[Laser] Re : 5 mw laser transceiver kit

Tue Sep 21 09:42:17 EDT 2010

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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