[Laser] Coherent Source - back on my soap box

Tue Nov 10 02:55:15 EST 2009

Charles:

Our definitions of "inexpensive" must be at variance.

You mention reception by "summing signals from 
several 12" telescopes a few meters apart".

Our Fresnel/Luxeon optical duplex transceivers, 
used to set world records in Utah and Tasmania
were each buit to a materials cost of about US$75.
If PhlatLight LEDs are used, maybe $200 per transceiver.
We are amateurs. Our budget is limited. Our chief
concern is not ultimate performance, but maximising
signal-to-noise against cost. As the title of our
paper implies, it's "dollars for decibels". And PRACTICALITY.
We can provide a duplex audio link over 174 miles, and
could probably transmit video over 100 miles or more.
What range have 10mW (or lower) lasers achieved
on speech or video? And with what scintillation noise?
And for what cost? Can you do it for $75? Has anyone?

For the cost of ONE 12" reflecting telescope we
could make around thirty (30) of our transceivers.

I would repeat my statement about terrestrial atmospheric
optical communications, with extra emphasis: not only
are coherent beams contra-indicated in the application, 
but their necessary ancillary equipment (optics, directing 
gear, interference filters, heavy mountings with micrometer
adjustment, beam steering servos) are needlessly and impractically
expensive. In general, the desirable effects of beam
expansion and using diffusing filters to reduce spatial
coherence are poorly understood or ignored, even by
laser advocates. Applying an ounce of basic atmospheric 
physics easily beats thousands of dollars in capital cost.

In presenting our paper at SPIE Photonics West 2008 I
was struck by one outstanding aspect of commercial
R&D which had never been a concern to us. Frequently,
in optical R&D, especially that involving potential military
application, the name of the game is to spend the MAXIMUM
yearly budget allocation. So all of the "trimmings" were
applied to their systems - huge gold-coated optics machined
to better than quarter wave accuracy, frighteningly powerful
high-tech lasers made by purposely obfuscated sources with outputs
in the far-infrared, and with complex cooling systems. Heterodyne
reception systems with beam combiners costing TENS of
thousands of dollars. Then we came in from the boonies
to tell these guys that a lot of what they were proposing
could be done for the cost of one meal at a better Silicon
Valley restaurant. The reaction was extremely amusing.
Some were alarmed. You can't justify the employment of
an R&D team on the low capital cost gear we were using.
And - horror of horrors - our optical mountings were not
nano-engineered to micron accuracy from bomb-proof
titanium. They were PLYWOOD! If this "got out", their
R&D departments might have to shed a few PhD's.
Oh, the shame of it!!!

This is one case, I'm afraid, where the low-tech approach
wins. The reasons for that are given in our paper, and
in the references appended to that paper:

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

Scintillation is better and far more cheaply overcome
by optical means (large apertures, non-coherent sources,
spatial diversity reception, increased beam flux combined
with moderate beam divergence) than by expensive information
redundancy, "burst" coding, beam steering or similar exotic 
and costly schemes.

I doubt that many of the advocates of high-tech laser
systems have actually sat on mountain tops, looking at
a modulated beam coming to them through a hundred
miles of air. If they had, they'd notice one very obvious
problem. Laser light clearly looks "granular", while non-coherent
light looks more even, with far less "twinkling". 

Granular = noisy.

I hope that I've made my point - best wishes,

Chris Long VK3AML.

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

> Date: Mon, 9 Nov 2009 22:00:31 -0800
> From: ckpooley at sbcglobal.net
> To: laser at mailman.qth.net
> Subject: Re: [Laser] Coherent Source - back on my soap box
> 
> Chris:
> 
> Thank you for the article.  I will study it, compose off line a more detailed response and post it.  
> 
> But first a note:  From the outset the intended application is very long distance in space and a short steep pass through the atmosphere rather than a long pass through dense air, and that a narrow beam is assumed therefore the need for active steering, and there are inexpensive ways to do this.  
> 
> A possible test might be to have a contact with a weather balloon going to over 30 km and giving a slant distance over 100 km with most of the path in thin air.  The balloon moving would demonstrate tracking.  For this the beam width might be 60" rather than the assumed space width of 2" (10^-5 radian).  The signal strength will be about 1000 times less than that for a given distance.
> 
> My references are centered on lasers and space communications, ie Deep Space Optical Communication, Hemmati, 2006, and Laser Satellite Communications, Katzman ed, 1987, others.
> 
> You mentioned interference filters being not needed at night.  In the situations you've described I would agree, but in the space situation the received signal will be in the photon counting area, and a filter will help to avoid confusing background stars etc.  
> 
> The scintillation you mention will make the receiving noisy, but is to be dealt with in two important ways:
> 1.  the data rate requirement is >0 for start (then several thousand bits/s later.  Recall the 1975 Altair was barely more than 0 as a computer, 
>     but it could evolve.
> 2.  Probably the receiving might be done by summing the signals from several 12" telescopes a few meters apart.
> 
> I have worked with LEDs in the past (drivers for the Luxeon LEDs on the Luxor hotel in Las Vegas are my design), and am familiar with their properties.
> 
> Charles      KD6HKU,   Microalunchers
> 
> 
> 
> 
> ________________________________
> From: Chris L <vocalion1928 at hotmail.com>
> To: laser at mailman.qth.net
> Sent: Mon, November 9, 2009 10:21:48 AM
> Subject: Re: [Laser] Coherent Source - back on my soap box
> 
> 
> 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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