[Laser] Re : Re : 5mw laser tranceiver kit (resent because mailing error)

[f1avyopto at aol.com]

"The statistical noise can only INCREASE upon encountering
a reflecting surface including moving foliage or (with clouds or atmospheric
dust) dynamic reflection variations."

This point is not right !
The optical noise depends of the density fluctuations of the photon flux in a cutting plan into the beam.
The averaging of this area intensity, in all this plan, reduces this noise but does not cancel it perfectly because the photon density fluctuation into this beam is not a flat structure but a 3 x D structure at a "t" defined moment.
Because the focusing and the defocusing effects, the light beam is noised by bright or dark irregularities.
At a macroscopic scale this random structure is very the same in all angular cutting plans.
The scale of these light irregularities is perfectly integrable into the optical thickness of many diffusing target types.
It is why clouds are able to integrate all these fluctuations with a  nearly fully cancelled scintillation noise in the global scattered light.
The targets movements do not add extra noise because the light has lost its coherence well before to reach the bouncing media.
No beating phenomenon can occur on this target and only albedos changing are possible with the movement.
All our experiments confirm this point and they are very easy to verify !
The bounced signal returned by a laser beam from a far away diffusing target is nearly perfectly noise free !!!
This effect occurs if the detector can "see" all the laser enlighted area on the target, if this target is really diffusing without any specular component and if the enlighted area is view as a non ponctual area to avoid secondary scintillation from the path "target to detector".
(Viewed from the earth a star is flickering when a planet is not flickering because its apparent diameter...)
73 Yves F1AVY
    

-----E-mail d'origine-----
De : Chris L <vocalion1928 at hotmail.com>
A : laser at mailman.qth.net
Envoyé le : Vendredi, 24 Septembre 2010 0:49
Sujet : Re: [Laser] Re : 5mw laser tranceiver kit (resent because mailing error)



Yves,

Whatever your argument - and I find these unconvincing, on the basis of several 
decades of our group's experiments, the momentary additions and cancellations 
produced by atmospheric turbulence within a fully coherent beam do not just 
"cancel out", they produce STATISTICAL NOISE, before they reach any reflecting 
OR scattering surface. The statistical noise can only INCREASE upon encountering 
a reflecting surface including moving foliage or (with clouds or atmospheric 
dust) dynamic reflection variations. Low frequency noise is scintillation. High 
frequency noise is hiss. The variations of cancellation and addition of a laser 
(coherent) beam readily produce both effects, as well as turbulence-produced 
beam steering, focussing and defocussing - which are most severe when the 
transmitted beam is sharply collimated, as in laser practice. Because laser 
radiation is spatially coherent, atmospheric turbulence can result in COMPLETE 
cancellations and additions of beam flux from moment 
 to moment, producing a beam 100% modulated with noise, and therefore incapable 
of reliably conveying modulation, except at very low frequencies. With a 
non-coherent beam, the additions and cancellations result in far less 
statitistical modulation variations, as Korotkova et al have shown:

http://pegasus.cc.ucf.edu/~okorotko/SPIE4976.pdf

Furthermore, the usage (as in your Yves' case) of extremely expensive LIDAR 
large-aperture diffraction limited optics (25cm+ diameter in your equipment, I 
believe), high power diode lasers, avalanche photodiode receivers, and their 
employment at near I-R wavelengths easily collected by a dark-adapted eye to 
produce potential retinal damage, results in an expensive, impractical, 
unreproducible (for the majority) and potentially dangerous communication 
system. Surplus optics are great - IF you can get them, and that does not happen 
often, resulting in superb but unreproducible outfits/ You may be able to buy an 
occasional surplus LIDAR or large telescope at prices beyond the expense account 
of most people, but why would anyone even consider this, when much larger molded 
Fresnel optics are available at a fraction of the cost and with far higher 
optical gain? Or when PhlatLight LEDs posessing an unprecedented visual 
intensity and a total flux output exceeding one radiometric Watt
  - several orders of magnitude higher than most diode lasers at 5 milliwatts or 
so - are cheaply (USA$35) available, and when modulation at a GHz rate is not 
needed. In fact, the transit time spread involved in reflection from irregular 
surfaces, clouds or haze would limit the modulation rate to 1/4 of the 
wavelength of variation in the reflection surface. Let's say you're reflecting 
from a cloud with a thickness of 30 metres, a scatter field 30 metres in 
thickness - that's a high frequency rolloff of 10 MHz produced by the transit 
time spread in the reflected or scattered modulation. PhlatLight LEDs can exceed 
that modulation bandwidth, so is there any need for the greater bandwidth of 
diode lasers for that service?

