[Laser] Re : 5mw laser tranceiver kit (resent because mailing error)

[Chris L]

Yves,

Whatever your argument - and I find these unconvincing, on the basis of several decades of our group's experiments, the momentary additions and cancellations produced by atmospheric turbulence within a fully coherent beam do not just "cancel out", they produce STATISTICAL NOISE, before they reach any reflecting OR scattering surface. The statistical noise can only INCREASE upon encountering a reflecting surface including moving foliage or (with clouds or atmospheric dust) dynamic reflection variations. Low frequency noise is scintillation. High frequency noise is hiss. The variations of cancellation and addition of a laser (coherent) beam readily produce both effects, as well as turbulence-produced beam steering, focussing and defocussing - which are most severe when the transmitted beam is sharply collimated, as in laser practice. Because laser radiation is spatially coherent, atmospheric turbulence can result in COMPLETE cancellations and additions of beam flux from moment to moment, producing a beam 100% modulated with noise, and therefore incapable of reliably conveying modulation, except at very low frequencies. With a non-coherent beam, the additions and cancellations result in far less statitistical modulation variations, as Korotkova et al have shown:

http://pegasus.cc.ucf.edu/~okorotko/SPIE4976.pdf

Furthermore, the usage (as in your Yves' case) of extremely expensive LIDAR large-aperture diffraction limited optics (25cm+ diameter in your equipment, I believe), high power diode lasers, avalanche photodiode receivers, and their employment at near I-R wavelengths easily collected by a dark-adapted eye to produce potential retinal damage, results in an expensive, impractical, unreproducible (for the majority) and potentially dangerous communication system. Surplus optics are great - IF you can get them, and that does not happen often, resulting in superb but unreproducible outfits/ You may be able to buy an occasional surplus LIDAR or large telescope at prices beyond the expense account of most people, but why would anyone even consider this, when much larger molded Fresnel optics are available at a fraction of the cost and with far higher optical gain? Or when PhlatLight LEDs posessing an unprecedented visual intensity and a total flux output exceeding one radiometric Watt - several orders of magnitude higher than most diode lasers at 5 milliwatts or so - are cheaply (USA$35) available, and when modulation at a GHz rate is not needed. In fact, the transit time spread involved in reflection from irregular surfaces, clouds or haze would limit the modulation rate to 1/4 of the wavelength of variation in the reflection surface. Let's say you're reflecting from a cloud with a thickness of 30 metres, a scatter field 30 metres in thickness - that's a high frequency rolloff of 10 MHz produced by the transit time spread in the reflected or scattered modulation. PhlatLight LEDs can exceed that modulation bandwidth, so is there any need for the greater bandwidth of diode lasers for that service?

I think the time has come - as Clint has emphasised before me - in the assemblage of any kit for practical purposes, and especially in assembling a kit that may be used by young experimenters, to avoid past mistakes and drawbacks:

(1) Avoid the coherent (laser) beam option, owing to the high cost of glass diffraction-limited optics necessary for their usage, especially the cost of the large-aperture optics necessary for beam spreading to eye-safe flux densities.
(2) Avoid the coherent (laser) beam option, owing to beam noise and beam steering effects in atmospheric turbulence.
(3) Avoid the usage of near-IR. A visible beam can be much more easily focussed, collimated and aimed, and when eye exposure occurs, it produces an iris-down reaction in the recipient. To make an IR beam eye-safe, one really has to shift to operating in the middle-IR, above 1.5 micron wavelength. At those wavelengths, the detectors become much more expensive than Si photodiodes, emitters are very expensive and often limited to usage by the military by legislation, the thermal noise in detectors greatly increases over that of detectors designed for visible radiation, and optics assumed to be transparent at visual frequencies can often exhibit opaque behaviour.
(3) Use molded plastic optics, not expensive 1/4 wave accurate glass lenses or parabolic mirrors (and especially not coated optics - totally unnecessary). A 10" telescope objective, even a simple parabolised telescope mirror from a Newtonian, can cost upwards of USA$500. A 10" Fresnel lens can be had for about $2, or (as stated by a recent English correspondent to this thread) about 99p.
(4) Use an optical receiver with an inexpensive P-I-N diode like the BPW34, working into a simple voltage amplifier (NOT a transimpedance amp) of a type designed by Clint - refer sections 7.3, 7.4 and 7.5 of this paper:

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

(5) If a PhlatLight is too powerful a device for a mass-produced kit, consider the usage of a red LuxeonRebel (around 1 watt input) or even a red Cree high-output LED of more standard design.
(6) If only night operation is contemplated, the Fresnels and their electronics can be mounted in a simple wooden skeleton framework. Focussing to infinity is all that is required - focussing adjustment is unnecessary, except in initial setup.
(7) Ideally, the tx and rx should be co-aligned (but optically baffled from each other by using an opaque board or cloth between them) in a single unit, to simplify alignment and aiming. Full duplex operation is then provided - and this is a major unrecognised advantage of optical comms.
(8) The optical unit can be mounted to a 1/4" whitworth T-bolt, to facilitate attachment to a standard photographic tripod (I use a Velbon VGB-3 or a wooden surveyor's tripod).

These are just suggestions, but as Clint has emphasised, the poor performance of existing optical comms kits and a general misunderstanding of the incompatibility of coherent light with atmospheric turbulence has led to performamnce expectations dismally below current possibilities.

Best wishes, and happy (practical) constructing,

Chris Long, VK3AML, 2 Newton Street, Surrey Hills 3127, Victoria, Australia.
    Tel: +61 3 9890 8164.

    http://www.modulatedlight.org

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

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

    

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