[Laser] Fresnel lenses etc from Chris Long

[Chris L]

Dear laser people,

An excellent monograph on fresnel lenses, their materials, their manufacture 
and usage may be found at:

http://www.fresneltech.com/pdf/FresnelLenses.pdf

I saw a posting suggesting that the 3dLens 39.5 cm square 22 cm FL lens 
would be better to use than the 33 cm FL lens. An 'f' number lower than 1 is 
actually a disadvantage in this case.

With respect to transmitted beam intensity, I refer to my collaborator Dr 
Mike Groth's article ''Photohones Revisited'' in Australian ''Amateur 
Radio'' magazine, April 1987, page 14. The intensity of the transmitted beam 
from a surface-emitting non coherent source is given by the following 
(hoping that the .txt format will not scramble things):

Beam intensity = (G x objective diameter squared) divided by (source 
diameter square) all multiplied by source intensity.

G is a geometric correction factor for the f/D ratio of the optical system, 
and relates to a graph published by Mike on the page of the article 
aforementioned. In brief, approx G = 0.95 for f2; G = 0.85 for f1 and G = 
0.75 for f0.7. The apparent contradiction arises because beam divergence 
increases at a greater rate than the total beam power as the focal length is 
reduced. An 'f' ration  of 1 or something slightly greater is desirable. At 
very low f numbers, lens illumination also becomes uneven, which may lead to 
increased scintillation of the received beam.

Provided that the focal length is not too short and that the lens has 
reasonable accuracy, the output lens will have approximately the same 
luminance as the source. An observer at a great distance will effectively 
observe the source enlarged to the size of the objective. Taking a practical 
example for gain calculation:

Luxeon emitting surface = approx 0.5 mm x 0.5 mm = 0.25 square mm
Fresnel collimator surface = approx 400 mm x 400 mm = 160,000 square mm
'Antenna' (fresnel) gain approx = 160,000 divided by 0.25 = 640,000 (!)

The gain would be much higher for a diffraction limited source like a laser, 
but the lens would have to be several orders of magnitude more accurate and 
your bank account would have to = Bill Gates!

And yes, I agree, the coherent source requirement of ARRL is silly as the 
received beam will decohere through any distance of dusty and turbulent 
atmosphere. The major research groups of the late 1960s and early 1970s 
working on laser comms - Hughes Aviation etc - gave up on atmospheric 
optical comms when they found that superhet operation at these frequencies 
was made impossible by atmospheric turbulence. There would also be a maximum 
possible transmitted modulation frequency set by the transit time spread of 
parts of the beam travelling through an atmosphere constantly varying in 
instantanous refractive index. The max modulation frequency would reduce 
with increasing atospheric distance, and would vary from day to day 
according to weather coinditions, temperature etc.

Don't take my word for it - read the refs and do the experiments! In this 
manner, Karel, I have admiration for your experimental efforts. Mike and I 
are not trying to operate any sort of commercial enterprise so we can't 
offer to 'merge' with Ronja, but you're welcome to our experimental results.

All the very best to all experimenters,

Chris Long (for himself and for Dr Mike Groth VK7MJ, currently visiting 
Adelaide).