The secret to keeping costs down for this interferometer are: use a cheap laser; keep the number of precision mechanical mounts to a minimum; keep the spots small on the beam splitter and mirrors and then blow the beam up for viewing at the end. We use a laser pointer because you get a surprisingly good and reliable light source for under $25. By keeping the distances between the end mirrors and the beam splitter nearly equal, we mitigate problems caused by multimode behavior of the laser. There are only two critical alignment angles in a Michelson interferometer and we use a single precision mount to make these adjustments. The rest of the mounts are simple and cheap. To get good looking fringes, you need for the laser wavefront and the surfaces of the two end mirrors and the beam splitter to be flat to within a small fraction of an optical wavelength. This can be accomplished by expensive extremely high quality optics for large-diameter laser beams or, as we do, by keeping the laser beam small inside the interferometer and only expanding the beam for viewing after the interference occurs at the beam splitter. Relatively cheap optics with flatness of 4-6 waves/inch are actually very flat (lambda/4 or better) over the small size of our laser beam (typically 1mm on a side).
Here is a list of the parts you will need to demonstrate laser interference:
You will need a flat piece of particle board (or other material) on which to mount the parts. A 1-ft diameter particle-board "round" was used purchased from a local hardware store for this demo. The baseplate needs to be smooth and flat to make mounting and adjusting parts easy. We painted our baseplate with a thin coat of flat black paint for purely cosmetic reasons. (Do not spoil the flatness with a thick, crummy paint job!)
This is the light source for the interferometer. Almost
any over-the-counter laser pointer will work for this demo. For safety
reasons, make sure that the pointer is Class III-a or lower power as indicated
on the labeling. You need to invent a way to hold down the "on" button
easily. We have successfully used clothes pins, paper clips and even the
broken tine from a plastic fork, held down by tape.
|Mirror on Adjustable Mount:
The adjustable mirror mount is the most expensive, but necessary, part of the demo. It allows you to make the fine alignment adjustments needed to get a nice fringe pattern. We purchased our mirror mount from www.optosigma.com (part number 112-0250) for under $50. We purchased our two mirrors from www.edmundoptics.com. We used "first-surface" mirrors with an optical quality of 4-6 waves/inch. For reference, our mirrors were 26 mm diameter, which makes handling easy. Since the laser beam uses only a small fraction of this area, one could experiment with smaller mirrors. We used Scotch "double-stick" tape on the back of the mirror to hold it to the mount.
Aluminum Angle for Fixed Mounts:
This part can be found at most hardware stores. It is one inch aluminum angle. (Costs about $1.25 a foot.) Cut it into 1 inch pieces for mounting the other optics. Use a hacksaw or something of that nature to cut it. (Filing the edges after cutting is necessary for safety, but also will keep the optics from rocking on rough edges or cutting into the baseplate. The beauty of aluminum angle is that it is made by an extrusion process that holds pretty close to a 90-degree angle. So, be sure not to bend the metal when cutting! You need to preserve the 90 degree angle to keep the beams aligned well enough (within a degree or so). If you do bend it, you can use shims to work around poor alignment, or just cut new pieces more carefully.
Mirror on Fixed Mount:
The second of the two mirrors gets mounted to the piece cut from the aluminum angle as shown, using Scotch "double stick" tape. For the 26-mm diameter mirrors, we let about one third of the mirror overlap the mount. If you use smaller mirrors, you need to experiment a bit. The advantage of double stick tape is that mistakes are easily corrected by using a razor blade to get the mount dismounted.
| Beam Splitter on Fixed Mount:
The beam splitter splits the laser into two separate beams and also recombines the beams after they strike the mirrors. We used a 12.5x17.5 mm 50:50 beam splitter. (Beam splitters are sometimes called "two-way" mirrors.) The term 50:50 means that 50% of incident light is reflected and 50% is transmitted by the beam-splitter coating. You want a non-polarizing beam splitter. Optical quality of 4-6 waves/inch is sufficient, just as for the mirrors. Our beam splitter had an antireflection coating on the side opposite the beam-splitter coating.
|Small diverging lens:
This lens spreads the laser interference pattern out for easy viewing on a screen. We used a planoconvex (PCV) lens from www.emundoptics.com with diameter of 12 mm and a focal length of -12 mm. (The minus indicates that the lens causes parallel light rays to diverge.) Our lens had an antireflection coating on both sides.
