White light to laser?

[Murudai]

This is a little twist on the old 'can you focus down a spotlight into a laser?' question.

A friend of mine was asking that old spotlight question, and I said it wouldn't work because it wouldn't be coherent, etc, etc. But then he said what about filters and things? If you had the right filters, could you filter out all the incoherent light and all the other stuff to form a laser?

And I didn't know how to answer that. I mean, in theory, since white light consists of everything, you can filter out everything you don't want so you just have monochromatic coherent light left. And that would make a laser. You would need a HELL of a lot of white light, but the question is would it be possible?

So if we got, say the sun, rigged up a huge amount of filters and things, could we form an uber laser?

 

[knimrod]

You can't make filter incoherent light to make coherent light. Coherence is a characteristic of light waves that are in phase.

High End Systems used to market an "Emulator" that did a pretty darn good job of simulating laser beam effects and even limited projected graphics. Check out this video. No lasers were used.

[media width=480]http://www.youtube.com/watch?v=teT3da6w320[/media]

 

[Murudai]

Yeah, I know I know.

But surely some of the photons are in phase with each other, even in incoherent light, right? So for every gazillion photons, there might be 2 that happen to be in phase, but that's what I'm asking

So if you had an infinite supply of white light, and enough filters to filter out the 99.9999999% of photons that aren't in phase, could you get coherent light

 

[Chad]

No. You can't get coherent light out of uncoherent light, as knim said. There probably WOULD be a few photons that were in phase, but there wouldn't be any feasible way to find those few photons and block the rest. Even if you could, it still wouldn't really be coherent. Go pick up a book on the physics of light.

Stick with actual lasers, as well.  

 

[likewhat]

Well, sunlight is coherent, but the coherence length is only a few microns or something like that, so it wouldnt make a very good laser. But yeah, if you filtered it to be one color, with something like a prism or a diffraction grating then took the single color and focused it through a pinhole "spatial filtering" then put a lens after it to collimate it then it would be monochromatic, coherent, and traveling in the same direction. So it would be essentially the same as a laser. But you could also just get a laser and it would be way easier and better.

 

[RA_pierce]

As for using the sun as a laser, it might be best to get a 1meter diameter parabolic mirror (or convex lens) with a focal point of more than 5 feet and mount it on an adjustable frame... there you go... 1kW of sorta-coherent white light.

 

[Murudai]

Okay, so the answer is that it is theoretically possible, but there is no practical way to filter out non-coherent light from coherent light. Thanks

I'll tell my friend that

 

[nikokapo]

that video was incredible!!!


but you can make incoherent light with coherent one, cant you?