[Laser] 5mw laser tranceiver kit

Thu Sep 23 06:17:56 EDT 2010

Chris 
I agree most of the points of your comments. 
It stays a point can be discussed about the scintillation on the global signal received from a target. 
The scintillation noise is a very complex process in a laser beam. 
Many phenomenons occur because cahotic air density changing. 
(Angular changing, ponctual beam speed changing etc...) 
However something stays always right. 
You cannot cancel the energy of a photon pair in a beam point by destructive beating (phase combination) without egal increasing of the energy of an other photon pair at an other point by constructive beating. 
It is the energy conservation principle. 
Korotkova and the others analyse the beam structure at the RX point in a without thickness plan. 
A target is never a perfect plan and its equivalent thickness can be very wide (eg clouds). 
In these conditions the scintillation noise seems fully canceled. 
F1ORL F5PNP F8DO and me did many experiment from 5 Km to 10 Km to various targets to test this effect (factory chimney, grassy hill, local cloud layers etc) with a simple direct audio voice amplitude modulation. 
In these condition the signal can be heard clearly directly at the RX output without FFT process. 
As you will notice, this signal is absolutly noise and scintillation free. 
One can hear only the flat thermal noise of the receiver. 
http://sd-1.archive-host.com/membres/up/22679775843705539/F5PNP.wav 
73 Yves 
Yves F1AVY
http://f1avyopto.wifeo.com

--- En date de : Mer 22.9.10, Chris L <vocalion1928 at hotmail.com> a écrit :

> De: Chris L <vocalion1928 at hotmail.com>
> Objet: Re: [Laser] 5mw laser tranceiver kit
> À: laser at mailman.qth.net
> Date: Mercredi 22 septembre 2010, 23h52
> 
> Let me quote a part of Tim's statement about Yves'
> non-line-of-sight optical transmissions in his previous
> post:
> 
> "he [Yves] removes scintillation by looking at the beam
> spot reflected by some object. The entire beam spot is
> focused on the detector. And so most of the scintillation
> averages out and goes away."
> 
> Actually, the scintillation caused by the turbulent air
> between Yves' transmitting laser and the reflecting surface
> will remain, owing to the total beam cancellations that must
> occur with any coherent transmission system through
> turbulent air. These cancellations may be averaged, to an
> extent, by the imaging of what is left of the whole beam,
> but it will still appear as overall communication system
> loss. Refer Korotkova's [et al] experiments and
> masthematical analyses in removing a part - or the whole -
> of a communication laser's spatial coherence PRIOR to
> atmospheric transmission:
> 
> http://pegasus.cc.ucf.edu/~okorotko/SPIE4976.pdf
> 
> The point is, after transmission through 50 or more
> kilometres of turbulent air - or even 5 km - the beam is not
> and cannot be spatially coherent any more. The air has
> removed the beam's spatial coherence, but in an
> unpredictable way, full of complete cancellations and
> additions producing severe beam noise and scint additional
> to that encountered with a non-coherent beam, and the
> radiation then diverges like any other non-coherent beam, to
> an extent partially (dynamically and unpredictably) dictated
> by the degree of air turbulence, and partially by the usual
> dictates of tx aperture versus diffraction. We have already
> shown that optical apertures achieved with molded (plastic
> Fresnel lens) collimating optics can be achieved for perhaps
> 0.1% of the cost of the equivalent size aperture of an
> accurate glass optic, and often with less spherical
> aberration.
> 
> This disadvantage of coherent (laser) beams will affect any
> beam projected horizontally through the atmosphere. If,
> however, the beam is pointed straight up, as it would be in
> earth-to-space comms, the beam is only going through "thick"
> atmosphere through its first couple of kilometres of its
> travel, and most of the air laminae are then at right angles
> to the beam direction. The optical advantage with lasers is
> then indisputable - LIDAR, satellite ranging, and even
> optical moonbounce, possibly even from the retroreflectors
> left at the Apollo 11 site, but the returned signals will
> then be exceptionally tiny, and expensively recovered.
> 
> The principal objection that experimenters like Clint
> (KA7OEI) and I have to lasers is one of extreme cost - not
> only of a high-powered laser itself, if it's to have
> anything even approaching the available modulated flux of a
> PhlatLight LED, but of the 1/4 wave accurate large-aperture
> glass collimating or beam-spreading optics that lasers
> necessitate. Today, the high output LED and the
> exceptionally low price of molded optical acrylic Fresnel
> lenses permits cheaply achieved tx and rx beam apertures up
> to about 1.5 metres square. To get a beam aperture 1.5
> metres diam with a laser involves the cost of a major public
> astronomical observatory instrument - if that instrument can
> be swung down far enough to aim horizontally to another
> comms site!
