[Laser] Updated URL for my optical web pages

[Chris L]

Just a note on all of that, Barry -

Mike VK7MJ and I were the first hams to use the Luxeon for speech-on-light DX, setting a 168 km full duplex record in January 2005, on that occasion using then-new Luxeon I (1 watt input) LEDs:

http://www.bluehaze.com.au/modlight/

Lumileds recently stopped producing the red Luxeon III, concentrating on production of their slightly lower intensity (and much lower priced) Luxeon "Rebel". The red Luxeon product is therefore no longer the optimum choice for long-distance optical TX with a Fresnel collimator. Also, the truncated inverted pyramid ('TIP') structure of the red Luxeon chip was always a difficult emitting surface from which to collimate a beam, as its emitting surface does not lie in a single, well-defined optical plane. With Clint, KA7OEI, VK7MJ and I presented a scientific paper on our DX experiments for the SPIE in California in January 2008. Section 6 of our paper deals with the usage of molded Fresnel lenses for modulated light systems under the heading of "large apertures at minimum cost". The optimisation of the Fresnel lens collimator system involves the placing of a small, high-diopter plano-convex or positive meniscus secondary lens between the LED and the Fresnel collimator - refer figures 8 and 9 of our paper:

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

The usage of a secondary lens, just in front of the high power LED, (first included in our equipments in 1989) is partly dictated by the need to gather the maximum available flux from the Lambertian polar pattern of the LED's emission, throwing it into the collecting area of the Fresnel objective. It is also needed to take account of inevitable inaccuracies in any cheaply molded Fresnel lens - especially the cheap "page magnifiers" that are commonly available - by optically increasing the effective source size to fill the focal point blur circle of such a molded lens. The requisite secondary PMN (positive meniscus) lens is inexpensively available (at around USA$5) from American sources like Surplus Shed, or rather more expensively from Edmund Optical (the optics department of the old Edmund Scientific Company). This lens has the additional benefit of increasing the TX beam divergence WITHOUT simultaneously decreasing the far-field flux intensity, thereby reducing the need for accurate aiming of the optical transceiver. In fact, our group has found that the usage of a moderately stable medium-sized Velbon VGB-1 aluminium photographic tripod on which to mount the optical transceiver is quite stable enough.

Please note that the usage of the new high-power red LEDs for optical TX have several overwhelming advantages over laser diode technique:
(1) The elimination of coherent output from the TX greatly reduces atmospheric scintillation due to the break-up of coherent optical wave fronts in atmospheric turbulence.
(2) The larger effective source area of the LED permits the usage of cheap, inaccurate optics that need not be diffraction-limited. Very large aperture molded plastic Fresnel collimators, insufficiently accurate to collimate laser sources, can be readily used with LED sources with their much larger effective source dimensions. Molded plastic Fresnel collimators are a tiny fraction of the cost, weight and mounting inconvenience of glass optics of similar size.
(3) By increasing the tx and rx optical aperture to the size of an A4 sheet, one achieves a degree of aperture averaging across atmospheric turbulence cells in the transmission medium (the atmosphere), thereby minimising the received beam scintillation that is the single greatest problem in the atmospheric usage of modulated laser (including diode laser) sources.
(4) The increase of the effective TX source diameter also maximises the distance at which the received beam effectively becomes coherent due to diffraction effects (refer to Michaelson's famous experiment in determining stellar diameter via the coherence/aperture method). When the turbulent atmosphere is the transmission medium, optical coherence is the tx beam parameter that it is most necessary to AVOID.
(5) Lasers have a very narrow half-power bandwidth, which may or may not coincide with the equally narrow absorption lines of the atmosphere's constituent gases. If one could simply chose a laser wavelength to avoid these absorption wavelengths, one might be able to set up a reliable communications link. Unfortunately, laser emission wavelengths drift unpredictably (by hundreds or even thousands of MHz) according to their operating conditions and operating temperature, making the atmospheric transmissivity of a laser beam unpredictable and frequently unreproducible. By comparison, LEDs typically have a half-power bandwidth of 20 nM @ 630 nM, straddling several narrow atmospheric absorption bands, and ensuring the transmission of those parts of the emission wavelengths that pass between the narrow absorption lines.

