[Laser] laser optics questions

Sun Mar 21 12:15:32 EDT 2010

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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