[Laser] Re: Fresnel lenses

[Tim Toast]

for purposes of discussion or for anyone who hasen't
seen them yet, i posted these pics from Paul Cianciolo
  (W1VLF) on one of my backup websites:
http://www.angelfire.com/space2/toast/

Thank you for the info and pics paul 

That was very interesting to see how the signal varies
in amplitude over time with the moving clouds relative
to the noise level in an adjacent band (980 Hz). I
haven't done any of these "Lidar" type experiments yet
(I didn't even realize spectrum lab had an amplitude
plotter! much less a multi-channel one.. DOH! I've
been learning how to set it up tonight and playing
around with the settings and all)

I was thinking that the relatively wide FOV isn't
hurting you much with this, as you can aim it near the
"edge" of the detector to eliminate all but the
"point" signal of the laser beam spot. I would think
too, that the extra wide FOV does not contribute much
extra noise, at night anyway and with those nice
return signal levels. I had read somewhere that its
normal to have a wider receive FOV than the laser spot
diameter on the target with "lidar" type setups. I
assume that's mostly because it's possible, with
lasers, to have a very narrow spot on a target. You
can always widen the beam depending on what your doing
i guess.

For looking at different heights though, i can see how
a narrower FOV would help discriminate and confine the
return signal to the desired beam height better - and
also to move the transmitter further away from the
receiver to make it easier to change the heights
looked at and to further narrow the length of beam
viewed by the receiver.

I assume the clouds moving through the field of view
are causing most of the signal strength changes in the
graph but some of that may be due to scintillation and
path loss slowly changing with time. I assume with a
very narrow laser spot, most of your resolution is
coming from the spot and not your receiver FOV. In
other words, it's the laser spot size and not the
receiver FOV size that is the limiting factor in the
smallest details your system can "see". So your
effective FOV IS the laser spot size for all practical
purposes even though the actual receiver FOV is many
times greater than this spot size.

The very pronounced "dip" in the middle of the chart
looks like it might have been an "edge" of a cloud
moving out of the field of view, or more properly, out
of the way of the laser spot illuminating it. 
The slow rise in amplitude is cloud related i'm sure
as well as most of the short term peaks and valleys. 

The odd rise in amplitude towards the edge of a cloud
(if that's what caused the dip) though seems strange
to me but it may be due to the angle with which you're
looking at them and the way they were moving across
the FOV. I would almost expect that to look different
when clouds are moving by at other angles to the beam
and the differing shapes/types of clouds etc..? 

On the very steady noise floor in the adjacent band
(980 Hz) I was thinking most of the general "noise" in
the sky is due to 120 Hz street light buzz and the
harmonics, so i wouldn't expect this noise level to
change unless you were measuring on or close to one of
the lines. Most of them are quite weak up near 1000 Hz
i'm sure. The closest ones being 960 and 1020 Hz...
yes,' your laser is right on one of the lines but the
line is so weak i doubt it's bothering things much. 
I haven't actually tried to measure the 1020 Hz
harmonic here, maybe it's stronger than i'm thinking
it is... another experiment to try


Some things this makes me want to try:

To set up a stationary target some distance away and
make an amplitude graph of it over time, to see if
those small term variations are present and that
they're mostly due to changing path losses etc.. But
if nothing else, a stationary target test would let
you see the pure scintillation and path loss over time
all other things being equal. Maybe that slow creeping
upwards is a purely atmospheric effect and not so much
the actual cloud reflectance changing. Repeat the test
in clear skies or a stationary ground target maybe to
clarify that some.

Another thing i wanted to try or a suggestion to try,
would be to have two lasers illuminating the same spot
on a target but modulated at different frequencies, so
you could be assigning each to a separate channel of
the software and displaying them both on the graph
side by side. The idea being to see if both signals
change the same way or not. If the two transmitters
are spaced apart enough, they would be sampling
markedly different beam paths as they travel to the
target. Any difference between the two signals would
be due mainly to the different path each took... At
the same time, a similar amplitude of each beam would
indicate purely the cloud reflectance. 

At any rate, I hope you'll be posting this stuff on
your website eventually so everybody can see. And good
luck with the rest of the experiments you try!

-toast


>
on Mon, 13 Jun 2005 14:41:31 -0700 (PDT) "Paul
Cianciolo" wrote:
>
>
Hey Tim,
 
Thanks a bunch for the time to answer my email. I was
never sure if the answer came out in mR or in degrees.
 
Where does the constant 57.3 come from?  I have seen
formulas  that state FOV = detector dia/ focal length.
 
I have a 1 watt laser diode that I want to collimate a
single 5" lens. The diode has a fiber output angle
that closely matches a 5" lens I have.
 
I have been doing some experiments with an 808nm at
150 mw if you want to see a pix of cloud bounce at
straight up I have that I can send you. Also Have a
clear sky night trace of signal return as well.

Both are graphs showing noise floor and the 1 Khz
square wave.  Boy does the signal pic up when you
follow the light column up and finally hit the dot on
the clouds.
>
>From: "Paul Cianciolo" 
>


Tim Toast
http://www.aladal.net/toast/exp.html


		
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