These are used to prop the laser up to the correct height for the mirrors. We used this method because it allows nearly infinitely height variation, by adding or removing sheets. You can also use the post-its along one edge of a mount as a shim to make up for alignment difficulties in case your fixed mounts are out of whack. You will need 2 to 3 packs depending on your exact set up.
This is used to bond most of the parts onto the baseplate. It can be found at any hardware store. The directions to use it should be on the back. Get this almost anywhere. Do not use the epoxy between two flat surfaces or you will never get them apart without breakage. We position our parts until we get them in the right places and then we mix a very small amount of epoxy, place a small dab (a few cubic millimeters) on each edge of the joint and let it cure. The photos above show the epoxy bonds on one edge holding the fixed and adjustable mirror mounts to the baseplate.
How to build it:
Note: Throughout these instructions it is very helpful to take a look at the pictures to get an exact idea of what we are talking about. Take an especially good look at the "In action" photo. Click on the links provided to jump up to the appropriate picture.
After you have the baseplate, laser, mirrors and lens and you have cut 4 similar 1 inch wide pieces from your aluminum angle, then you are ready to begin. Take one of the pieces of angle and tape the piece of beam-spliting glass down the way we have shown in the picture above. Make sure that most of it is exposed over the top. Do the same with the round mirror. Then, take one of the two remaining pieces of angle and tape a piece of cardboard onto the inner wall of one side. Make sure that most of it is exposed over the top (see picture above). Then punch a hole that is a little smaller than the lens into the exposed cardboard. Then mount the lens to the hole with some double sided tape or epoxy. Make sure that the tape or epoxy barely touches the lens. Once you have all three pieces made, look closely to make sure they look the way that they do in the pictures above. Move on to step two.
Set the baseplate on a table in front of you. Measure approximately 1 inch in from a quadrant of the circle. Use double stick tape to down the adjustable mirror mount to that spot making sure that it is facing the center of the circle. Later, you will epoxy this mount down. Using double stick now allows you to make small adjustments until you get it all right. Look at the picture above and see how we mounted our mirrors. You will want to mount your mirrors in the same arrangment, using double stick tape to hold them in place. You need to make sure that both of the mirrors are about the same distance away from the beam splitter at the point where the laser light is split. We used a wooden ruler to verify the distances were the same to within 1/16 inch. The actual size of your baseplate will affect how far apart the mirrors will be.
Get your laser out and use a clip or tape a small piece of plastic or a screw over the switch to hold it "on". BE CAREFUL NOT TO SHINE THE LASER INTO ANYONE'S EYES! Set the operating laser onto the post-it notes. Remove or add post-it notes to get the laser light to shine straight at the black mirror mount. You should be able to see the laser beam reflected from the mirror, so that it almost goes back into the laser. Ideally, it should hit the laser next to where the laser beam originates.
Take the mounted beam splitter and place it in between the laser pointer and the adjustable mirror mount. You should now be able to see a spot reflected from the beam splitter. Align the beam splitter so that this reflected beam travels at roughly 90 degrees to the transmitted beam. Use a pencil to outline the mount onto the baseplate. Now place double stick tape on the bottom of the beam splitter mount and place it back down within your marks.
The double stick tape will hold the beam splitter in place while you measure the distance between the adjustable mirror and the laser spot on the beam splitter. Now place the fixed mirror mount at the same distance from the laser spot on the beam splitter. Align the fixed mount so that you can see the mirror reflect the laser light back to the beam splitter. You should be able to get this return beam to go through the beam splitter close to where it originated. Mark the outline of the mount on the baseplate, apply double stick tape and tape the mount down to the baseplate.
You will now see that the laser light from the two mirrors returns to the beam splitter so that some light is directed back toward the laser and some light goes off into space. Put up a screen to stop the beams going off into space. Adjust the adjustable mirror so that the two beams on the screen overlap. Now place the mounted lens between the beam(s) and the screen, but keep the lens close to the beam splitter. You will now see one or two big red splotches on the paper. Typically there will be two splotches, slightly misaligned. Adjust the knobs slowly on the mirror mount to line up the two splotches on the screen better and better. Once you get them very well lined up you should notice black lines running through the splotches. These are your fringes.
Now with the fringes visible, epoxy the mounts down to the baseplate using a small amount (bead) of epoxy on two opposite sides of the mount. You can see an example of one of these epoxy beads in this picture.
This "cheap" adaptation of Michelson's interferometer was developed
by Fred Raab, who thanks Andri Gretarsson and Rick Savage for helpful design
This page last modified on May 10, 2001.