interferometry

nikokapo said:

[quote author=pullbangdead link=1212512408/12#14 date=1212819454]Also, interferometry can be used to measure the thickness of thin films, without even doing anything else with the light. Doesn't even have to be coherent. Light hits the surface at a non-perpendicular angle: some is reflected at the air-film interface, and some goes through. Then, some light is reflected at the film-substrate interface. Light from both paths then hits the detector, but since the paths were different lengths, the light from the different paths will be out of phase, causing interference. With some more complication and a little math, you can calculate the difference in path lengths, and therefore the film thickness.

This is also exactly what you do with x-ray diffraction, kind of. Except it's x-rays, and the x-rays reflect off of different planes of atoms in the material, giving you the interatomic (or "d") spacing in a material. Bragg's law is basically derived straight from the picture I described above.

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So they use x-rays to determine the difference/distance from something to something else?[/quote]

You can do a lot of things with x-ray diffraction, but one of them often used is to find the distance between neighboring planes of atoms in a material.

I'm sorry for bumping this, but RoadRunner from LC made a Michelson interferometer some time ago.

Link: http://lasercommunity.com/lc_view_project.php?aid=8

If you have been banned from LC, use this link: http://209.85.173.104/search?q=cach...e:lasercommunity.com&hl=en&ct=clnk&cd=1&gl=us

I'll just throw in a monkey wrench and watch the action. Are you aware, and I find this a really neat aspect of the use of lasers and something most hobbyists are not really aware of, but using a laser to generate interference patterns opens the fundamental nature of quantum mechanics to the everyday Joe with a laser pointer. Interference is the basis of quantum mechanics. The interference of probability waves creates the randomness and the schizophrenic nature of quantum uncertainty. What I am saying is that if you understand the physics of photon interference you have no choice but to understand the big picture of quantum mechanics. Having a direct relationship to quantum mechanics is a very unusual aspect of a macro object.

By the way, if you really want to learn about "big" interferometers look up the gravity wave interferometer at CalTech.

I want to measure the coherence length of my lasers. I have a ton of mirrors, dichro's, beam splitters and lenses. Can anyone give me an example setup and explanations?

^^Yes, ground-based interferometers for gravitational wave detections are extreme.  

The LIGO set, with one in Louisiana and two in Washington, have perpendicular arms whose length is precisely measured.  The arms are 4 kilometers long, which is just insane.  The even more insane part is that the most a gravity wave could be expected to do to the arms is to change the length of the arms by 10^-17m, or 10 attometers, and that these things can supposedly detect that change in length.  Detecting a .000000000000000001% (that's 17 zeros) change in length on a 4 kilometer long tube.  What will they come up with next?

EDIT: Actually, I think Frothy and I are talking about the same interferometer(s), because the LIGO instruments are operated partly by CalTech. Great minds think alike.

I think your information source is inaccurate. For starters, temperature changes would expand and contract a four kilometer arm by much more and secondly I think it's impossible to measure that small of amounts with lasers with a wavelength still in the hundred of nanometer range. That would require sensors to measure differences of 10^-10 (assuming 100nm). btw, the change in length in % is not correct. (1 aM / 4 kM is not a ratio ending with a 1)

Zom-B said:

I think your information source is inaccurate. For starters, temperature changes would expand and contract a four kilometer arm by much more and secondly I think it's impossible to measure that small of amounts with lasers with a wavelength still in the hundred of nanometer range. That would require sensors to measure differences of 10^-10 (assuming 100nm). btw, the change in length in % is not correct. (1 aM / 4 kM is not a ratio ending with a 1)

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They take temperature, seismic events, the mean air speed of european swallows, all of those things into account. It's all adjusted for already before they ever begin measurements. They are precise enough that they even have to account for the curvature of the earth in constructing the things, and it's all inside some of the world's largest high vacuum systems. They have multiple systems to compare with temperature and seismic events, and by the time at which events happen (since gravity waves move a the speed of light, that's why they are so far apart). Temperature is a problem, yes, but that's a VERY easy thing to control for.

10^-18 meters is correct. http://www.ligo.caltech.edu/LIGO_web/PR/scripts/facts.html under "How will the detectors sense gravitational waves?". And it actually uses IR, not visible light, at 1064 nm, so they're measuring even smaller than you think with an even higher wavelength than you think. Go read about it, it works.

And yes, the ratio would not end in 1 precisely. But what they measure will never end in 25 precisely, or anything precisely for that matter. I was simply expressing the order of magnitude of the problem. When you're working on the scale of 10^-20 or 10^-21, a factor of 4 is nothing. I even said that that was the MOST a gravity wave would be able to do, so all real data would be even less than that. So expressing an exact quantity would be quite silly.

Not to toot my own horn but the LIGO lab at CalTech has 11 pair of my laser protective glasses for maintenance and operation of the laser. Hooah!

Either they work great or I blinded everyone and they can't dial a phone or send me an email now, lol.

FrothyChimp said:

I'll just throw in a monkey wrench and watch the action.

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