[Laser] 5 mw laser transceiver kit

[Chuck Hast]

On Tue, Sep 21, 2010 at 08:41, Chris L <vocalion1928 at hotmail.com> wrote:
>
> 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.
>
I guess I was not cynical enough, they spent all of that money to get
beam spreading and tracking which is what comes as part of the pack-
age with a LED based system. Someone will argue that they have the
higher speed, but I bet there is a way to get those high speeds out of
LED's too.

The articles were very interesting, but they did spend (and are spen-
ding) a boat load of money we don't have.

Yes lasers are neat to work with, and I believe that experimentation
should continue, but LED's offer a quick and efficient solution to the
issues that we are all looking at dealing with. I for one am interested
in high speed data links for disaster recovery while the normal links
are being brought back on line. Since I live in the state of Florida, we
have to deal with hurricanes, and I think that those of us who have
been around microwave dishes know what those winds can do to a
big dish, figure that there are a lot of those paths that could use a
optical link while the MW stuff is being re-aimed or the dish is being
replaced. I see now that every cell tower in my area has a small family
of dishes springing up somewhere below the cell sector antennas.

If I can do it with LED's I will certainly give it a try as it sure takes care
of a load of problems with exposure and fading.

I am interested in this as a part of my amateur radio disaster recovery
kit.

-- 

Chuck Hast  -- KP4DJT --
When the righteous are in authority, the people rejoice: but when the
wicked beareth rule the people mourn.  Prov 29:2 KJV
---
Home web site:
www.wchast.com

ZoneMinder Demo and test site
www.wchast.com/zm
Login = guest
pwd   = guest

FSE Emhart Veritas: ViM, ViB, and ViC.