[Laser] Target Mt. Blanc

Tue Apr 20 17:54:01 EDT 2010

I bet that is the reason Yves. The fluctuations are smaller than the beam diameter. The reflected light from the target spot would be coming from random areas inside the spot diameter as the laser beam scintillates. But because the receiver field of view is seeing the entire spot, the scintillations average out. And as you say, this also averages out any "fringe" modulations as well.
If the beam spot on the target was smaller than about 20 to 30cm diameter, then i would expect the received scintillation to increase. This would be due to the total returned signal coming from a "source" smaller than 20 to 30cm. 
If your optics are capable of focusing the laser spot to less than 20cm on a target then you could compare the signal qualities with the small spot and a larger spot, directly. 

---Yves wrote---
> 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 d?coherence noise and
> the scintillation noise may be because these chaotic 
> fluctuations are well smaller than the beam diameter ?

--- On Tue, 4/20/10, laser-request at mailman.qth.net <laser-request at mailman.qth.net> wrote:

> From: laser-request at mailman.qth.net <laser-request at mailman.qth.net>
> Subject: Laser Digest, Vol 67, Issue 6
> To: laser at mailman.qth.net
> Date: Tuesday, April 20, 2010, 11:00 AM
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>    1. Re :  Target Mt. Blanc (f1avyopto at aol.com)
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> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Mon, 19 Apr 2010 18:19:01 -0400
> From: f1avyopto at aol.com
> Subject: [Laser] Re :  Target Mt. Blanc
> To: laser at mailman.qth.net
> Message-ID: <8CCAE25727296A6-1944-1A5F at webmail-stg-d14.sysops.aol.com>
> Content-Type: text/plain; charset="utf-8"; format=flowed
> 
> 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 d?coherence 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
> Envoy? 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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> End of Laser Digest, Vol 67, Issue 6
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