However, with the Luxeon III high-output LED no longer available, which high power LED sources are now the ideal alternative? 

The development of the new PhlatLight ("photonic lattice") LEDs by Luminus Devices of Massachusetts represents as much of a leap in available flux and intensity above Luxeons, as the Luxeons did above that of standard LED's! Refer:

http://www.luminus.com/stuff/contentmgr/files/0/5d1ae46c1a76dcc9f3de5b546fef72fc/miscdocs/pds_001229_rev_02_cbt_40_product_datasheet_illumination.pdf

These PhlatLight LEDs have the highly desirable characteristic of delivering all of their output from a single, well-defined flat plane, with a flux output that is much easier to optically collect and collimate than that derived from a red Luxeon TIP chip. New developments in thermal and optical management permit these PhlatLights to have lower junction capacitance than the Luxeon, while simultaeously increasing conversion ("wall plug") efficiency and increasing the possible chip driving current density. PhlatLights will be available in units ranging from 16 to 120 watts, principally intended as replacements for xenon arc lamps in video projectors. The red (625 nM) PhlatLight has a much faster rise time and is more linear than than the white emitter. The red PhlatLight also provides the closest spectral match to the response curve of silicon photodetectors. In an optical communication system, far-field communication beam intensity is related, principally, to the intensity of the light source. There is therefore no point in using the 120-watt PhlatLight for our purposes, as its larger source area would only result in a broadening of the transmitted beam without any increase in far-field intensity. Consequently, the interest of Clint's Utah group and ours in Australia concentrates on the low-powered LEDs in the PhlatLight range - the CBT-40 (4 square mm emission area) which is already commercially available, and which we use:

http://www.luminus.com/stuff/contentmgr/files/0/5d1ae46c1a76dcc9f3de5b546fef72fc/miscdocs/pds_001229_rev_02_cbt_40_product_datasheet_illumination.pdf

...and this SBT-16 (1.6 mm square source) model, which will offer the prospect of equal far-field intensity for much less driving power, but which is not yet commonly available:

http://www.luminus.com/stuff/contentmgr/files/0/2a3f5d2a6a07f25cdd38e273a9ea5237/miscdocs/pds_001479_sbt16_summary_datasheet__rev_01.pdf

Out Australian group is currently constructing fold-up optical transceiver units, each using two of the CBT-40's in series to make maximal usage of a 12 volt rail through a series PowerFET modulator, with the LED's output focussed through co-lined Fresnel collimators, spaced either side of a central receiving collimator. All three collimators in the transceiver unit will use Fresnel lenses on the same mounting board, with black flexible (cloth) separators ("bellows") between the optical enclosures. The spacing of the two tx collimators will, according to our measurements in actual field tests, provide maximal freedom from atmospheric scintillation by providing the maximum effective tx aperture (spatial diversity and aperture averaging) in a single, manageable transceiver unit avoiding the cumbersome demands of a single, larger collimating optic.

If the group would like to be kept informed of our progress on these, we would be willing to give progressive reports if there's sufficient interest.

With best wishes,

Chris Long, VK3AML, Melbourne, Victoria, Australia.
Mike Groth VK7MJ, Hobart, Tasmania, Australia.

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

> Date: Mon, 13 Sep 2010 17:18:48 +0100
> From: b.chambers at sheffield.ac.uk
> To: laser at mailman.qth.net
> Subject: [Laser] Updated URL for my optical web pages
> 
> Please note that the URL for my optical comms web pages has been changed 
> to :
> 
> http://www.barry-chambers.staff.shef.ac.uk/laser_files/laser.htm
> 
> 
> I'm currently rebuilding my optical Tx and Rx. The Tx will use one or 
> more LuxeonIII mounted behind A4 sized Fresnel lenses and using VK7MJ's 
> electronics. The Rx will be based on KA7OEI's V3.02 design.
> 
> 73
> 
> Barry, G8AGN
> 
> 
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