[Laser] Coherent Source - back on my soap box

Mon Nov 9 13:21:48 EST 2009

Charles, in reply, I can only suggest that you've not read our scientific,
paper, its list of references, or its conclusion. The need for DX linking
is probably slight in your digital links - but we are talking of bridging
mountain tops more than 100 miles apart. Again refer:

 http://www.modulatedlight.org/Dollars_vesus_Decibels_colour.pdf
A few basic points:

(1) The need for expensive beam tracking devices,
or adaptive optics, is only required when the beam
divergence is impractically small. For the DX paths
that we've achieved (104 miles) and that Clint's
Utah group subsequently achieved (over 174 miles),
one hasn't the luxury of a mounting based in tons of
concrete, or of unnecessarily complex tracking devices
that would be necessary to maintain such a finely 
collimated beam. It is far better and more practical
to have a beam with higher total flux but greater
beam divergence, to permit slight misalignment through
mounting wobble, wind disturbance, and aiming accuracy.
We find that a beam divergence of
about 0.5 degrees makes our optical transceiver unit
compatible with mounting on simple photographic tripods,
or on average custom leg or table mountings. A Luxeon/Fresnel
combination provides that divergence of beam admirably - a collimated diode
laser does not. That lack of divergence can be more of
a practical liability than an asset. The Luxeon also provides a much higher
total flux output than all but the most powerful and 
expensive laser diodes.

(2) Even if you eliminate the beam steering problem, the spatial
coherence of wave fronts in laser sources passing through
atmospheric turbulence provides cancellations and additions
of instantaneous beam flux. This can result in complete loss
of beam flux at random times ata kHz rate - received white 
noise modulation, completely masking any attempt to carry
signal information. Reference fifteen (15) at the
end of our .pdf article link above - "Phase diffuser at the
transmitter for lasercomm link: effect of partially coherent
beam on bit-error rate" by Korotkova, Andrews and Phillips,
(Proceedings of SPIE, 2003) describes how THE LINK QUALITY IMPROVES
WHEN A MILD DIFFUSER SCREEN IS PLACED OVER THE LASER TO REDUCE
SPATIAL COHERENCE. There is a point at which the bit-error
improvement is offset by beam divergence - but the implication
of these measurements and the graphs provided are clear.
That is, the link's capacity to carry data through atmospheric
turbulence is greatly IMPROVED when spatial beam coherence is
REDUCED OR ELIMINATED.

(3) We have achieved our DX results WITHOUT Peltier cooling,
WITHOUT expensive avalanche diodes (simple PIN diodes suffice)
and WITHOUT the expensive, diffraction-limited optics that
laser sources demand. Go to a non-coherent source, and the
need for diffraction limited optics vanishes. Go to moulded
Fresnel optics, and for a given cost you can increase
transmit and receive apertures tremendously. In this way,
one can avoid the beam coherence that creeps in when
one is viewing a light source at great distance (refer
section 3.1 of our SPIE paper); while simultaneously the
large apertures facilitate aperture averaging over several
turbulence cells, reducing scintillation and hugely
increasing optical gain.

(4) For DX operation at night, narrow band interference
filters are generally unnecessary, and indeed they often introduce
more loss than they improve sig/noise ratio. For daytime
operation, the gain provided by Fresnel collimators offsets
ambient light from extraneous sources, and a half-power emission width of
20nm at 630nm (typical for Luxeons) may still fairly effectively pass through an
appropriate dye or interference filter.

(5) The modulation bandwidth achieved with a red (630 nm) Luxeon LED
may be in the order of 15MHz. Sure, laser diode modulation may go up
into the GHz range, but how often do radio hams need THAT
much bandwidth?

I am sure that for links of under a mile, diode laser links
are quite viable, but we're operating over distances more
than one hundred times greater. The optical, electronic
and practical demands of our DX links are completely different.

What surprises me is that these basic advances have not
yet seeped out into practical application. Again, please
refer to our article and particularly its appended list of
scientific/optical references:

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

Sorry it had to be two visits after a four-year gap, but
I honestly do not think the full implication of the experimental
results of Clint, Mike or myself have really sunk in. With
Charles' comments about standard "last mile" link hardware,
the different approach required for anything approaching a
DX link required emphasis - which our .pdf gives.

Please do give it a look.

Best wishes,

Chris Long VK3AML.
===========================================

> Date: Mon, 9 Nov 2009 09:17:36 -0800
> From: ckpooley at sbcglobal.net
> To: laser at mailman.qth.net
> Subject: Re: [Laser] Coherent Source - back on my soap box
> 
> I have been a long time lurker, posting only a couple times, but do want to add 2c here.
> 
> As an advocate of laser diode data comm (see http://www.microlaunchers.com/7816/L3/laser/laser-link.html )
> The diode laser is not meant to be coherent (they are only to on order of a cm or so), but the better collimation and narrower optical spectrum.
> 
> The former allows the transmit optics to be very small; the latter use of interference filters to reduce background light.
> 
> Atmospheric effects are an aspect to experiment with by using active tracking of beam wander and if needed, multiple receive optics spaced a few meters apart.
> 
> The receiver detector can be PIN or avalanche diodes before going on the more expensive photon counter diodes.
> 
> Tim's mention of multiple LED colors being available is available for lasers.  They come in 50 or so wavelengths in near IR and red.
> Also you can "tune" them by controlling the temperature with a peltier cooler.  
> 
> Interference filters are tunable by tilting from normal to the optical axis.
> 
> If anyone is interested in the space comm situation described in Microlaunchers, or in any aspect, I would like to hear.
> 
> Charles Pooley                  KD6HKU, Microlaunchers
> 
> 
> 
> 
> ________________________________
> From: Tim Toast <toasty256 at yahoo.com>
> To: laser at mailman.qth.net
> Sent: Mon, November 9, 2009 6:46:12 AM
> Subject: Re: [Laser] Coherent Source - back on my soap box
> 
> I dont claim to understand the underlying math completely, 
> but the degree of coherence of a source is directly related 
> to the source's spectral bandwidth mathematically. In the 
> simplest form the formula is the reciprocal of bandwidth. 
> (coherence = 1/bandwidth) Other formulas include the 
> refractive index of the medium (air), center frequency and C. 
> 
> Technically, requiring coherence without specifying a number, 
> means that you cannot use a source with an infinite bandwidth 
> or zero coherence length. So technically any real light source 
> qualifies as "coherent" because none have infinite bandwidth.
> 
> At the other extreme, a hypothetical laser producing a single 
> frequency which does not drift (ever) would have perfect 
> temporal coherence and infinite coherence length. 
> 
> These two imaginary extremes are the only two that can be 
> described fully with "coherent and not". In the spirit of the 
> rule though, i would say they imply a small bandwidth and a 
> resulting degree of coherence from a real light source. 
> 
> Besides LEDs having usually less than 50nm bandwidth, i 
> thought a 50nm spacing would divide up the visible spectrum 
> nicely into 6 bands (Blue, Cyan, Green, Yellow, Orange and 
> Red) easily estimated by eye, and with a few UV and IR bands 
> beyond each end. 
> 
> 
> 
> 
> 
> 
> 
>       
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