[Laser] laser optics questions

[Chris L]

OK Yves, but I feel that you are still missing
the point I'm making 
about lasers and
propagation through atmospheric turbulence.

The
 receiving Fresnel, by virtue of its aperture
area, can only 
integrate the INTENSITY variations
across the received beam. It does 
NOT integrate
the phase decoherence effects caused by
atmospheric 
disruption of the PHASE of the
spatially coherent wave fronts. Once 
the noise
of phase decoherence has affected the beam,
it cannot be
 recovered by any spatial integration
technique currently devised. At
 optical wavelengths,
and with atmospheric transmission, the phase 
coherence
 in the transmitted beam is an UNDESIRABLE
characteristic of lasers 
as a modulated source.
As Korotkova and her collaborators on this 
paper 
indicate:

http://pegasus.cc.ucf.edu/%7Eokorotko/OptEng43.pdf

...the
 only way to reduce the effect of phase
decoherence noise in a 
transmitted laser beam
is to make it LESS coherent PRIOR to 
atmospheric
transmission. Korotkova proposed placing a mild
diffusing
 filter in front of the laser to reduce
spatial coherence, thereby 
reducing the amount
of noise due to phase cancellation via turbulent
atmospheric
 transmission. She and her collaborators
applied and measured the 
noise reducing effects of
various degrees of diffusion. The lowest 
noise was
always obtained when the amount of remnant
spatial 
coherence in the beam was LEAST. In
this way, they could retain the 
narrow bandwidth
of a laser source but THEY CLEARLY IDENTIFIED
SPATIAL
 COHERENCE AS AN UNDESIRABLE 
CHARACTERISTIC OF A MODULATED BEAM
THROUGH
 THE ATMOSPHERE AT OPTICAL
WAVELENGTHS. In other words, lasers are 
NOT
the best method of tackling atmospheric
optical communication 
unless some important
optical characteristics of laser beams are
modified,
 notably (1) reduction of spatial
phase coherence via diffusion 
filtering and
(2) by tx beam spreading through a large-
aperture 
collimator.

Clint, Mike VK7MJ and I took all of that theory
one
 step further, by eliminating the possibility
of spatial coherence. 
We use a NON-coherent
(high power LED) source, and with tx and rx
collimator
 apertures sufficiently large as to 
eliminate the possibility of the
 beam becoming 
effectively coherent at great distances,
as with 
starlight - (refer Michaelson's method
of the determination of 
stellar diameter.)
Refer section 3.1 of this paper for details and
analysis
 "Effects of distant aperture size on
signal quality":

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

We
 found tremendous measurable improvements 
in beam noise reduction 
and recoverable 
modulation, in comparison with coherent (laser)
comms
 beams under these conditions. This has
been confirmed by our setting
 of world DX
records with these techniques, firstly by
Mike Groth 
VK7MJ and myself here in
Australia, and latterly by Clint KA7OEI and
his
 group in Utah.

The "speed of modulation capability" of which
you
 speak only becomes a concern when
the requisite modulation bandwidth
 exceeds
about 10 MHz. There, a high power LED's
large effective 
junction capacitance makes
modulator design somewhat difficult, and
a
 laser diode's smaller junction becomes
an advantage. However, a 
laser beam
without the reduction of spatial coherence
at the tx 
end will ALWAYS have poorer noise
immunity than a LED-derived beam. 
As a
result, such broadband laser links have
always been limited 
to a "last kilometer"
role in local area networtking. We have
demonstrated
 that LED links are much
better for virtually every atmospheric
link
 application.

How many amateur operators need links
with more 
than 10 MHz of bandwidth?

Best wishes,

Chris Long VK3AML.

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


> Date: Sat, 20 Mar 2010 11:28:13 -0700
> From: f1avy at yahoo.fr
> To: laser at mailman.qth.net
> Subject: Re: [Laser] laser optics questions
> 
> 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
> 

 		 	   		  
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