[Laser] 5 mw laser transceiver kit

[Chris L]

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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