[Laser] Re: Fresnel lenses and dispersal angle

[Chris L]

OK re:source size and fresnel focal patch match.

No, I did not imply that a larger source will focus to a point at infinity, 
HOWEVER:

Dr Groth and I use a compound instrument - fresnel objective and small 
plano-convex lens
of high diopter (short focal length) in front of the Luxeon or the PD, 
whatever the case may
be. Most Luxeon source sizes in the red (AlInGaP) chip are of roughly 
similar intensity. The old
Luxeon I had a chip size of just under 1mm by 1 mm. This was insufficient to 
fill the fuzzy
focal patch in the inaccurate fresnels.

By optically enlarging the virtual source area to fit the focal patch on the 
fresnels, far-field
intensity and beam flux homogenity were both optimised. I only wish it were 
possible to
include a ray diagram of what we're describing, and the situation would be 
clarified
considerably.

With fresnels, inaccuracies in the moulded lens set the minimum possible 
beam dispersal angle. If you reduce your source size to be smaller than the 
fuzzy focal patch, the beam dispersal does NOT reduce, as the lens quality 
is the limiting factor.

The new Luxeon III has a source size about 2mm square, fits our fresnel 
focal patch
perfectly, beam intensity is way up, dispersal angle roughly THE SAME, beam 
cross-section
homogenity is excellent and atmospheric scintillation is minimised.

The "2 inch" or "1 inch" focal patches are typical when you use a twinned 
fresnel intended
for use in an overhead transparency projector. They're not designed for 
far-field collimation. However from 3DLens.com you can by such a lens, 39 cm 
square, which provides an image patch around 1.5 mm diameter.

Dr Groth and I have arranged a set of nine (9) new web pages on these and 
other subjects relating to optical communication, and will be posting these 
on the web by
the end of this month. Full optical design details, calculations, 
photographs of equipment
and recordings of results up to our current record of 167.7 km duplex audio 
will be
included.

Our present, rather limited web coverage can be accessed via our "index" 
page:

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

Please feel free to visit or to comment...

I will also be presenting an audio-visual lecture at the forthcoming 
"GippsTech" confab at
Monash University's Churchill Campus, La Trobe Valley, Victoria, 9-10 July 
2005. All
welcome. Further details will be posted closer to the date.

Best wishes,

Chris Long (Melbourne) and Dr Mike Groth VK7MJ (Hobart) - Australia.