[Laser] laser optics questions

Garnier Yves f1avy at yahoo.fr
Sat Mar 20 14:28:13 EDT 2010


Hi Chris !!
As very often I see I was not clear...
I never wrote a Fresnel lens can be used as laser collimator !
A laser diode needs a very good aspheric lens (1/10 lambda is ideal) and to avoid astigmatism it also needs a complementary long focal cylindrical lens or an elliptical diaphragm.
This is fully obvious to take the efficiency from the phase coherence to create a parallel beam.
But after a few hundred meters, in atmosphere, the laser light has nearly the same properties a LED one.
The advantages are a nearly parallel very concentrate beam and a nearly monochromatic light and, if useful, a very high speed modulation capability.
It is only at the RX side that a Fresnel lens can be used with a tronconic to parabolic concentrator lens near its focal point.
Far away the laser the light is fully decoherated and the focal point can be small, gaussian and clean with this device.
Of course the energy is not concentrate in phase but the very large capting area can integrate the decoherence scintillations created bay air at the biginning of the beam ! :o))
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, 16h57
> 
> No, Yves, that is wrong. Fresnel lenses cannot be used
> as collimators for *LASER* tx sources for modulated
> atmospheric
> communications with any reasonable efficiency, because:
> 
> (1) The effective source size of any diffraction-limited
> source, such as a laser, will be too small to effectively
> filll the blur circle of any practical Fresnel collimator,
> regardless of the Fresnel's diameter, focal length,
> groove pitch or accuracy. The use of such a tiny source
> would result in an unevenly illuminated Fresnel tx
> collimator,
> if that collimator was observed from within the received
> beam, at "infinity". This, in turn, would produce severely
> compromised optical "gain" and collimation efficiency,
> as well as increased beam scintillation via a beam of
> inhomogenous cross-sectional flux.
> 
> A compound collimator can correctly be used with a 
> surface emitting-LED to vary the effective source
> size to match that of the blur circle of a given Fresnel 
> objective lens. However, a diffraction-limited source of 
> a size determined by the wavelength of an optical
> source cannot be viably varied with any secondary
> lens, and the Fresnel blur circle can never be made
> small enough to match a diffraction limited source
> with secondary hemispherical lenses of practical
> sizes and refractive indices.
> 
> (2) As Clint has suggested, and as both of our experiments
> confirm, the groove pattern of the Fresnel produces
> wild diffraction patterns and areas of unpredictable 
> beam cancellation/addition with any coherent source.
> Furthermore, the diffraction patterns become stronger 
> and more severe as the groove pitch becomes finer.
> 
> To summarise again: high power LED beams collimated
> via Fresnels seem to be most effective where a link 
> system of up to about 10 MHz of modulation bandwidth 
> is required. They would be suitable for audio and 
> analogue video. For multi-channel video or digital
> video, the LED source may not be quite fast enough.
> 
> Laser sources only become desirable when modulation 
> bandwidths significantly in excess of 10 MHz are
> required. Even then, as Korotkova et al have shown,
> the reduction of spatial coherence of the source laser
> is desirable by means of a thin diffusing filter when
> the bit-error rate is to be maximised with an atmospheric
> transmission channel.
> 
> If a full set of references to Korotkova's SPIE
> publications
> on these matters are required, I can provide Internet
> url's. You could start with this:
> 
> http://pegasus.cc.ucf.edu/~okorotko/OptEng43.pdf
> 
> I am sorry to have to be so dogmatic about this, but
> a lot of this optical theory tends to be
> counter-intuitive,
> and it runs against the general thinking commonly
> assumed in optical communication discussions.
> 
> It just comes to a point where I have to say,
> "sorry, but this is wrong", and to provide the
> reasoning, the experimental measurements, and 
> the references.
> 
> Best wishes,
> 
> Chris Long VK3AML.
> 
> =================================
> 
> > Date: Sat, 20 Mar 2010 12:39:10 +0000
> > From: f1avy at yahoo.fr
> > To: laser at mailman.qth.net
> > Subject: Re: [Laser] laser optics questions
> > 
> > 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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