[Laser] Newbie

[Chris L]

That is not *completely* true, Tim.

Fresnels can be used for focussing the
received laser light onto the photoreceptor
after the beam has lost its coherence via
transmission through atmospheric turbulence,
but you will still have to face the major laser
problem of atmospheric decoherence noise.

The usage of a Fresnel for laser collimation
at the transmitting end has at least two major
problems:

(1) Fresnels are not diffraction-limited optics.
To provide adequate tx beam-spread with an
homogenous cross-sectional beam flux, ensuring
minimal beam scintillation, the light source must 
fill the blur circle at the focal point of the Fresnel. 
In general, laser diodes or focussed laser sources 
present too small an effective source to fill the
Fresnel's blur circle, while Luxeons or PhlatLight 
LEDs have adequate effective source size to 
permit the Fresnels to be efficient collimators.

(2) The spatially coherent radiation emitted
by a laser, even of partially coherent visible
laser diodes, demands collimating optics accurate
to at least 1/4 of the working wavelength. This is
necessary to avoid random cancellations and 
additions of light flux across the collimator's
area.

The cost of setting up an atmospheric link
using lasers, in comparison with LED's, is far more
dictated by the cost of the ancillary optics than
of the laser and its matching photodetector.

For links demanding moderate bandwidths, 
up into the tens of megahertz, modern high 
power LED's present a far superior solution to
lasers in terms of atmospheric scintillation.
LED's are now capable of far higher total
radiated optical power than are lasers of the
same cost. LED's require FAR cheaper and
rougher optics, and LED's are much more electrically
robust than diode lasers.

The advantages of lasers as an atmospheric
transmission link scheme only start to become
significant when one is trying to attain bandwidths
exceeding about 20 MHz, and even then, only
for transmission ranges in the order of a few km's.
High power LED's have greater atmospheric range
potential, by virtue of their ease and economy
of interface to large, cheap, molded Fresnel optics.
With a high power LED, the chief modulation
problem is the relatively high capacitance of
the diode junction, which can be in the order
of 0.01 uF. Even if the effective junction
resistance of the high power LED is in the order
of 1 ohm, a 0.01uF capacitance will present
the same load at a frequency around 10 MHz.
At those modulation frequencies, a very low
modulator source impedance becomes necessary 
and design problems increase. For bandwidths
of hundreds of MHz or even GHz, laser diodes
present a far easier alternative - but at the
expense of high-cost optics, much higher
atmospheric scintillation, greater potential
atmospheric absoption (when the wavelength
coincides with an atmospheric spectral absorption
line) and lower source flux for the same cost.

The narrow bandwidth of a laser source through
atmospheric transmission is also a problem. Atmospheric
absorption spectrum lines tend to be vary narrow in
bandwidth, and as a laser source's emission frequency
drifts (as ALL practical lasers do), the beam wavelength
may randomly encounter these bands. The broader
spectral bandwidth of the LED generally ensures that
at least some of the emitted radiation will pass
between the narrow absorption bands.

I fully support Clint's conclusions regarding lasers
in this application. They may be "cool" but they
probably aren't practical, unless you want hundreds
of megahertz of bandwidth.

Chris Long, VK3AML, Melbourne, Australia.


> Date: Wed, 17 Mar 2010 12:26:50 -0700
> From: toasty256 at yahoo.com
> To: laser at mailman.qth.net
> Subject: Re: [Laser] Newbie
> 
> Hi Keith and Clint
> I just wanted to jump in here and point out that you don't have to use expensive optics to use a laser. You can collimate a laser at least as well or better than the LED with a cheap fresnel lens. The type of laser coherence Clint is talking about is lost shortly after it leaves the transmitter. Another aspect of coherence is the bandwidth of the light source. Both the laser and the LED have a certain bandwidth. The bandwidth stays the same regardless of the type of optics used.
> 
> -toast
> 
> >
> >
> >
>    - Coherent light.  The use of "coherent" light from a laser can 
> dramatically increase the degree of scintillation on the transmitted 
> beam.  The use of coherent light also limits the sorts of optics one can 
> use to collimate the beam:  The use of "diffraction-limited" optics 
> (e.g. extremely accurate and precise lenses) is required to avoid 
> disrupting the beam from a laser.  If one wishes to obtain a large beam 
> diameter, you'll need a piece of glass that large, along with the 
> weight, expense, fragility and awkwardness that goes along with it.  
> After traveling some distance through the atmosphere (a couple of km - 
> at most - will do it!) the beam loses its coherent properties, anyway.
> >
> >
> >
> 
> 
> 
>       
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