> 
> I think I speak for Clint's (KA7OEI's) group as well as our
> own in Australia (Dave VK3QM, Mike VK7MJ and myself) when I
> state that our aim is to make something facile, portable,
> affordable, REPRODUCIBLE (note that emphasis) and useful for
> at least voice bandwidth and, if possible, relatively broad
> bandwidth. We are not interested - at the moment, anyway -
> in sub-noise level systems dependent on wsper, WSJT or any
> of the ultra-slow CW so-called "digital" systems. We leave
> that to those investigating marginal scatter paths, to those
> wishing to meet the bare minimum definition of a "DX
> contact", or to those wishing to measure system loss in such
> marginal paths. The prospect of inexpensive, license-free
> links with full duplex and/or a continuous return error
> correction path is far more attractive - to us. 
> 
> Ultimately, we believe that an affordable solution to such
> practical ground-based communication demands can only be
> affordably met by avoiding coherent beam communication
> systems at optical wavelengths. Refer:
> 
> http://www.modulatedlight.org/Dollars_vesus_Decibels_colour.pdf
> 
> Best wishes to all of those experimenting in this field,
> 
> Chris Long VK3AML, 2 Newton Street, Surrey Hills 3127,
> Victoria. 
> Tel: +61 3 9890 8164.
> 
> http://www.modulatedlight.org
> 
> http://www.bluehaze.com.au/modlight/
> http://www.modulatedlight.org/Dollars_vesus_Decibels_colour.pdf
> 
> ========================================================
> 
> > From: vocalion1928 at hotmail.com
> > To: laser at mailman.qth.net
> > Date: Wed, 22 Sep 2010 14:08:09 +0000
> > Subject: Re: [Laser] 5mw laser tranceiver kit
> > 
> > 
> > Already done!
> > 
> > Point 1/ Clint KA7OEI has exactly that type of
> tone-based anti-scint option on some of his transceivers. We
> used it on the occasion on which we communicated across
> Yucca Vallay in Southern California in February 2008, when I
> manned one station and Clint joined Bob Legg W6QYY on the
> other. However it is desirable to maximise the tx and rx
> beam aperture to reduce scint optically, even before
> applying these electronic "cosmetic" systems to the signal.
> If the signal completely goes to zero, as it can with
> coherent systems where total cancellation can occur, no
> degree of AGC can recover anything from zero signal.
> > 
> > Point 2/ Our current fold-up design will employ two
> PhlatLight LEDs working through two co-aligned Fresnel
> lenses, placed either side of a receiving Fresnel. By
> splitting the source in this way, one achieves the maximum
> possible aggreagate tx aperture (spatial diversity) without
> recourse to separate optical units demanding separate
> alignment on the target. You will note by reading our
> account of our Tasmanian test in 2005, reproduced here:
> > 
> > http://www.bluehaze.com.au/modlight/
> > 
> > ...that we indeed had TWO complete optical
> transceivers on Mount Barrow to receive our signals over 168
> km. Listening to the signal, effectively, in stereo, with
> each receiver separated from the other by a couple of
> metres, we could easily hear that the interpretation of
> signals from both was more intelligeable when clouds
> intervened than it was in listening to either one. A metre
> or two is all it takes, possibly even less for effective
> spatial diversity.
> > 
> > Best wishes,
> > 
> > Chris Long VK3AML.
> > 
> > ======================================
> > 
> > > Date: Wed, 22 Sep 2010 05:40:09 -0700
> > > From: toasty256 at yahoo.com
> > > To: laser at mailman.qth.net
> > > Subject: Re: [Laser] 5mw laser tranceiver kit
> > > 
> > > 
> > > Hi Tom
> > > 
> > > Yes, i believe there might be a couple ways to
> partially cancel 
> > > out atmospheric scintillation by using a full
> duplex link. I 
> > > think it would amount to a fast ALC at the
> transmitter using 
> > > feedback from the distant receiver "target". Or a
> fast AGC at 
> > > the receiver which uses a pilot subcarrier tone
> from the 
> > > transmitter to control it. A kind of real-time
> active feedback 
> > > of channel conditions from one station to the
> other.  
> > > 
> > > 1.
> > > The distant receiver supplies the "feedback" by
> sending out a 
> > > pilot tone, say 1 khz, with constant amplitude.
