Why isn't my Michelson interferometer working?
May 10, 2011 2:08 PM Subscribe
Why isn't my homemade Michelson interferometer working?
Nutshell: 5 mW laser, beam splitter, two first surface mirrors, beam is going through the splitter, reflected off the two mirrors, back through the splitter, and combining on a surface. I don't see an interference pattern. Also tried a green laser pointer, no difference.
There's a bunch of YouTube videos with homemade interferometers that get patterns, e.g. here or here or here. What could I be doing wrong?
Nutshell: 5 mW laser, beam splitter, two first surface mirrors, beam is going through the splitter, reflected off the two mirrors, back through the splitter, and combining on a surface. I don't see an interference pattern. Also tried a green laser pointer, no difference.
There's a bunch of YouTube videos with homemade interferometers that get patterns, e.g. here or here or here. What could I be doing wrong?
Response by poster: Ah yes, the one in the video does -- I tried defocusing it with no effect, and also removed the lens (in which case I got a bath of red light), and that didn't do anything interesting either.
posted by trevyn at 2:32 PM on May 10, 2011
posted by trevyn at 2:32 PM on May 10, 2011
Any idea what the coherence length of your laser is? Your path lengths to the two mirrors need to be matched to within that length, which could be on the order of a mm or so for a cheap diode laser.
posted by doctord at 3:09 PM on May 10, 2011
posted by doctord at 3:09 PM on May 10, 2011
I wonder if the second maximum is just too weak (because it is too far from the center). If you lengthen the arms of the interferometer (I.e. move the two side mirrors further from the partial mirror) I think that should decrease the angular distance between the center and the second maximum. Or maybe there's another problem :)
Cool project.
posted by Salvor Hardin at 3:11 PM on May 10, 2011
Cool project.
posted by Salvor Hardin at 3:11 PM on May 10, 2011
Oh, also, you might try putting a pinhole right in front of the laser; if the laser is too much of a non-point source, that could wash out the interference pattern.
posted by Salvor Hardin at 3:18 PM on May 10, 2011 [1 favorite]
posted by Salvor Hardin at 3:18 PM on May 10, 2011 [1 favorite]
Response by poster: Hmm, preliminary tests with a pinhole and attempts to even out the path length haven't resulted in anything useful, but I suspect the coherence length might be what's getting me here.
posted by trevyn at 3:48 PM on May 10, 2011
posted by trevyn at 3:48 PM on May 10, 2011
Well, just looking at other similar interferometers on youtube, I would probably try:
- Moving the projection surface w-a-y back (and also darkening the room to check for any interference patterns in the outer areas, ie outside the main beam area)
- Putting some kind of lense to spread the beam--as here, here, and here.
It's just possible you're getting a perfectly nice interference pattern but you're only seeing the part of it right in the middle of the center dot.
posted by flug at 5:32 PM on May 10, 2011
- Moving the projection surface w-a-y back (and also darkening the room to check for any interference patterns in the outer areas, ie outside the main beam area)
- Putting some kind of lense to spread the beam--as here, here, and here.
It's just possible you're getting a perfectly nice interference pattern but you're only seeing the part of it right in the middle of the center dot.
posted by flug at 5:32 PM on May 10, 2011
Best answer: Coherence length is unlikely to be your problem, as long as path lengths are equal within a few mm or so. Some diodes have longer coherences than this.
Without a beam filter (usually a focusing lens, pinhole at the focus point, and collimating lens afterwards), the laser beam is likely to be too shitty to see patterns. This doesn't mean the beam isn't coherent; it means with the beam this tight (small in diameter), the noise overlaps, ruining any fringe patterns you might have seen. Hell, fringes across a beam under a few mm in size are too small to see in a glaringly bright spot, anyway!
Also, nudging the mirrors back and forth by hand like that is moving them by hundreds or thousands of wavelengths - that's like expecting to clearly see a bullet pass by!
So, a cheap plano-convex or biconvex glass lens, a pinhole (good quality is desirable, but a reasonable handmade one that looks clean under magnification), and another lens on the other side.
