[Laser] laser optics questions

Sun Mar 21 09:01:41 EDT 2010

Hello Chris
It is a great pleasure to exchange many mails with you because your very strong technical background.
May be you have to separate the pure decoherence noise from the scintillation noise.
The decoherence noise is rather a light speed effect due to the air density fluctuating that combines the waves fronts from their radiative point.
The waves phase beating into the beam builds numerous light picks and light holes with a high speed propagation effect that produces a wide bandwidth noise into the receivers.
The air density fluctuation gives also refraction gradient fluctuations that produce low frequency light density changing (angular modulations).
With laser diodes many other problems occur because the radiate volume into the AsGa layers change with the current during modulation.
The light source area changes in size, moves its origin points with a changing astigmatism. 
The real decoherence noise has a large audio spectrum and its cumulative effect is more serious in a wide band.
The BF scintillation is easy to integrate by a large optical aperture to the RX side.
The decoherence noise is beam power linked.
When you modulate a laser beam in amplitude this noise is also modulated in amplitude.
A slow receiver integrates this noise flux that participates to the LF useful amplitude because a photodiode with its RC effect accumulates the photons flux out its 1/4RC bandwidth as an averaged free noise LF component.
It is why if you receive a bounced laser on a large target to 10 km, it seems absolutely noise free.
To illustrate these properties the following links give two examples from a laser signal.
The first is F1CDT I received to 75 Km via a less than 3 mW laser with a 24 cm mirror on the receiver.
The scintillation is deep and the decoherence noise give strong “cliks” during the modulation but, because the large aperture, the message stays globally reading.
The second example is the 800 nm F1ORL laser bounced from a to 10 km factory chimney.
You will notice that because the large area target integration effect, the signal is weak but very stable and perfectly clear and noiseless.
http://sd-1.archive-host.com/membres/up/22679775843705539/F1CDT.mp3
http://sd-1.archive-host.com/membres/up/22679775843705539/F5PNP.wav
I am not an absolute advocate of the laser for communications experiments but lasers are difficult to exchange with LEDs for experimental video links or bouncing links via very distant and small targets.
For a far away point to point line of sight optical link, the main problem seems not really technical.
- You need a powerful team with a very good availability.
- You need to find two line of sight high ASL areas with easy access.
- You need stable frames with their associated accurate aiming devices.
- You need clear sky and very frequent good weather conditions.
If these conditions are assured, the link can succeed with LEDs, lasers, heliostats, wood fires… everything and so on because the receivers sensitivity can be quite sufficient nowdays.
73
Yves F1AVY
http://f1avyopto.wifeo.com


--- En date de : Dim 21.3.10, 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: Dimanche 21 mars 2010, 6h40
> 
> 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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