[Laser] AW: coherence and scintillation

[James Whitfield]

Yves

The comparison of a red laser to a star is an interesting experiement and
suggests that there are two mechanisms at work: light path bending and wave
cancelation.  For lack of better terminology, call them "twinkle" and
"cancelation".  Twinkle would (more or less) equally affect all light
sources, as is obvious from the twinkling of the stars.  Cancelation would
need at least some coherence.

What I started to suggest was another experiment.  If you had a low power
laser with optics to expand the beam to 100 mm ( or some other convenient
size and a cluster of LEDs the same diameter, at the same wavelength, and
emitting similar power, you could compare the two for how much atmospheric
noise is associated with each.  It is a better comparison to a star because
it would have the same path through the atmosphere and be monochromatic.

In theory, twinkle should increase with longer path through the atmosphere,
and if there is a local concentration of turbulence, the effect should be
greater as you get farther away from the concentration. ( You should get
different results if you pass a light beam through 10 Km of turbulent air
then 10 Km of calm air when compared to calm air first then turbulent. )
On the other hand, coherence is degraded the further a light beam passes
through the air.  This degradation results in cancelation.  But it would
seem that you should get less effective cancelation as the coherence is
degraded.


I am starting to suspect that "twinkle" is influenced much more by the
apparent diameter of the source than I previously thought.  It is well known
that planets do not twinkle, at least not as much stars.  If that is the
case, the experiement that I described above may not provide the "twinkle"
effects that I first thought.

Anyway, if there are two mechanisms, it does not seem to help solve the
problem of atmospheric noise on a light communication channel.  The
Australian experiments may be better for using LEDs instead of lasers.  I
started using LEDs instead of lasers for my demonstrations, but that was
because kids kept finding ways to look down the light beam.  LEDs were
easier to AM modulate and had all the range that I needed, indoors and out.


James
 n5gui

----- Original Message -----
From: <F1AVYopto at aol.com>

>
> I experimented this receiving conditions with a red laser  to more 70 Km
> range and the scintillation is well stronger the one created by a  star
near the
> same elevation.
> When the convergent beam is adjusted not  exactly at the perfect focusing
> point on the photo detector I noticed the  scintillation decreases.
> I noticed also if the laser is beamed on a  bouncing diffuse area the
> scintillation totally disappears on a RX beamed to  this bouncing area.
> A diffusing media fitted in front of the photodiode will  give losses but
it
> is also possible it could reduce a little bit the  scintillation.