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Michelson interferometer-Coherence required?

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Good day.&nbsp;&nbsp;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).&nbsp;&nbsp;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).&nbsp;&nbsp;Here's the problem:&nbsp;&nbsp;The (large and difficult to move) YAG is in one room, and my experiment is in the next room.&nbsp;&nbsp;So my professor says &quot;no problem, rig up a series of mirrors to reflect the light from one room into the other and use it in your experiment&quot;. 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.&nbsp;&nbsp;She says to try it anyway.<br><br>Who's crazy here?&nbsp;&nbsp;Do I have a prayer of generating clear interference patterns with this arrangement?&nbsp;&nbsp;Thanks!
 
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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.&nbsp;&nbsp;The nice laser beam may end up hideously deformed after so many reflections off inperfect mirrors.&nbsp;&nbsp;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!!&nbsp;&nbsp;Expect lots of lovely fringes but you should be able to see your dynamic changes with some clever optics!!!<i>(N.B. if you can still see anything that is - HeNe's are easier on the eyes)</i>.&nbsp;&nbsp;Sounds fun - all the best.<br>If it works send us a pic - <A HREF="mailto:eatmore.toast@physics.org">eatmore.toast@physics.org</A><br>Vik
 
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?

Cheers
 
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.

Bob
 
You should look into using HP's two-frequency HeNe Laser
interferometer. <nbucska@pcperipherals.com>
 
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