[Laser] laser optics questions

Sat Mar 20 08:39:10 EDT 2010

Hi all
Very big Fresnel lenses can be used with lasers also.
The lines number by inch must be as high as possible with a long focal lenght.
The Fresnal lenses for TV screen magnifier is a good choice.
As you say, with a laser, the light coherence is fully lost away the TX but, because the monochromatic light, the focal zone is very diffused into concentric fringes.
To avoid this effect it is mandatory to use a Compound Parabolic Concentrator followed by a small spherical lens.
The additionnal optical gain can be very strong into a small sensitive area photodetector.
http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productid=3213
It stays a rather moderate cost solution for a high speed laser video experiment.
I full agree, for audio voice coms, LEDs are well better than lasers. 
73
Yves F1AVY
http://f1avyopto.wifeo.com

--- En date de : Sam 20.3.10, Chris L <vocalion1928 at hotmail.com> a écrit :

> De: Chris L <vocalion1928 at hotmail.com>
> Objet: Re: [Laser] laser optics questions
> À: laser at mailman.qth.net
> Date: Samedi 20 mars 2010, 9h20
> 
> 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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