[Laser] LED vs Laser diode

[F1AVYopto at aol.com]

>I know almost nothing about light communication, but this summer I  will 
perform some experiments.
>I have just a question which will show how  ignorant I am in this field.
>I bought at an ham fair a Luxeon 3W white  LED. What I would like to know is 
how does it compare, in >terms of quantity  of light received at distance, 
with a laser diode ? Suppose that both are  reasonably >focused with a lens.
>So, what is the mW equivalent from a  laser diode of that 3W Luxeon LED ?

Alberto,
The question  is difficult because the answer depends of the optical devices 
available  for you.
Because light coherence and small emitting size active area, a laser  is 
always able to be focused in a very narrow beam : easily less than five  
milliradiants of beam divergence !
At long range, for this reason, a 1 mW  laser fitted in a good optical 
collimator gives in theory always a more  concentrated light than a 1 W power LED in 
all the optics you can choose.
But  a laser beam is very noisy (phase cancelling scintillation) and gives a 
very  difficult aiming.
A power LED fitted at a big Fresnel lens focal point will  give a very easy 
beam aiming, less noise, and a very sufficient power for a more  than 150 Km 
point to point LF optical QSO.
Lasers could be better for long  range echoes and NLOS communications but LED 
are better for point to point low  frequency coms.
For high speed digital coms the problem is very different  because the high 
level and high bandwidth needed.
73 QRO.
Yves F1AVY 
http://pageperso.aol.fr/yvesf1avy/