The dictionary definition goes something like:
"INTERFEROMETER: An instrument designed to produce optical interference
fringes for measuring wavelengths, testing flat surfaces, measuring small
distances, etc."
As an example of an interferometer for making precise physical measurements, split a beam of monochromatic coherent light from a laser into two parts, bounce the beams around a bit and then recombine them at a screen, optical viewer, or sensor array. The beams will constructively or destructively interfere with each-other on a point-by-point basis depending on the net path-length difference between them. This will result in a pattern of light and dark fringes. If one of the beams is reflected from a mirror or corner reflector mounted on something whose position you need to monitor extremely precisely (like a multi-axis machine tool), then as it moves, the pattern will change. Counting the passage of the fringes can provide measurements accurate to a few nanometers!
A simple version of a Michelson interferometer is shown below:
_____ Mirror 1 (Moving)
^
|
| Beam
| Splitter
+-------+ | / |
| Laser |=========>/<---------->| Mirror 2 (Fixed)
+-------+ / | |
|
|
|
v Screen (or optical viewer,
------- magnifier, sensor, etc.)
A long coherence length laser producing a TEM00 beam is generally used for this application. HeNe lasers have excellent beam characteristics especially when frequency stabilized to operate in a single longitudinal mode. However, some types of diode lasers (which are normally not thought of as having respectable coherence lengths or stability) may also work. See the section: "Interferometers using inexpensive laser diodes". Even conventional light sources (e.g., gas discharge lamps producing distinct emission lines with narrow band optical filters) have acceptable performance for some types of interferometry.
Such a setup is exceedingly sensitive to EVERYTHING since positional shifts of a small fraction of a wavelength of the laser light (10s of nm - that's nanometers!) will result in a noticeable change in the fringe pattern. This can be used to advantage in making extremely precise position or speed measurements. However, it also means that setting up such an instrument in a stable manner requires great care and isolated mountings. Walking across the room or a bus going by down the street will show up as a fringe shift!
Interferometry techniques can be used to measure vibrational modes of solid bodies, the quality (shape, flattness, etc.) of optical surfaces, shifts in ground position or tilt which may signal the precursor to an earthquake, long term continental drift, shift in position of large suspended masses in the search for gravitational waves, and much much more. Very long base-line interferometry can even be applied at cosmic distances (with radio telescopes a continent or even an earth orbit diameter apart, and using radio emitting stars or galaxies instead of lasers). And, holography is just a variation on this technique where the interference pattern (the hologram) stores complex 3-D information.
This isn't something that can be explained in a couple of paragraphs. You need to find a good book on optics or lasers. Gordon McComb's: "The Laser Cookbook [1} and the Scientific American collection: "Light and its Uses [2]" include various type of interferometers which can be built with (relatively) readily available parts. Hewlett Packard (among others) manufacture 'Laser Interferometry Measurement Systems' based on these techniques. Information and application notes are available by searching for the key words: "Laser" or "Dimensional Measurement" at the HP Test & Measurement Web Site Search Page.
(From: Randy Johnson (randyj@nwlink.com)).
I'm an amateur telescope maker and optician and interferometry is a technique and method that can be used to quantify error in the quality of a wavefront. The methods used vary but essentially the task becomes one of reflecting a monochromatic light source, (one that is supplied from narrow spectral band source i.e. laser light) off of, or transmitting the light through a reference element, having the reference wavefront meet the wavefront from the test element and then observing the interference pattern (fringes) that are formed. Nice straight, unwavering fringe patterns indicate a matched surface quality, curved patterns indicate a variation from the reference element. By plotting the variation and feeding the plot into wavefront analysis software (i.e., E-Z Fringe by Peter Ceravolo and Doug George), one can assign a wavefront rating to the optic under test.
The simplest interference test would involve two similar optical surfaces in contact with each other, shining a monocromatic light source off the two and observing the faint fringe pattern that forms. This is known as a Newton contact interferometer and the fringe pattern that forms is known as Newton's rings or Newton's fringes, named for its discoverer, you guessed it, Sir Issac Newton. If you would like to demonstrate the principle for yourself, try a couple of pieces of ordinary plate glass in contact with each other, placed under a fluorescent light. Though not perfectly monochromatic, if you observe carefully you should be able to observe a fringe pattern.
Non-contact interferometry is much tougher as it involves the need to get a concentrated amount of monochromatic light through or reflected off of the reference, positioning it so it can be reflected off of the test piece, and then positioning the eye or imaging device so that the fringe pattern can be observed, all this while remaining perfectly still, for the slightest vibration will render the fringe pattern useless.
(From: Mark Kinsler (kinsler@froggy.frognet.net)).
