[Laser] Scintillation and Adaptive Optics

Wed Aug 22 18:34:30 EDT 2007

In the 60's there was an article in the Amateur Scientist section of
Scientific American that showed what might have been one of the first
Starlight Twinkle Suppressors. It was a basic design that used tube
amplifiers, Split photocells and servo motors to move the primary mirror
around. I'll dig it up tonight. It is on the Amateur Scientist CD collection
that is available out there. I'm not sure how effective that design was, but
I'm sure it can be modernized.

I'm changing my amplifier circuit to accept a second [and third] detector
channel so that I can bring in signals [from separate optics] spaced some
distance apart. 

We are also cooking up a ring modulator that we can use with pre-recorded
scintillation to evaluate different circuit approaches to the problem.

Next week, some of us from San Diego will be heading out away from the
lights to record moon noise during the full eclipse. According to the maps
I've seen, we should just be looking at Earth illumination coming from the
West Coast USA. Any suggestions? 

73, Lee KD0IF

-----Original Message-----
From: laser-bounces at mailman.qth.net [mailto:laser-bounces at mailman.qth.net]
On Behalf Of Terry Morris W5TDM
Sent: Wednesday, August 22, 2007 3:01 PM
To: laser at mailman.qth.net
Subject: RE: [Laser] Scintillation and Adaptive Optics

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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