I think the time has come - as Clint has emphasised before me - in the 
assemblage of any kit for practical purposes, and especially in assembling a kit 
that may be used by young experimenters, to avoid past mistakes and drawbacks:

(1) Avoid the coherent (laser) beam option, owing to the high cost of glass 
diffraction-limited optics necessary for their usage, especially the cost of the 
large-aperture optics necessary for beam spreading to eye-safe flux densities.
(2) Avoid the coherent (laser) beam option, owing to beam noise and beam 
steering effects in atmospheric turbulence.
(3) Avoid the usage of near-IR. A visible beam can be much more easily focussed, 
collimated and aimed, and when eye exposure occurs, it produces an iris-down 
reaction in the recipient. To make an IR beam eye-safe, one really has to shift 
to operating in the middle-IR, above 1.5 micron wavelength. At those 
wavelengths, the detectors become much more expensive than Si photodiodes, 
emitters are very expensive and often limited to usage by the military by 
legislation, the thermal noise in detectors greatly increases over that of 
detectors designed for visible radiation, and optics assumed to be transparent 
at visual frequencies can often exhibit opaque behaviour.
(3) Use molded plastic optics, not expensive 1/4 wave accurate glass lenses or 
parabolic mirrors (and especially not coated optics - totally unnecessary). A 
10" telescope objective, even a simple parabolised telescope mirror from a 
Newtonian, can cost upwards of USA$500. A 10" Fresnel lens can be had for about 
$2, or (as stated by a recent English correspondent to this thread) about 99p.
(4) Use an optical receiver with an inexpensive P-I-N diode like the BPW34, 
working into a simple voltage amplifier (NOT a transimpedance amp) of a type 
designed by Clint - refer sections 7.3, 7.4 and 7.5 of this paper:

http://www.modulatedlight.org/Dollars_vesus_Decibels_colour.pdf

(5) If a PhlatLight is too powerful a device for a mass-produced kit, consider 
the usage of a red LuxeonRebel (around 1 watt input) or even a red Cree 
high-output LED of more standard design.
(6) If only night operation is contemplated, the Fresnels and their electronics 
can be mounted in a simple wooden skeleton framework. Focussing to infinity is 
all that is required - focussing adjustment is unnecessary, except in initial 
setup.
(7) Ideally, the tx and rx should be co-aligned (but optically baffled from each 
other by using an opaque board or cloth between them) in a single unit, to 
simplify alignment and aiming. Full duplex operation is then provided - and this 
is a major unrecognised advantage of optical comms.
(8) The optical unit can be mounted to a 1/4" whitworth T-bolt, to facilitate 
attachment to a standard photographic tripod (I use a Velbon VGB-3 or a wooden 
surveyor's tripod).

These are just suggestions, but as Clint has emphasised, the poor performance of 
existing optical comms kits and a general misunderstanding of the 
incompatibility of coherent light with atmospheric turbulence has led to 
performamnce expectations dismally below current possibilities.

Best wishes, and happy (practical) constructing,

Chris Long, VK3AML, 2 Newton Street, Surrey Hills 3127, Victoria, Australia.
    Tel: +61 3 9890 8164.

    http://www.modulatedlight.org

    http://www.bluehaze.com.au/modlight/

    http://www.modulatedlight.org/Dollars_vesus_Decibels_colour.pdf 

    

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