Interferometry

The Michelson Interferometer

http://www.dragonseye.com/interferometer.jpg

This is one of the most useful tools in a holographer's laboratory.

Placed on the holographic table and observed carefully, it can answer many questions -

• how long does it take for the table to settle after components are moved? (this could be minutes to hours to a day or more!)
• does the table ever completely settle?
• what outside factors influence the stability of the set up? (like hearing a car going by and seeing if the fringes move)
• what times of day are the most stable?
• are the movements due to air movement, vibrations, temperature etc. ?
• how long does it take the table to settle after a plate is loaded?

and a variety of other useful vibration feedback.

If the Michelson is rigidly fixed on an optical rail and allowed to equalize then the interferometer can be used to study properties of the laser such as mode hop behavior, drift and coherence length.

How It Works

The Michelson Interferometer uses a beam splitter to split a laser beam into two beams. Each beam takes a different path towards its own mirror which reflects it back into the beamsplitter which now acts to combine the beams so that they overlap. The overlapped beams are fed through a lens and expanded onto a wall or white card. The beams interfere to form fringes which can be seen and studied.

The beams in the diagram are slightly offset so that the laser light isn't reflected back into the laser cavity. This reflection back into the laser can cause gas lasers to fluctuate in power and cause fringe movement and even ruin diode lasers.

Diagnosing Fringe Behavior

• Fringes drift slowly back and forth, either continuously or sporadically - This is most likely due to air currents. Create a breeze and see if it affects the drift.

• Fringes move slowly and continually in one direction - This is drift and most likely an optic is drooping or the table is sagging in one spot (this is most often seen on sand tables).

• Fringes jitter rapidly - This is most likely caused by vibrations coming through the table legs or a fan or other electrical component on the table is vibrating.

• Fringes stay stable then jump - This is called a mode hop. It is common when lasers are changing temperature. Allow your laser to warm up before use. If you aren't using a diode laser this can also be caused by air pressure changes. Make sure the AC is turned off and doors in your building aren't being opened and closed while testing with the interferometer or making an exposure.

Inserting a plate into the plateholder - the time it takes for the fringes to stop moving is the minimum settling time needed after you load a plate for table movement to stop.

Coherence Length - start with both legs of the interferometer set to exactly the same length (to within a millimeter if your coherence length is unknown). When you get all your mirrors aligned properly, you should see fringes. If and when you do, they should be contrasty, not washed out. If not, ask the holography forum for help. After the fringes stabilize, increase the length of one of the legs by a small amount. If fringes remain and are still contrasty your laser is still coherent to the length of the difference in path lengths. Continue to increase the length of the same leg until the fringes are not contrasty or disappear entirely. At this point you have exceeded your coherence length. To find the exact length start to go back from this length to the laser known good length in small increments.

Tests

Once the fringes are stable, do a variety of tests. Tap the table slightly and see how long it takes for the fringes to stop moving. Tap the floor with your foot and see how long it takes for the fringes to stop moving. Have someone tap the floor with their foot at different locations remote from the table, i.e. next room, upstairs, downstairs, further away etc. while you are viewing the fringes. Watch the fringes at different times of day - at night when streets are quiet, daytime when streets are busy etc.

An excellent way to evaluate your table is to set up a camcorder aimed at the fringes and set a walkie-talkie next to it. Now move around your house/building and walk around, open and close doors, start machinery (if in a home, start your dishwasher etc) and generally cause vibrations while talking into a second walkie-talkie. You can then review the tape and see exactly what, if anything affects your ability to make a hologram in your lab. This tape also serves as a record of table behavior that can be used at a later date to compare current and past behavior.

References

Great references for using holograms as measuring tools:

Schumann, W. and Dubas, M., Holographic Interferometry, Springer, 1979.

Schumann, W., Zürcher, J. P., and Cuche, D., Holography and Deformation Analysis, Springer, 1985.