Lining up the pinhole on the focal spot is a real pain; as you approach the right position, bright off-center fringes and patterns will appear. These are your warnings that you are (finally!) in the vicinity of the focus spot. Setting up the beamspreader lens on the other side first can help see these more clearly, but a paper target 1-3 cm behind the pinhole should show the Fraunhofer patterns easily enough.
Another way to find the focal spot is to dim the beam WAAAAY down, so when it drops to a tiny dot, it doesn't flood your eyes with glare. Then, you can actually see it getting smaller - normally the beam is so bright it never seems to get small, because even the "dim" outer reaches are too bright.
Having said all that, you may find the focus in 4 seconds the first time, and think me an idiot. S'fair.
Now, the other side of the pinhole is easy. Move the lens back&forth until the beam stays the same size far away (against the far wall). Too close or too far, it will get bigger or smaller than the lens diameter. If the lens is really big, "stop" it down with a circular mask the size of the beam you want (a little bigger than your mirror size is handy!).
Bear in mind that you are likely to see a gajillion fringes horizontally across the dual-beam target, which indicates gross angular misalignment (since you haven't insured perpendicularity to within nanoradians!). Initially, this will be hard to tell from a slightly dim target (but you won't realize it's dim, because you haven't seen it in mid-fringe yet). Then, you'll see a fine stripe pattern. If you can get it down to some weird, irrregular, curved lines, you're damn near there.
Finally, to see fringe movement of red light (1/4-wave of approx 600nm in wavelength), you are talking mirror motion on the order of 0.1 microns! Try pressing lightly on the mirror supports from above, or blowing on the mirror to momentarily heat it - that should be enough. If you can see the mirror movement, it's too damn much.
posted by IAmBroom at 9:50 PM on May 10, 2011 [3 favorites]
Without a beam filter (usually a focusing lens, pinhole at the focus point, and collimating lens afterwards), the laser beam is likely to be too shitty to see patterns. This doesn't mean the beam isn't coherent; it means with the beam this tight (small in diameter), the noise overlaps, ruining any fringe patterns you might have seen. Hell, fringes across a beam under a few mm in size are too small to see in a glaringly bright spot, anyway!
Also, nudging the mirrors back and forth by hand like that is moving them by hundreds or thousands of wavelengths - that's like expecting to clearly see a bullet pass by!
So, a cheap plano-convex or biconvex glass lens, a pinhole (good quality is desirable, but a reasonable handmade one that looks clean under magnification), and another lens on the other side.
Lining up the pinhole on the focal spot is a real pain; as you approach the right position, bright off-center fringes and patterns will appear. These are your warnings that you are (finally!) in the vicinity of the focus spot. Setting up the beamspreader lens on the other side first can help see these more clearly, but a paper target 1-3 cm behind the pinhole should show the Fraunhofer patterns easily enough.
Another way to find the focal spot is to dim the beam WAAAAY down, so when it drops to a tiny dot, it doesn't flood your eyes with glare. Then, you can actually see it getting smaller - normally the beam is so bright it never seems to get small, because even the "dim" outer reaches are too bright.
Having said all that, you may find the focus in 4 seconds the first time, and think me an idiot. S'fair.
Now, the other side of the pinhole is easy. Move the lens back&forth until the beam stays the same size far away (against the far wall). Too close or too far, it will get bigger or smaller than the lens diameter. If the lens is really big, "stop" it down with a circular mask the size of the beam you want (a little bigger than your mirror size is handy!).
Bear in mind that you are likely to see a gajillion fringes horizontally across the dual-beam target, which indicates gross angular misalignment (since you haven't insured perpendicularity to within nanoradians!). Initially, this will be hard to tell from a slightly dim target (but you won't realize it's dim, because you haven't seen it in mid-fringe yet). Then, you'll see a fine stripe pattern. If you can get it down to some weird, irrregular, curved lines, you're damn near there.
Finally, to see fringe movement of red light (1/4-wave of approx 600nm in wavelength), you are talking mirror motion on the order of 0.1 microns! Try pressing lightly on the mirror supports from above, or blowing on the mirror to momentarily heat it - that should be enough. If you can see the mirror movement, it's too damn much.
posted by IAmBroom at 9:50 PM on May 10, 2011 [3 favorites]
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posted by Confess, Fletch at 2:25 PM on May 10, 2011