> While traveling 
> > > through the atmospheric path, the tone is
> amplitude modulated 
> > > with the attenuation that is occuring at that
> instant. The 
> > > transmitting station uses that data to
> "pre-adapt" its own 
> > > modulation amplitude according to the current
> conditions of the 
> > > channel. If the channel is fading, the
> transmitted audio is 
> > > increased. If the channel is peaking, the audio
> is reduced. 
> > > The result is an outgoing audio stream that tries
> to pre-
> > > compensate for channel conditions in real time.
> Since the 
> > > transmitter and receiver both share aproximately
> the same 
> > > atmospheric path, signals coming from either
> station would 
> > > contain nearly the same path-loss data. 
> > > 
> > > This ties up one of the duplex paths of course.
> On the reverse 
> > > communication, the roles are reversed and the
> other station 
> > > transmits the tone signal. So whichever station
> is listening, 
> > > sends out an unmodulated tone for the other
> station to use as 
> > > feedback. 
> > > 
> > > To restore the full duplex, the feedback pilot
> tone could be 
> > > moved out of the baseband, to a higher frequency
> where it would 
> > > not interfere. Like a 20 khz tone. This amounts
> to an unmodulated 
> > > subcarrier. At the transmitter where the tone is
> used, it is 
> > > detected and the low frequency envelope is used
> to drive the 
> > > transmitter ALC. So, each receiver has two
> channels, one for the 
> > > tone and the other for regular audio. Idealy if
> the the channel 
> > > fades 10 dB, the transmitter increases amplitude
> by the same 
> > > amount, 10 dB. 
> > > 
> > > 2.
> > > Another way to do this would be for the
> transmitting station to 
> > > send the reference tone signal piggy-backed on
> its own audio as 
> > > a subcarrier. The receiver would use this pilot
> tone to drive its 
> > > AGC, so that it's receiver gain is changed to
> compensate for the 
> > > transmitted tone scintillations (and thus the
> received baseband 
> > > audio signal since it shares the exact same path
> literally). So 
> > > the receiver station modulates its receiver gain
> in accordance 
> > > with the pilot tone variations. In this case the
> transmitter sends 
> > > plain audio that is uncompensated and the pilot
> subcarrier. It is 
> > > at the receiver where the scintillations are
> cancelled. This is 
> > > really the tradishional receiver AGC although it
> is not derived 
> > > from the base band audio but the pilot carrier or
> subcarrier 
> > > signal instead. This second method may be a bit
> better than the 
> > > first since it uses a reference tone that travels
> in the same 
> > > path through the entire atmospheric path, whereas
> the first 
> > > method is offset a little from the same path and
> so is a bit 
> > > different. A combination of both a receiver
> subcarrier AGC and 
> > > feedback ALC at the transmitter could increase
> the combined 
> > > effects. In this case both stations are supplying
> a reference 
> > > signal the other uses. 
> > > 
> > > Other ways to do this are diversity reception and
> actually using 
> > > the whole signal instead of only part of it.
> Diversity reception 
> > > amounts to using more than one small piece of the
> light beam. An 
> > > extreme case of diversity reception is: the
> receiver lens captures 
> > > the entire light beam from the transmitter, where
> it will be 
> > > averaged on its photodetector (if it is big
> enough). This is the 
> > > type of thing Yves talks about when he removes
> scintillation by 
> > > looking at the beam spot reflected by some
> object. The entire beam 
> > > spot is focused on the detector. And so most of
> the scintillation 
> > > averages out and goes away. 
> > > 
> > > Maybe this type of thing is practical to do, i
> dont know for sure 
> > > since i havent tried it. Maybe it has already
> been tried before by 
> > > those who know more about it than me. When the
> observitories use 
> > > the artificial star method, and the complex
> adaptive optics, they 
> > > are compensating for the direction the light is
> coming from in 2
> > > dimensions x-y. A third channel would be the
> amplitude but i think 
> > > the detector itself is integrating the amplitude
> directly (with 
> > > film or a ccd) and they are not worried with
> amplitude changes in 
> > > the signal for the most part.
> > > 
> > > 
> > > > 
> > > > I wonder if scintillation can be corrected,
> akin to a
> > > > terrestrial 
> > > > observatory using a laser to produce an
> artificial star
> > > > whose motion is 
> > > > removed from the objective image? 
> Could, for
> > > > instance, twin paths - or 
> > > > full-duplex colinear paths - be useful to
> remove the
> > > > effect?
> > > > 
> > > > Tom
> > > > http://www.ustream.tv/channel/bowcam
> > > > http://www.ustream.tv/channel/cape-coral-marine-radio
> > > > VHF
> > > > http://67.207.143.181/vlf9.m3u Lightning, spherics
> > > > 
> > > 
> > > 
> > > 
> > >       
> > >
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