(probably a bit OT) Laser question

[bod]

Being as a laser maintains its power in its beam. Does anyone know if a laser beam is infinite in how far the beam shines or is there a limit?

[Tim Watts]

On Monday 29 July 2013 20:47 bod wrote in uk.d-i-y:

There's a limit.

Very slight beam divergence is one, and for most applications, atmospheric absorption and scattering.

[bod]

Ok, thanks for that.

[Dave Liquorice]

Unless it absolutely parallel the beam will diverge (even if it converges first) and matter in the path will scatter the beam. So in practical terms a laser beam is not infinite but with very careful collimation and could be very long.

[bod]

Gotcha, cheers.

[Bob Eager]

[newshound]

You could see examples of this in the demonstrations in Egypt where people were shining red and green laser pointers at helicopters resulting, IIRC, in beam diameters approaching a metre at helicopter height. My guess is that most of this was divergence, rather than scattering, associated with cheap laser pointer optics.

ISTR that US military experiments were getting something like metre divergence at ~ 100 miles. (The "diffraction limit" sum is trivial).

In fact let's do the sum. Assume a wavelength of half a micron and a laser pointer with a 1 mm diode, the diffraction angle is of the order of 1 in 2000 or a metre diameter at 2 kilometers. Suppose a military laser has a 100 mm "objective", now the divergence is 1 in 200,000 or a metre in 200 km.

[John Rumm]

They estimate that if you project a laser spot on the moon, its about

40m wide by the time it gets there...

[newshound]

Same sum says that means a 5 metre lens or mirror. Which, by chance, is the size of the Hale telescope on Mount Palomar.

[The Natural Philosopher]

It doesn't maintain it THAT well

[The Natural Philosopher]

ISTR we (humans) bounced one off the moon and it was detectable with an optical telescope

[bod]

Some useful info in that link, thanks.

[Brian Gaff]

Not quite sure what you mean. Its not really any different to any other light except its coherent much like a radio wave, and can be better focussed of course, but it still diverges and is affected by turbulenceand impurities in the medium it is going through.

Brian

[bod]

I asked the question after hearing a quote from a presenter on a documentary about lasers. He stated that "a laser does not lose any power over distance".

[Tim Watts]

On Tuesday 30 July 2013 09:24 bod wrote in uk.d-i-y:

..."in a vacuum". But that's true of any light.

And it will still lose power per area as the beam diverges.

[bod]

Ah! the presenter didn't elaborate about divergence etc. He just made that sweeping statement, hence, my question.

[whisky-dave]

"At the Moon's surface, the beam is only about 6.5 kilometers (four miles) wide[6"

[Dave Liquorice]

The moon isn't very far away. At least one of the Apollo missions left a retro-reflector on the surface fairly sure that they regularly fire a laser at that and look for the reflection to measure the distance between earth and moon to a cm or three. B-)

[bod]

Cheers.

[Dave Liquorice]

Care to explain this "diffraction limit"?

I can't see a (theoretical) reason why you can't construct an optical system that will produce a parallel beam. If there is some physical reason to do with photons that means that optics that should create a parallel beam don't you just put a "correction" into the optics to take that into account.