[time-nuts] Using lasers for data transmission

[jimlux]

On 5/1/16 9:48 AM, Mark Sims wrote:
> There are "eye-safe" wavelengths that some laser diodes can operate at (generally greater than 1300 nm).  These are much less prone to damage eyes.  Basically your eyeball juice blocks the wavelength.  Still, there is some potential for cornea and lens damage at higher powers.
> http://www.laserfocusworld.com/articles/2008/03/photonic-frontiers-eye-safe-lasers-retina-safe-wavelengths-benefit-open-air-applications.html
>
If they eye blocks (as in absorbs) the power, fine, you don't get a 
retina burn, but you get cataracts or other damage instead (more like 
"looking into the waveguide with your remaining good eye").


It's all about energy deposition and there's copious literature on what 
safe levels are (on Wikipedia even)


This kind of thing is fairly easy to do safely, you just have to work 
through the scenarios and recognize that doing it outside isn't like 
doing it the relatively controlled environment of a lab.

With reference to my previous comment about binoculars and telescopes, I 
don't think you have to worry about someone at the intended receiver 
site looking back at the transmitter (any beam divergence at all will 
make the energy density so low at the receiver, it's probably not an issue).

It's someone who's a lot closer than you expected turning around and 
looking back at the transmitter or an unexpected specular reflection off 
something.


BTW those geophysics measurements with red visible lasers across 
California were mostly done with HeNe lasers, because that's what was 
available back then.  They also have a nice long cavity (The 1mW 
SpectraPhysics 155 that I had back in 1978 had a 30cm-ish cavity), so 
the spectral purity is quite good and the beam divergence is low.  The 
HeNe are also a lot brighter visually than the red diode laser pointers 
for the same optical power.