Real time active aiming?( was Re: [Laser] Laser comm stuff)

[Jim Moss]

1)corner cubes: For paths where the beam diameter is much greater than the
target, the corner reflector will only return its amount of power (area), and
then disperse it over the path back.  The return signal is much much smaller
than 1/2! Autoalignment systems with corner cubes is really only practical over
shorter distances.

2) scintillation:  There are a few papers talking about air turbulance "cell
size".  From what I can recall 6" is the average size.  While you may think
that a larger beam size will help... the problem is that each cell causes
random direction changes resulting in random power per unit area variances at
the RX end.  This can be compensated by increasing the RX power capture area
(larger RX lenses), but quickly becomes impractical or uneconomical.  For short
hauls, beams are kept less the cellsize. This keeps the beam power more
concentrated and going in the same path.  However, wander becomes more evident.
I think I have some links from my webpage under PROPAGATION.
www.qsl.net/wb9ajz/laser/laser.html

Note that once again.. if you are going for LONG paths... you WILL end up with
beam size greater than the turbulance cell size... so it is something we need
to deal with. Large detectors/lens systems will optimize total power captured,
and average distribution variances across the area.  You may also recall that
large detectors result in greater capacitance, therefore lower data rates...

3) active aiming: I use a pair of compumotor micro steppers controlled via
parallel port for raster scanning of the target. Feedback via radio.  I have
not tried short distance compensation for vibration etc. With a good tripod (on
the ground) and a good optical bench, those effects are essentially
eliminated...

Also so far only been using the full 1-2mR, haven't used any 10x expanders
(which reduce to .1 to .2 mR). At 20 miles 1-2mR is incredibly strong, but has
the problem of scintillation, which I don't think you will be able to
compensate for with x-y directional changes alone.  Some good stuff on this on
the net if you read about guide stars and adaptive mirrors for telescopes.

Jim
N9JIM/6

--- G0MRF@aol.com wrote:
> In a message dated 7/23/03 6:58:01 AM GMT Daylight Time, 
> derekw@alphalink.com.au writes:
> 
> 
> > Sure, but the tighter the beam the faster the possible data rate, all
> > else being equal.
> > _______________________________________________
> > 
> 
> Well......Almost equal. I found that working a signal from a station using 
> beam expanders was much more difficult than a signal with a 1 or 2m Rad 
> divergence.  The problem appeared to be scintillation of the beam as it
> passed through 
> the atmosphere. The 'scattering' or 'refraction' caused the narrow beam to 
> flutter between no signal and full saturation. While the normal beam rarely 
> moved out of the receiver lens.
> 
> 73
> 
> David
> 
> 
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