[Laser] Re : Target Mt. Blanc

[f1avyopto at aol.com]

Hi Tim
  From the Beaujolais (near my home JN26SE), the Mont Blanc is rather 
often
viewable to 170 Km just before a rainy day, during a south wind period.
For the YungFrau it is an other story and this mountain is visible to
270 km a few days each year in very good optical air transparency
conditions especially in autumn.
The top is just visible because the atmospheric refraction but the pure
geometric path is normally impossible because the earth curvature.
In theses conditions the air transparency gives a loss well lower than
0,1 dB / Km in red and near infrared … !
I was very surprise to notice a laser beam that illuminates a diffusing
area gives a very noiseless bounced signal.
This area integrates the most of the décoherence noise and the
scintillation noise may be because these chaotic fluctuations are well
smaller than the beam diameter ?
An other advantage is a well better linearity of the photo detection
versus the laser diode current modulation.
Look at a red laser pointer on a far away flat white target :
When you vary slowly its current, you will notice the laser “spot”,
that is a roughly elliptical patch with many concentric interference
fringes, changes in intensity but also in size and area !
It is because the pumped volume change in the laser diode junction
versus the current.
The global beam energy follows rather well the laser diode current but
the energy crossing a small area into the beam does nor vary with the
same law.
Even the fringes moves with the modulation and if the detector is near
the beam limit, the smallest modulation index can gives very powerful
pulses in the receiver !
The laser pulses width modulation reduces this phenomenon because the
current stays constant during the “on” periods.
In NLOS, the laser can be modulated linearly in current and the quality
stays very good without noises and distortions.
During the audio voice recording attached into my last mail, the used
laser was modulated with this very simple circuit :
http://f1avyopto.wifeo.com/images/K0-AUDIO-VOICE-LASER.jpg
About the ash cloud from the volcano eruption in Iceland, nothing seems
viewable from the ground :o))
73 Yves F1AVY
http://f1avyopto.wifeo.com

-----E-mail d'origine-----
De : Tim Toast <toasty256 at yahoo.com>
A : laser at mailman.qth.net
Envoyé le : Lundi, 19 Avril 2010 12:40
Sujet : [Laser] Target Mt. Blanc


Hi Yves,
In that video, i half expected to be able to see the laser spot on
snowy Mt.
Blanc even though it is over 160km away :-) From that same location, it
is
tempting the way Mt. YungFrau sticks her peak out just visible at 270km
like
some kind of mountainous burlesque show!
It is very interesting about the way a NLOS signal can integrate the
coherence
noise and scintillation of the laser. By paying a price in signal
strength via
NLOS, you have reduced the coherence noise to almost nothing. I wonder
how this
relates to the diffusing screens talked about in those papers by Olga
Korotkova
on direct paths? In the case of NLOS, it is as if the bouncing point is
acting
as the diffusion screen.

Also, have you noticed any effects from that ash cloud from the volcano
eruption
in Iceland?

-toast






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