[Laser] laser optics questions

Sun Mar 21 12:29:08 EDT 2010

Hi Chris.

This formatting option worked - as you can see below. Thanks for 
making your words easier to read. They are well worth reading.

73 de Glenn WB6W

At 09:15 AM 3/21/2010, you wrote:

>To take your points in turn:
>
>(1) Tim, you are confusing the concepts of spatial coherence and 
>monochromaticity. Spatial coherence refers to the radiation from a 
>laser source being in the form of plane waves, propagated in phase - 
>"in step", if you like. Only a laser source, or a point source at 
>very great distance from the observer can produce spatially coherent radiation.
>
>Monochromaticity, on the other hand, refers to the BANDWIDTH of the 
>waves propagated by the source. A highly monochromatic source can be 
>either noncoherent or coherent - or coherent to an intermediate 
>degree. The diffusing filter shown in Korotkova's papers removes the 
>spatial coherence from the laser radiation, but leaves the source's 
>monochromaticity (bandwidth) substantially unaffected. A beam can be 
>highly monochromatic (like the output of a diode laser), but it need 
>NOT be coherent. In fact, it is desirable that this spatial 
>coherence be avoided for atmospheric transmission, as highly 
>spatially coherent beams, in the presence of atmospheric turbulence, 
>can encounter constructive and destructive interference producing 
>extremely deep and rapid scintillation. Refer Korotkova's papers on 
>the controlled removal of spatial coherence of an atmospheric laser 
>comms beam to reduce the bit-error-rate of digital comms:
>
>http://pegasus.cc.ucf.edu/~okorotko/SPIE4821.pdf
>http://pegasus.cc.ucf.edu/~okorotko/SPIE4976.pdf
>http://pegasus.cc.ucf.edu/%7Eokorotko/OptEng43.pdf
>
>It may be difficult to grasp the fact that coherence is undesirable 
>in atmospheric optical communication, but if you read the three 
>papers above and refer to our success in applying those basic 
>principles of atmospheric physics, I think you'll see the logic of 
>what we're doing. Please do read these references. You need not 
>undersrtand the math fully to appreciate the principles being 
>expounded. We described the practical applications of these 
>principles in our field tests in this paper for the SPIE in 2008:
>
>http://www.modulatedlight.org/Dollars_vesus_Decibels_colour.pdf
>
>(2) The scintillation encountered in beams received from LED-derived 
>non-coherent beams collimated, at the tx and rx ends, by 
>large-aperture lenses (and I'm talking 20 cm Fresnels here, or 
>larger) is derived from sources other than decohering noise. The 
>scintillation occurs owing to atmospheric effects including (a) beam 
>wandering or steering, (b) beam focussing and defocussing, (c) 
>changing angle of beam arrival or image jitter. For an excellent 
>reference on atmospheric scintillation effects, I refer you to:
>
>SPIE FIELD GUIDE SERIES, Volume FG02: "Atmospheric Optics" by Larry 
>C Andrews; SPIE Press, Bellingham, Washington, USA, 2004. (refer 
>website:  www.spie.org/press/fieldguides  )
>
>(3) Tim, the usage of Fresnels for laser beam collimation is 
>unsuitable for several reasons, and there are very good reasons for 
>their not working properly with coherent sources, as Clint has 
>already described on the basis of personal demonstration. (A) To 
>adequately and predictably collimate a COHERENT beam without 
>variations of constructive and destructive interference across the 
>collimator area, the collimating optic must have a surface accuracy 
>considerably finer than 1/4 wavelength. In other words, for laser 
>collimation it must be a DIFFRACTION LIMITED optic. Practical molded 
>Fresnels produce an image spot, or "blur circle" of between 0.25 and 
>0.5 mm. This is much too large for the diffraction-limited effective 
>point source that a laser produces. So there is, in fact, very good 
>reason for coherent (laser) sources producing more scattering, more 
>beam inhomogenity and radomly unpredictable phase cancellation with 
>Fresnels than there is from any poractical LED source.
>
>(4) To Yves: Clint Turner (KA7OEI) demonstrated the transmission and 
>reception of NTSC video signals via a Luxeon LED source in his 
>workshop more than three years ago. There is NO problem in 
>modulating high power Luxeon LED's to 5 or 6 MHz, though the higher 
>powered PhlatLight LEDs have a somewhat higher junction capacitance. 
>The problems begin to occur when one aims for a broader bandwidth 
>than about 10 MHz with present high power LED hardware.
>
>Incidentally I'm trying one more form of layout for this posting, 
>hoping that in doing so, I'm not going to run into the "line break" 
>problem affecting my earlier postings. If this doesn't work, I've no 
>idea what to do about the problem!
>
>Chris Long VK3AML.
>
>=================================
>
> > Date: Sun, 21 Mar 2010 06:27:32 -0700
> > From: toasty256 at yahoo.com
> > To: laser at mailman.qth.net
> > Subject: Re: [Laser] laser optics questions
> >
> > Hi all,
> > I just want to be clear, i'm not trying to argue one method is
> > better than some other here. From what i've read about all this
> > so far, it looks like most, if not all, of the de-coherence noise
> > is generated in the first kilometer or two as the spatial
> > coherence of the laser is being destroyed by passage through
> > the atmosphere. The de-coherence process has, in effect,
> > modulated the beam in amplitude and so the beam carries this
> > noise with it to the detector.
> >
> > I suspect an LED still suffers from some scintillation because of
> > its narrow bandwidth. Both the laser and LED have a finite
> > bandwidth and so both have a degree of "coherence" which shows up
> > as the noise and scintillation on both signals - the LED having
> > less because its bandwidth is greater and has less temporal
> > coherence. (Not because it has None) In the real world, no
> > light source can be made "non-coherent" or infinite bandwidth.
> >
> > I pick a random point in the discussion and run with it now...
> > I don't think a fresnel lens would scatter laser light any more
> > than it does with LED light. It may be that it is just more 
> notifiable with a laser because it is a sharply defined
> > reflection or scatter (caustic?). The LED light gets scattered
> > just as much but its reflections are fuzzy and diffuse due to
> > the wider bandwidth.
> >
> > -toast
> >
> >
> >
> >
> >
> >
> >
> >
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