[Laser] Coherent Source - back on my soap box

Mon Nov 9 16:27:37 EST 2009

I full agree the Chris points of view.
Lasers have absolutely no advantage on power LEDs for long range at low speed (audio) atmospheric communications.
Lasers stay the best only for short range at very high speed because high power density in the beam and nearly perfect monochromatic light for high background rejecting in daylight (very narrow dielectric filters use).
For ranging application that needs a very small lighted area on a target lasers stay great.
But the laser light becomes absolutely not coherent after a small air travel.
A very thin beam near its start point is extremely sensitive to the atmospheric scintillation
Safety problem at high power and easy aiming with low cost optical devices are resolved only by LEDs.
73's
Yves F1AVY
http://f1avyopto.wifeo.com

--- En date de : Lun 9.11.09, Chris L <vocalion1928 at hotmail.com> a écrit :

> De: Chris L <vocalion1928 at hotmail.com>
> Objet: Re: [Laser] Coherent Source - back on my soap box
> À: laser at mailman.qth.net
> Date: Lundi 9 Novembre 2009, 19h21
> 
> 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. 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> >       
> >
> ______________________________________________________________
> > Laser mailing list
> > Home: http://mailman.qth.net/mailman/listinfo/laser
> > Help: http://mailman.qth.net/mmfaq.htm
> > Post: mailto:Laser at mailman.qth.net
> > 
> > This list hosted by: http://www.qsl.net
> > Please help support this email list: http://www.qsl.net/donate.html
> >
> ______________________________________________________________
> > Laser mailing list
> > Home: http://mailman.qth.net/mailman/listinfo/laser
> > Help: http://mailman.qth.net/mmfaq.htm
> > Post: mailto:Laser at mailman.qth.net
> > 
> > This list hosted by: http://www.qsl.net
> > Please help support this email list: http://www.qsl.net/donate.html
>     
>         
>           
>   
> _________________________________________________________________
> Looking for a date? View photos of singles in your area!
> http://clk.atdmt.com/NMN/go/150855801/direct/01/
> ______________________________________________________________
> Laser mailing list
> Home: http://mailman.qth.net/mailman/listinfo/laser
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Laser at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>