[Laser] Re : 5mw laser tranceiver kit (resent because mailing error)
f1avyopto at aol.com
f1avyopto at aol.com
Thu Sep 23 13:47:48 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.
We 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.
http://sd-1.archive-host.com/membres/up/22679775843705539/F5PNP.wav
One can hear only the flat thermal noise of the receiver.
73 Yves
-----E-mail d'origine-----
De : Chris L <vocalion1928 at hotmail.com>
A : laser at mailman.qth.net
Envoyé le : Mercredi, 22 Septembre 2010 23:52
Sujet : Re: [Laser] 5mw laser tranceiver kit
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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