[Laser] laser optics questions

Sat Mar 20 04:20:21 EDT 2010

OK Charles, 

Your quotation of "76 mm" collimator for $50 as a "cheap" 
diffraction limited optic only serves to emphasise
how much cheaper the molded Fresnel optics are.

The SMALLEST Fresnels that we used for our 2005
two-way duplex 'phone optical DX record in Tasmania 
(104 miles or 167.8 km) had a working aperture of 
200 mm by 270 mm, and they were puchased
for Aust$3 = USA$2.70 apiece. Compare costs:

Glass 76mm diameter = aperture area 4537 sq mm @ $50.00.

Fresnel 200mm X 270mm = area 54000 sq mm @ $2.70.

Cost per working aperture area, even with this "cheap"
glass optic is therefore  54000/270 vs 4537/5000, or about
two hundred and twenty (220) times more costly to implement
with "cheap" diffraction-limited optics than by using equivalent area
Fresnels and high power LED's. The Fresnel tactic
eliminates the need for costly, hard-to-get diffraction-limited
cylindrical "secondary" optics. The Fresnel tactic also greatly
reduces the terminal optic's weight and ease of mounting.

The 850 nm laser with 1.5mW of optical output seems
fairly puny in comparison with the latest PhlatLight
LED's, which can produce an optical output exceeding 1 watt.
This is inherently eye-safe when it's spread into a minimally
divergent beam of 54000 sq mm effective aperture by
cheap Fresnels. We have also tried 830 nm LED's side-by-side
in equivalent optical housings with high-power red LED's, and
the difference of atmospheric transmission of the beams was 
so close in dB as to be insignificant. The difference in response
with an Si detector was also insignificant between 850 nm and
630 nm. Maybe less than 2 dB. The visibility of our beam (red light, 630 nm)
is also inherently easier to focus, collimate and steer.
Beam aquisition done by eye is beyond simplicity itself,
particularly in a binocular collimator with tx and rx inherently co-aligned,
and the typical 15 arc-minutes LED/Fresnel beam divergence allows
simple photographic tripods to be used for mounting. Sure, some
beam power is lost via divergence, but with far higher initial
source power and far greater aperture from the Fresnels,
that is not a concern. The system is practical, and the
complete reliability of the concept has been proven in tests
by Clint's Utah group and ours in Australia. In both cases we
have transmitted, with reasonable reliability, to the optical
horizon. In many cases we have reliably spoken over beams
where the intervening air was so murky that our beams were
not visible to the naked eye. In our case over 104 miles, in Clint's
to 174 miles.

When you mention beam divergences of 2 arc-seconds, I seriously
wonder if you have ever actually tried such a beam on a horizontal
atmospheric path, even over one of minimal turbulence? Apart
from the extreme expense and mass of the micrometer-fine mounting
adjustments necessary to reliably acquire such a beam,
the maintenance of the aim of such a beam would demand complex
feedback servo-mechanisms for continuously steering it to an incredible
accuracy and speed. Perhaps attainable, but certainly NOT cheap - not
by our standards, or those, I suspect, of 99.9% of hams worldwide.

Charles, I realise that there are some people who will remain
unconvinced that lasers and high tech could ever be challenged
by a low-tech approach. However, there are many good reasons
in basic atmospheric physics for avoiding a coherent beam. There
are also many basic safety considerations fulfilled by reducing
tx beam flux density via beam spreading. A combination of large
apertures (for aperture averaghing of beam flux) and the avoidance
of optical beam coherence has proven capable of breaking DX 
records by a considerable margin over laser equivalents. Fresnels 
present the safest, most practical, most economic and most portable
collimator for most amateur budgets. Refer the paper that
we presented to the SPIE Photonics West conference in January
2008:

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

Space communication or beam transmission in a vertical
direction may be a less critical case for coherent optical
beam communication, but you will still encounter decoherence
and beam steering effects at 850 nm.

Repeating the basic tenet of my previous posting, *unless
one is trying to set up an optical comms beam carrying in
excess of 10 MHz bandwidth*, the LED/Fresnel combination
seems to be far desirable to any expensive laser/diffraction
limited optic approach in the atmosphere.

I would have thought that our demonstrated results and DX
records would amply confirm that.

Chris Long VK3AML.

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

> Date: Fri, 19 Mar 2010 20:21:48 -0700
> From: ckpooley at sbcglobal.net
> To: Laser at mailman.qth.net
> Subject: [Laser] laser optics questions
> 
> Clint:
> 
> The single most outstanding feature of the usual edge emitting laser diodes (not VCSEL which are axisymmetric) the 2 orthoganol directions of the light are evvectively at 2 different distances from the face, and the widely spreading direction is because of the narrow emitting width.
> 
> With a cylindrical lense or that feature ground into a lense, the 2 locations can be both brought into focus, then the light acts as though coming from a small point, and conventionaql optics can expand the beam diameter then render it parallel with another larger lense of telescope mirror. 
> 
> I plan this for the space data link for Microlaunchers as seen in a short sample of the math:
> (  http://www.microlaunchers.com/7816/L3/laser/laser-link.html ).
> 
> If a separate cylinder lense is used, it can be focussed separately so the beam is axially symetrical, then, with conventional optics--a small lense and a thin parabolic mirror, focussed ot a diffraction limited 2 arc seconds or so.
> 
> This can be done on the ground, indoors or inside a straight length of pipe.  2" would be a focussed spot 100 micron diameter per 10 m of setup lengh.
> 
> VCSEL is one of the cheaper laser types, but most are limited to about 1.5 mw and 850 nm wavelength.
> 
> Cheap optics?  Orion has the $49.95 "FunScope"  76 mm Newtonian.  Has spherical mirror, but $49.95:
> 
>   http://www.telescope.com/control/telescopes/mini-dobsonian-telescopes/funscope-76mm-reflector-telescope
> 
>     Charles Pooley     KD6HKU               
>     ckpooley at sbcglobal.net
>     http://www.microlaunchers.com/
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