[Laser] Scintillation and Adaptive Optics

[Terry Morris W5TDM]

Hi James,

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.

73
Terry W5TDM


>From: "James Whitfield" <n5gui at cox.net>
>Reply-To: Free Space LASER Communications <laser at mailman.qth.net>
>To: "Free Space LASER Communications" <laser at mailman.qth.net>
>Subject: [Laser] Scintillation and Adaptive Optics
>Date: Tue, 21 Aug 2007 22:09:14 -0500
>
>I am puzzled by recent comments by Dieter, dl7udp, and Terry, W5TDM, about
>the possiblility of adaptive optics being able to reduce scintillation.  I
>know that my concept of optics is more geometry and Newtonian than waves
>with diffraction and phase characteristics, but I just don't "see" a
>mechanism how adaptive optics can help, much less be affordable for the
>experimenter or practical communications.
>
>Since I usually ramble a bit trying to explain myself, I will try to put
>forth the three scenarios that I have come up with, then try to explain two
>of them.  The first, and I believe the most responsible for scintillation,
>is a change in the photon flow into the receiver aperture.  For this I see
>absolutely no mechanism for adaptive optics to work.  The second, I will
>call "image dance", does have a mechanism that I believe would cause
>amplitude variance.  The problem with it is that it should not affect a
>practical optical receiver system and if it did, the effects should not be
>enough to account for the scintillation that is observed.  The third is a
>random phase distortion of the wavefront that results in time varying
>cancelation/reinforcment of the intensity on the sensor.  This third
>scenario is so fuzzy in my head I will not even try to describe it, much
>less suggest what adaptive optics might do about it.
>
>
>
>Now for the ramblings:   In layman's terms the twinkling of the stars is
>scintillation, just at a speed that we can see rather than hear.  Stars are
>so far away that we can treat them as point sources.  To model the flow of
>light to the eye, imagine a long isoceles triangle with the star at the
>"point" ( the angle between the two long equal sides ) and the "base" ( the
>short side ) equal to the diameter of the iris.  In this model we can
>represent the flow of photons from the source as an arc moving with time
>away from the source.  The photons that are inside the triangle can be 
>seen,
>those outside the triangle cannot.  Now to model the atmosphere we place a
>transparent object so that at least some of the object is inside and some 
>is
>outside the triangle.  This object changes randomly with time in shape and 
>/
>or density so that the photons that pass through it change direction.  Some
>of the photons will change direction enough that they will exit the 
>triangle
>if they were previously in it or will enter it if they were previously
>outside.  The more dense the atmosphere, the longer the path through the
>atmosphere, or the more turbulent it is, the more likely the photons will
>shift into or out of the triangle.  (  Stars twinkle less on mountain tops
>than at sea level, more close to the horizon than overhead.  )   If you
>increase the aperture ( the base of the triangle ) in this model you will
>increase the number of photons received.  ( The model is two dimensional
>instead of three so it would only increase in proportion to the aperture
>with no change in the number of in/out photons.  In the real world the
>capture area is proportional to the square of the diameter and the in/out
>photons would increase proportional to the perimeter, that is linearly. )
>The end result is that the twinkle becomes a smaller fraction of the 
>average
>light received.  ( For this reason children and the elderly probably see
>more twinkling of the stars.  Children have smaller eyes.  The ability of
>the eye to dilate decreases with age. )
>
> >From this model, which I believe accounts for most of the scintillation
>effect, I cannot see how adaptive optics would have any effect on the 
>number
>of photons entering the aperture of an optical instrument.
>
>
>The second scenario presumes that the distortion from the atmosphere does
>not change the photon flux reaching the instrument apeture, but rather
>changes the direction that the image enters the instrument.  If you could
>watch the image of the source, it would change the location where it falls
>on the instrument's focal surface ( in a camera it would be the focal
>plane ).  I think of it as the spot on the photo sensor as "dancing"
>arround.  Now for our purposes the optical sensor ( a one pixel camera )
>does not need to be, and I suggest there are valid reasons that it should
>not be, at the focal surface of the instrument.  Further the image of the
>source we are trying to detect can be a very fuzzy patch, though it would 
>be
>nice if it did fall entirely on the sensor.  In this scenario the varying
>amplitude detected could be caused by the fuzzy patch dancing to, and 
>partly
>over, the edge of the sensor.  The fraction of the light within the
>overshoot of the spot would be the amount of lost signal.
>
>I can readily see how adaptive optics would be able to stabilize the dance
>of the spot, which in an imaging camera would reduce the distortion caused
>by turbulence, but I do not see it as having useful value on real
>scintillation.
>
>To carry the analogy further, in the experiment by KD0IF and N6IZW the 
>range
>was 21 miles or 1.33 million inches with a transmit aperture of four 
>inches.
>The source would then be three microradians.  I forget what the receive
>instrument what, but assume that it had a 1000 mm focal length and a 1 mm
>sensor, yielding a field of view of 1000 microradians.   That is an awful
>lot of dancefloor for the received spot.  I do not see adaptive optics 
>being
>of any real benefit for communication.
>
>
>
>James
>  n5gui
>
>
>
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>
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