Combining beams of the same polarity..

[steve-o]

Hey guys,
Quick question:
If I knife-edged two laser beams of the same wavelength together very closely and they begin to overlap at 100 ft, then completely converge appearing as one beam at 200-300ft, and one is not rotated 90 degrees, will there be power losses or interference patterns or some other problem at the convergence point? Thanx alot in advance!
Cheerz,
Steve-0

[steve-o]

I think I got an answer to my question by digging deep thru PL archives..

buffo
11-15-2010, 14:05

surely two beams at the same frequency
one would cancel the other out ?? the same as it does with audio,
You are thinking about destructive interference. This is when one wave is exactly out-of-phase with another, so that as they combine they cancel each other out perfectly.

This is possible (with great difficulty) to achieve with audio. In theory, it would also be possible to do the same with light, but you have to understand the tolerances we're dealing with are *much* tighter. Audio wavelengths are measured in inches, or feet even. But with light we're talking about a wavelength of a few hundred nanometers. And remember, the two waves need to be *perfectly* aligned in all three axis in order for the destructive interference to work. We're talking about nanometer tolerances here, over all three axis. Not an easy thing to do!

Then too, remember that they'd have to be spacially coherent as well. (Well, actually anti-coherent, because they need to be exactly 180 degrees out of phase.) And finally, due to the relatively short coherence length of the diode's output, even if you managed to get the destructive interference to work, it would only be effective over a very short distance - probably no more than a few inches. And this is under the very best case in a carefully controlled laboratory.

Bottom line: There is far too much variation for destructive interference to ever have any measurable effect on combined beams at the wavelengths we're working with. It's not a problem with the theory, just the fact that in the real world you'd never be able to align the beams to that level of precision, nor be able to keep them that way long enough to get it to work.

Adam

Thanks Adam

[Xytrell]

I would think conservation of energy ensures you'll still get a spot. There is destructive interference in your microwave, but there is just as much constructive interference. The interference pattern shouldn't look much different from a single laser spot - "speckle pattern"

From Wikipedia:

[steve-o]

I would think conservation of energy ensures you'll still get a spot. There is destructive interference in your microwave, but there is just as much constructive interference. The interference pattern shouldn't look much different from a single laser spot - "speckle pattern"

From Wikipedia:

So you're saying that there is destructive interference present resulting in a degradation of power?

[planters]

No. I think he is suggesting that like speckle the interference,+ and- will redistribute the energy both over time and in space.

[steve-o]

+ and - polarity? Is that the same as S and P polarity? Redistribution of the laser energy in space I can understand, but over time has me a bit perplexed. Are you referring to C, the speed of light?