[linux-elitists] Must have free space lasers!

Ben Woodard ben@valinux.com
Fri May 4 13:30:26 PDT 2001

> >>>>> "BW" == Ben Woodard <ben@valinux.com> writes:
>     Me> I'm trying to think of how you'd do laser communication that
>     Me> -wasn't- line-of-sight. I mean, unless you don't count mirrors
>     Me> as line-of-sight. Can you do atmospheric bounce with lasers?
>     Me> Masers?
>     BW> The absorbtion of higher frequencies is quite noticable. That
>     BW> is why you really can't see that far.
> Say that again, to yourself, and determine if you really mean it.

That says pretty much what I meant it to. Notice that solid objects
tend to block visible light and often times reflect it. There are very
few solid things that absorb and reflect high frequency light
i.e. visible light. Things like glass are rare in that they don't
absorm quite as much light. On the other hand, lower frequency light
like radio waves move right through many solid objects. As you go down
in frequency electromagnetic waves tend to move things more easily. So
the closer you get to visible light the more absorbtion occurs.

>     BW> However, radio goes through buildings and finally short wave
>     BW> radio bounces all around the world.
> Let me try this again: laser, communication, not line-of-sight. Could
> you create a short-wave laser? How big would it be? How would you

Theoretically I don't see any problem. However it might not be worth
it. The way that the atmosphere bends and warps the EMF is what allows
it to move around the planet. If you had laser it would quickly go out
of sync. It would be like looking at a laser focused on a piece of
paper. It was coherant but hitting the uneven surface of the paper
scatters it.

The thing would be fucking huge. Like giant. Building size at least
maybe more.

> encode information into the laser signal? What would happen to that
> signal if you were bouncing it off the atmosphere?

There are four big data encoding methods: 
AM amplitude modulation - modulate the amplitude of the carrier
FM frequency modulation - modulate the frequency of the carrier
PM phase modulation - shift the phase of the carrier
CSM carrier sense modulation - turn the carrier on and off.

>     >> I guess if you had a powerful enough laser, eventually anything
>     >> that was blocking your line of sight would open up and you'd
>     >> have a nice clear path.
>     BW> You can use extremly low power if you use spread spectrum.
> Spread spectrum... lasers?

Higher power lasers have multiple frequenceies in them. They aren't
all monochromatic. e.g. the lasers they use for laser shows. That is
how they get color.

> ~Mr. Bad
> -- 
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