[Laser] Scintillation and Adaptive Optics

[Art]

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.