[Laser] Scintillation and Adaptive Optics

[James Whitfield]

Terry

Interesting the way you stated the mechanism.  I think what you are saying
is that the twinkling light from a star follows a straight path to the eye
and that the amplitude changes are explained by wave theory as the
atmosphere causes time varying changes in the phase on small sections of the
wavefront.  It would be easier for me to accept if we were talking about
coherent monochromatic light instead of the full spectrum and random phase
distribution of starlight.

I think that your explanation infers that stars would shift colors unless
the atmosphere has a mechanism that would synchronize the phase changes of
all colors.  It is my experience that stars twinkle on and off, not shifting
colors.

On the other hand, I think my explanation works better for random phase
multi-chromatic light.  If I may use terms from "old" optics, the light rays
are bent, each in a random small angle.  Sometimes they cluster together,
sometimes they dispurse.  To carry the comparison further, if I stand on a
dock watching a fish, your explanation implies that the fish is where I see
it, whereas my explanation says the light is bent at the air-water interface
so the fish is not where I see it.  ( Okay, I remember that from an article
on Bowfishing. )  The difference between the fish and the twinkling star is
one distinct density change instead of miles of density gradient.

This may be a communications problem originating from the dual nature of
light.  Newtonian optics, or ray tracing, may not explain all of the
observable phenomina of light, but I think it is quite adequate for
twinkling stars.  Newtonian gravity may not explain all that Relativistic
gravity does, but it is adequate for ploting planetary orbits and the design
of balistic missles.

I tried to think of an affordable experiment to demonstrate this.  This
might work:  Set up a light source and a receiver at two sites about five to
ten miles apart.  On is located on top of a tall building ( here in Wichita,
that is six to ten stories ) and the other so that the light beam
immediately travels less than ten feet above a road, beach, or open field
for about a quarter mile.  Now the turbulence over the field or road should
be much greater than a similar distance near the building.

>From the wave theory for scintillation, then the turbulence should cause the
same amount of change in either direction.  If the ray bending theory is
correct, the beam received at the building should show more ( the same
change in angle acts over a much larger distance, therefore more effect )
than the beam received at the field.




As an additional note, I have used this as an atmospheric rule of thumb:
The atmospheric pressure drops by 1/2 for every 18,000 feet in altitude.
For a crude estimate assume constant temperature, hence density is
proportional to pressure.  You can then model the atmosphere from sea level
to 100,000 feet in 1,000 foot increments ( go higher or more increments if
you like ) with a proportional pressure drop at each increment.  From that I
get one "atmosphere thickness" is about 5 miles horizontally.  This value
would be useful for absorbtion, but I would not trust it for turbulence ( or
scintillation ).


Another post suggested that water vapor makes air more dense.  Quite the
opposite.  Water has a molecular mass of 18 rather than the composite value
of dry air which is a bit less than 29.


James
 n5gui


----- Original Message -----
From: "Terry Morris W5TDM" <w5tdm at hotmail.com>
To: <laser at mailman.qth.net>
Sent: Wednesday, August 22, 2007 1:15 AM
Subject: RE: [Laser] Scintillation and Adaptive Optics


> 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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