You can get interferometer kits from several scientific supply houses. They are not theoretically difficult to build since they consist mostly of about five mirrors and a lens or two. But it's not so easy to get them to work right since they measure distances in terms of wavelengths of light, and that's *real* sensitive. You can't just build one on a table and have it work right. One possible source is: Central Scientific Company.
(From: Bill Wainwright (billmw@isomedia.com)).
Yes, you can build one on a table top. I have done it. I was told it could not be done but tried it anyway. The info I read said you should have an isolation table to get rid of vibrations I did not, and even used modeling clay to hold the mirrors. The main problem I had was that the image was very dark and I think I will use a beam splitter in place of one of the mirrors next time. The setup I had was so sensitive that lightly placing your finger on the table top would make the fringes just fly. To be accurate you need to take into account barometric presure and humidity.
The party line has tended to be that the coherence length of diode lasers is too short for interferometry or holography. While I was aware of CD laser optics being used with varying degrees of success for relatively short range interferometry (a few mm or cm), the comments below are the first I have seen to suggest that performance using some common laser diodes may be at least on par with that of a system based on a typical HeNe laser.
(From: Steve Rogers (scrogers@pacbell.net)).
I have been involved with laser diodes for the last 15 years or so. My first was a pulsed (only ones available at that time) monster that peaked 35 watts at 2 KHz with 40 A pulses! It was a happy day when they could operate CW and visible to say the least. Anyway, in the course of my working travels, I have built numerous Twymann-Green double pass interferometers for the wave front distortion analysis of laser rods, i.e., Nd:Yag, Ruby, Alexandrite, etc. The standard reference light source for this instrument has always been the 632.8 nm HeNe laser. Good coherence length and relatively stable frequency was its strong suit.
When visible diode lasers came out I often wondered aloud about their suitability as a replacement for the HeNe. I despise HeNe lasers. They are bulky and I have been shocked too many times from their power supplies.
I assumed that since CD player laser diodes at 780 nm could have coherence lengths on the order of tens of centimeters or into the meters (!!, see, for example: Katherine Creath, "Interferometric Investigation of a Diode Laser Source", Applied Optics (24 1-May-1985) pp. 1291-1293), Visible Laser Diodes (VLDs) could make excellent replacements. As it turned out, VLDs tend to have coherence lengths which are considerably shorter according to the latest technical literature and I held off on experimenting with them. Last week, I went through my shop and found enough mirrors, beam splitter, assorted optics to throw together my own double-pass interferometer for home use. This coincided with my acquisition of a 635 nm 5 mw diode module - a good one from Laserex.
To make a longer story shorter, I assembled said equipment with the VLD and WOW! excellent fringe contrast (a test cavity of four inches using a .250" x 4.0" Nd:Yag rod as the test sample.) When a HeNe laser was substituted for the VLD, virtually no difference in the manual calculation of wave front distortion (WFD) and fringe curvature/fringe spacing. The only drawback with the VLD is that it produces a rectangular output beam. When collimated you have a LARGE rectangular beam rather than a nice round HeNe style beam. My interferometer now occupies a space of 10" x 10" and is fully self contained. It probably could even be made smaller. Not only that, but it runs on less than 3 V!!!
I am just as surprised as you are with the results that I achieved. This is one reason why it took me so long to attempt this experiment (something like 4 to 5 years). I have always assumed that a HeNe laser would be FAR superior in this configuration than a VLD would be. Perhaps others may know more about the physics than I do. One thing is certain, these are "single mode" index guided laser diodes and typically exhibit the classic gaussian intensity distribution which is not so evident with the "gain guided" diodes. This in turn implies a predominant lasing mode which in turn would imply a (somewhat) stable frequency output. Purists would note that this VLD has a nominal wavelength of 635 nm +/- 10 nm while the HeNe laser is pretty much fixed at 632.8 nm. This variable could account for extremely minor WFD differences.
Extensive information on how to build lasers and how to use them, as well as info on building laser instruments. All of John Strong's (genius experimentalist) and Jeral Walkers columns on photonic devices are in this absolutely fabulous collection.
The book describes the construction of several types of lasers by amateurs including HeNe, argon ion, dye, CO2, and nitrogen - all from scratch (e.g., the HeNe and argon lasers require glassworking to fabricate the plasma tube.) Also, some hologram, interferometer, and other optics experiments. It is not for the absolute beginner but suitable for anyone who has some considerable hobbyist type experience with electronics and/or lasers.
Note: To actually construct most of these projects requires a fair degree of skill and determination; access to some machining, glassworking, and/or high vacuum facilities; a source of electronic, optical, and mechanical components; and a stock of chemicals and other materials. However, much of this can be provided without the assets of a major R&D laboratory but will require improvisation. Nonetheless, the book makes for some very interesting and educational reading even if you are not going to be building anything.