K feet an above, the amount of 
>water vapor contained at heights more than 20K feet is very minimal 
>(relative to the lower atmosphere). So, perhaps we might consider a 21 mile 
>line of sight path in the lower atmosphere to be as much as 5 or 6 
>atmospheres thick when the volume of water vapor is considered also. Or, 
>does water vapor enter into this issue at all? My guess is that water vapor 
>makes the 'air' more dense and since the water vapor isn't evenly 
>distributed, then it is valid to consider the absence of water vapor when 
>discussing the quantification of how many atmospheres the signal passes 
>through.
>
>Regards,
>
>Art
>
>
>
>>I think the first problem with your concept is that scintillation is cause 
>>by a change in photon flow into the receiver aperture. Light does not 
>>travel through space or other material as a photon, but as a wave. Light 
>>is said to be dualistic, meaning it is emitted and absorbed as a particle 
>>(photon), but it travels through space/air/glass etc as a wave. So it must 
>>obey the laws of wave propagation and wave optics.
>>
>>When light leaves a star at "infinity" it forms a spherical wavefront, but 
>>being at infinity it arrives here very much a plane wave. This appears to 
>>the eye as a point source. The same can be said for a Laser, it is a point 
>>source which produces a plane wavefront with all points on the wavefront 
>>in phase. As this plane wavefront from star or Laser propagates throught 
>>the turbelent atmosphear it is distorted by the changing density which 
>>causes phase distortions. The phase distortions result in both 
>>constructive and destructive interference. This causes increased amplitude 
>>distortions in addition to the ampltude distortions caused by absorption 
>>by the atmosphere. Large phase distortions also result in diffraction or 
>>beam steering. What you are calling dancing of the beam. I will concure 
>>that a larger receiver aperture will collect more light, however if that 
>>light consists of out of phase wavefronts you will have considerable 
>>destructive interferance at the focal plane of the lens. If you provide a 
>>way to correct the wavefront with adaptive optics you will reduce that 
>>destructive interferance. The wavefront correction will also reduce fringe 
>>patterns which will cause AM modulation as they move across the detector 
>>due to the "dancing". By using the correct modulation/demodulation this AM 
>>noise is eliminated. The signal that has already been lost between 
>>transmitter and receiver by absorption and interference is gone for good.
>>
>>I did state in my original post that adaptive optics were not in our 
>>budget. However the DOD does make use of adaptive optics for Laser 
>>communications to overcome the same problems we encounter. They do have 
>>the advantage that they generally only have to deal with one atmosphere 
>>thickness, where we are trying to deal with several atmospheric thickness. 
>>  Kerry and Lee's 21 mile link was just about one atmosphere thick. The 
>>one case I know of where the DOD has to deal with several atmosphric 
>>thickness is with the ABL and THEL programs.
>>
>>If you are interested, I can suggest some good books on wave optics.
>
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