[Laser] Laser comm and ARRL?

[laser@mailman.qth.net]

In a message dated 2/19/04 5:20:20 AM GMT Standard Time, glennt@attglobal.net 
writes:

> I find the "coherent" requirement particularly amusing as I suspect that 
> beam from a typical semiconductor or HeNe laser isn't even coherent a kilometer 
> away. It's still exceptionally well collimated at that distance, but not 
> coherent! How is a well collimated LED (which is also very monochromatic) all 
> that different?
> 
> 73 de Glenn WB6W
> 
> 

This seems to imply that there is a connection between COHERENT light and 
BEAMWIDTH.  There isn't.

A typical semiconductor laser diode when operated above its lasing threshold 
has a coherent light output and a beamwidth of around 20 degrees by 10 
degrees. It's the optics that give the nearly parallel beam. 

Over here in the UK the RSGB acknowledges all light sources as valid for 
QSOs. The requirement still insists on an "electronic detection stage" for the 
receiver. Unfortunately there is no difference in multipliers between 10GHz and 
light, so not many people are motivated to try a light band QSO.

Personally, I don't think that just removing the coherent source requirement 
is such a good idea. The comments on the reflector have mainly assumed that 
people will use an array of modulated LEDs instead of a laser. This would be 
fine as it would require construction adjustment and all of those things about 
home construction that we enjoy. However, consider how you would feel if someone 
on a nearby hill just plugged in a 2 million candlepower spotlamp into their 
cars accessory socket and  started sending CW by turning it on and off??  (A 
sort of contest lighthouse) OK, the receivers would need to be configured 
differently i.e. it would need to detect intensity and not 'switched modulation'  
But that's not a big problem. My point is that from the right location such a 
system could make a QSO over 20 to 50km in seconds and that buying your QRO 
transmitter from K-Mart (?) isn't really in the spirit of contests.

Perhaps defining a modulated source and removing the coherent requirement may 
work, but allowing any light source could be exploited.
BTW. Light sources are evolving greatly. The theatre industry is now begining 
to switch their spotlamps from Quartz Halogen to the new Ultra high output 
LEDs. I imagine these could be modulated very easily. Would certainly work for 
cloudbounce.....but still very expensive.

73

David  G0MRF 


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