Need Help with Michelson Interferometer Setup?

[Miphisto]

Hey there, I'm doing a Michelson Interferometer for my final Physics project, and my partner and I are having some difficulties.

We have our beam splitter set up 45 degrees from the light source (a simple red laser) and 2 mirrors set up in such a way that the one directly across from the beam splitter from the laser is 90 degrees to the laser, and the other parallel to the laser (so that the beam is also reflected off the surface at 90 degrees from the beam splitter).

Our problem is when our beams come back together on the beam splitter, the light projected on the piece of paper we have is much weaker, and the laser is the same size with no visible fringes.

This is very frustrating as we have spent a great deal of time trying to fix this over and over again. We have even puffed minute amounts of baby powder over our interferometer to see the laser beams reflecting off the mirrors to confirm that our angles were properly set.

We have spent many hours trying to figure out why we can't see anything and I'd really appreciate some advice from this community!

The following picture is basically the same set up as our interferometer. I will try to get a picture from my partner since the device is at his house currently so that you may have a better understanding as to what might be wrong with it.

http://www.clu-in.org/programs/21m2/openpath/op-ftir/images/exhibit1.gif

Thank you for your time and consideration.

 

[scupydog]

hi there, i am no expert in this field , but unless you get perfect angles and near perfectly stationless ie not moving equipment there will be no fringes.
hope this helps.

 

[Cthugha]

Michelson interferometers are sometimes pretty tricky to realize, so let me ask you a few questions:

Which wavelength are you working at?
What kind of beam splitter are you using - a cube? are you sure the beam splitter is a 50-50 divider at your wavelength?
Are your mirrors ok for your wavelength range?
How do you move your mirror? Do you use a dely line?
Do you use simple mirrors or retroreflectors?
Have you checked the divergence of your beam? in Michelson interferometers it is usually very important that the beam is parallel and does not change size of several meters.
Also check, whether your beams are really parallel after they are recombined at the beam splitter. It is usually best to take away the detector for a moment and check, whether the beams still overlap a few meters af the recombining beam splitter.
Also it is absolutely necessary to have a stable system. Even under lab conditions walking near the interferometer is sometimes enough to disturb the measurements.

 

[jtbell]

Are you using something to spread out the beam before it enters the interferometer? Typically you use the original narrow laser beam to get the initial alignment, by overlapping the two spots on the viewing screen. But the illuminated area is then too small to see any interference fringes. At that point you put a lens in the initial beam to diverge it before it enters the interferometer, so you illuminate a relatively broad area on the screen. Then the fringes are easier to see, probably after some further tweaking of the mirrors.

 

[Miphisto]

Michelson interferometers are sometimes pretty tricky to realize, so let me ask you a few questions:

Which wavelength are you working at?
I have no clue to be honest.
What kind of beam splitter are you using - a cube? are you sure the beam splitter is a 50-50 divider at your wavelength?
I am using a "2-way mirror" basically, half silvered other half has a darkish tint. Very expensive might I add, maybe we should have just used a glass cube?
Are your mirrors ok for your wavelength range?
Again, I'm not totally sure, I guess my interferometer looks really unorganized at the moment!
How do you move your mirror? Do you use a dely line?
We havn't mounted the adjustable mirror yet because we wanted to make sure everything worked properly first, we have a temporary holding device, and 90 bent piece of metal basically.
Do you use simple mirrors or retroreflectors?
Simple mirrors.
Have you checked the divergence of your beam? in Michelson interferometers it is usually very important that the beam is parallel and does not change size of several meters.
Also check, whether your beams are really parallel after they are recombined at the beam splitter. It is usually best to take away the detector for a moment and check, whether the beams still overlap a few meters af the recombining beam splitter.
Also it is absolutely necessary to have a stable system. Even under lab conditions walking near the interferometer is sometimes enough to disturb the measurements.
I think this might be the biggest issue. Our interferometer is infact not very stable and very "temporary" and nothing is really quite mounted right. I'll be going to my partners house in a couple days and i'll try to make the set up very precise and stable. I'll also take into account everything else mentioned, thank you for the advice. Feel free to reply again.

Thank you.

Are you using something to spread out the beam before it enters the interferometer? Typically you use the original narrow laser beam to get the initial alignment, by overlapping the two spots on the viewing screen. But the illuminated area is then too small to see any interference fringes. At that point you put a lens in the initial beam to diverge it before it enters the interferometer, so you illuminate a relatively broad area on the screen. Then the fringes are easier to see, probably after some further tweaking of the mirrors.
Actually we have about 4 or so diverging lenses with differen't focal lengths, and we haven't properly mounted them, infact we've been just holding it with our hands to see it first. I'm going to get everything properly aligned and temporarly mounted so we can have a more accurate set up.

Thank you for the advice, please feel free to reply and suggest some more ideas.