[Laser] Re : 5 mw laser transceiver kit

Tue Sep 21 10:10:06 EDT 2010

I am sure that "even with an ultra low cost laser system it is possible to do very interesting com experiments", but basic atmospheric physics tells us that with non-coherent beams, the results will be less subject to scintillation.

I've given the mathematically worked proof of Olga Kotokova, Larry Andrews and Ron Phillips:

http://pegasus.cc.ucf.edu/~okorotko/SPIE4976.pdf
http://pegasus.cc.ucf.edu/~okorotko/SPIE4821.pdf

I can also cite this moving image of the speckle pattern of a laser and a LED beam received in a telescope aperture, for comparison of scintillation, through several kilometres of atmosphere:

http://ka7oei.com/optical_comms/laser_pointer_led_fresnel_scintillation_2a1b1.mpg

Let me put this very simply, in a way that anyone with the most basic powers of observation will recognise:

Observe a non-coherent torch beam pointed at a wall. It is seen by the eye as a steady, unvarying light. A smooth "carrier wave" if you will.

Now reflect a laser pointer from the same wall. It twinkles, its reflection looks "granular", and frequently it will be seen surrounded by diffraction rings. These effects are produced by the break-up of coherent reflected wave-fronts, in the reflection off the irregular surface of the wall, and in the passage to your retina through your eye. The same constructive and destructive interference patterns occur in an optical comms beam transmitted through any appreciable distance of air, owing to turbulence. The last thing that one wants in a communication beam is "granularity". 

*Granularity = noise.*

Only a reduction of spatial coherence will minimise the scintillation. Refer:

http://pegasus.cc.ucf.edu/~okorotko/SPIE5160.pdf
http://www-ee.stanford.edu/~jmk/pubs/trans.com.ml.det.turb.pdf

So, Yves, we've demonstrated this effect via mathematics, via optics, in practical systems, by observation, by motion picture record and by measurement. If this is not accepted, I can do no more.

But meantime I wish you every good luck with your experiments.

