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Michelson interferometer-Coherence required?

Michelson interferometer-Coherence required?

Michelson interferometer-Coherence required?

Good day.  I'm using a Michelson interferometer constructed on a cushioned optical table to measure deflections of small, pressurized membranes (2mm square and a few micrometers thick).  We are currently using a small He-Ne for the light source, but would like to switch to a pulsed YAG (Continuum SureLite with the power significantly cut down) so that we can capture essentially instantaneous interferograms of the membrane being flexed dynamically (up to about 1kHz).  Here's the problem:  The (large and difficult to move) YAG is in one room, and my experiment is in the next room.  So my professor says "no problem, rig up a series of mirrors to reflect the light from one room into the other and use it in your experiment". Riiigght, I say, but the laser light needs to be coherent for effective interferometry, and I question if the coherence will be maintained after reflection off 6 mirrors (each having lambda/10 flatness at best) and travelling a distance of about 60 feet.  She says to try it anyway.

Who's crazy here?  Do I have a prayer of generating clear interference patterns with this arrangement?  Thanks!

RE: Michelson interferometer-Coherence required?

OK, your coherence isn't the problem - laser light stays as coherent as it started over a theoretically infinite distance - the problem is wavefront and/or phase distortion.  The nice laser beam may end up hideously deformed after so many reflections off inperfect mirrors.  Your boss is right to say try it - if possible deliver the laser beam 'as is' to the experiment and then do all your beam expansion/collimation to make that lovely perfect wavefront!!  Expect lots of lovely fringes but you should be able to see your dynamic changes with some clever optics!!!(N.B. if you can still see anything that is - HeNe's are easier on the eyes).  Sounds fun - all the best.
If it works send us a pic - eatmore.toast@physics.org

RE: Michelson interferometer-Coherence required?

I don't think it'll work with a YAG. the road to what you want is usually: the YAG pumps a colorant cell, and you use that induced laser light for ultrashort time probing. Guess why?
You are right: the YAG has a TERRIBLE wavefront; pulsed lasers and especially solid state ones have this behaviour.

That's why u use He-Ne in school for Michelson: gas laser, plus low power = nice wavefrnt profile, good for interferometry.

Your adviser wouldn't know anything about optics, would she?


RE: Michelson interferometer-Coherence required?

I understand why you you're thinking of using a pulsed laser... but have you considered either using some sort of beam chopping scheme with the He-Ne or a shutter on the film?

You may be able to achieve what you want without the extra trouble.  Coherence won't be a problem with the YAG, but there are a lot of other factors in trying to pipe the beam over that could distort your results.


RE: Michelson interferometer-Coherence required?

You should look into using HP's two-frequency HeNe Laser


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