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

============================================

> To: laser at mailman.qth.net
> Date: Tue, 21 Sep 2010 09:42:17 -0400
> From: f1avyopto at aol.com
> Subject: [Laser] Re :  5 mw laser transceiver kit
> 
> Chris,
> I know I never convince you but even with an ultra low cost laser 
> system it is possible to do very interesting com experiments.
> The recording in appendix is from F1CDT to 73 Km with a 3 mW red laser 
> (a modulated 5 dollars laser pen with its native small plastic lens)
> The RX has only a 22 cm spherical mirror (Low cost bathroom magnifier 
> :o) and a basic K3PGP RX.
> The scintillation is not so strong and the global cost very low.
> http://f1avyopto.wifeo.com/musiques/QSO.mp3
> With a very large RX aperture the scintillation is reduced by area 
> integration and far away the TX the laser light has lost its coherency 
> and can be received very well even by a Fresnel lens and a large area 
> photodiode !
> When the bouncing area used to receive a laser is large enough, the 
> scintillation fully disappears.
> We are working on a very powerful infrared laser that will be used in 
> full compliance with regulation rules into a 1.5 m telescope.
> The power density and the pulses duration and rate will be eye safe 
> even at the telescope output...
> It is a full team work with the help of responsive and competent 
> engineers.
> 73 Yves
> 
> 
> -----E-mail d'origine-----
> De : Chris L <vocalion1928 at hotmail.com>
> A : laser at mailman.qth.net
> Envoyé le : Mardi, 21 Septembre 2010 14:41
> Sujet : Re: [Laser] 5 mw laser transceiver kit
> 
> 
> 
> Sorry Yves, but...
> 
> (1) Daytime operation with red PhlatLight LEDs (50% power bandwidth
> 19nm @ 625nm
> bandwidth, allowing moderately narrow optical filtering) is easily
> possible when
> high-gain, large-aperture molded Fresnel optics are used.
> (2) The cost of molded high-gain Fresnel optics are several hundred
> times less
> than glass optics, for the same aperture area, therefore more
> practical, and
> more reproducible. How much would it cost to buy (retail) your LIDAR
> optics,
> Yves? Could any of us afford to duplicate that? Whatever extra
> directivity the
> laser optics may have, the ability for high output LEDs to be used with
> non-diffraction limited Fresnels gives them a huge advantage in
> possible and
> affordable transmission aperture and optical gain.
> (3) The flux output of a PhlatLight can be several thousand times that
> of a 5mW
> laser. Whatever light is lost via the broadening of the tx beam
> actually aids
> practicality, by reducing the need for continuous beam steering and
> ultra-steady
> optical mountings.
> (4) PhlatLight LEDs can be modulated with relative ease to at least
> 20MHz
> bandwidth. Laser diodes only start to have a modulation advantage in
> excess of
> about 50MHz. I know of no radio ham needing such a bandwidth. Laser
> diodes may
> be faster, but does any ham need THAT kind of speed?
> 
> Chuck, you're probably more "on the money" when you say:
> 
> "I think that the laser gives it a gee whiz bang view, you can blow
> smoke at the
> beam and it looks very impressive, usually the LED beam is broad and
> harder to
> show, but that is where it's value is."
> 
> In other words, "hang practical communication systems, lasers are
> cool!" Maybe
> cool, but maybe not financially or environmentally practical -
> especially in
> terms of atmospheric physics, eye safety, scintillation, range and
> ancillary
> optical cost.
> 
> I also have grave reservations about encouraging young and possibly
> irresponsible people to build laser-based links, even of the 5mw
> variety. Maybe
> use Luxeon "Rebels" or similar high output LEDs, suitably spread to low
> unit
> area flux by a cheap molded Fresnel collimating system, but NOT laser
> diodes in
> a night environment where dark-adapted eyes can intercept the beam -
> and ALL of
> a pencil-thin beam through their open iris to their unprotected retina
> - too
> easily.
> 
> You go on to say:
> 
> "There are those who are actually doing interesting work with
> atmospheric FSO
> links, but it is I guess complicated."
> 
> You then quote some Navy experiments with lasers operating in the
> middle
> infrared (1535 - 1565nm) followed by fiber optic light amplifiers
> producing 5
> WATTS (NOT milliwatts) of radiometric flux output (and NOT just 5 watts
> of
> INPUT), collimated through 10cm diameter glass optics ground accurate
> to better
> than 1/4 wavelength @ 1535nm, detected by an InGaAs avalanche
> photodiode 200
> microns in diameter. The military needs link systems that are
> undetectable,
> using cutting-edge encryption and capable of communication at a GHz
> rate. Do we?
> This high power Navy system achieves a rather underwhelming range of
> 16.3 km.
> Maybe more in the vacuum transmission medium of outer space, where
> perhaps its
> eventual application is intended. And have you any idea of how many
> hundreds of
> thousands of dollars such a system costs? Or how far from the realities
> of ham
> optical communication at visual frequencies this is?
> 
> I was invited to SPIE in California to give this paper in 2006, partly
> because
> Clint KA7OEI, Mike VK7MJ and I had achieved - for less than $70 per
> duplex
> transceiver - what the military had failed to do for almost a million,
> in terms
> of range and reliability:
> 
> http://www.modulatedlight.org/Dollars_vesus_Decibels_colour.pdf
> 
> Some basics to be faced:
> 
> Lasers suffer badly from atmospheric scintillation. If you want to use
> them for
> FSO, it's best to remove their spatial coherence while maintaining
> their narrow
> bandwidth, by the use of a diffusing filter as specified in these
> papers by Dr
> Olga Korotkova of UCF, who measured the way in which bit error rate
> increased as
> the percentage spatial coherence of a comms beam increased (refer
> especially
> figure 14):
> 
> http://pegasus.cc.ucf.edu/~okorotko/OptEng43.pdf
> 
> Unfortunately, such a diffusing filter also increases dispersion and
> loss --- so
> one gets back to the usage of non-coherent sources. Refer Clint's
> comparisons of
> laser and non-coherent beams:
> 
> http://ka7oei.com/Coherent_versus_noncoherent_test.html
> 
> Well fellas, if you want to go no further than a kilometre or two,
> expensively,
> with 5mW and far from optimum signal to noise ratio, the use of a laser
> diode
> pointer is a great way to achieve a mediocre result.
> 
> Chris Long, VK3AML, 2 Newton Street, Surrey Hills 3127, Victoria. Tel:
> 9890
> 8164.
> http://www.modulatedlight.org
> http://www.bluehaze.com.au/modlight/
> http://www.modulatedlight.org/Dollars_vesus_Decibels_colour.pdf
> 
> ==================================================
> 
> > Date: Tue, 21 Sep 2010 05:09:11 -0700
> > From: toasty256 at yahoo.com
> > To: laser at mailman.qth.net
> > Subject: Re: [Laser] 5 mw laser transceiver kit
> >
> >
> > I guess for me it's mainly the low power aspect of it. Not just
> > that though, the prospect of running it on 2 "aa" batteries
> > that could last for weeks of intermitant use. Also the challenge
> > of making it cheap and simple. If i could make one using four
> > transistors and a laser diode dug out of an old dvd player, all
> > the better. For an extra bell and whistle, maybe a digital I-O
> > and a small solar pannel glued to one side to charge the
> > battery!
> >
> >
> > ---- Chris L <vocalion1928 at hotmail.com> wrote:
> > >
> > >
> > > ...why do people persist with lasers for atmospheric optical
> communication?
> > >
> >
> >
> >
> >
> >
> >
> >
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