DIY Optical Bench With Kinematic Mounts / Michelson Interferometer

This is a DIY optical bench that can achieve sufficient precision for an interferometer. The sand in the bottom reduces vibrations significantly. I did some informal vibrations tests in the video, hitting the side of the box quite hard, but the interference pattern only shakes briefly before it returns to a stable position.

There are two types of kinematic mounts and are made up of several pieces. They generally hold elements with a diameter of 50mm, but you can print the included adapters to hold smaller pieces (1 inch diameter) or the laser (19mm diameter) if necessary. I made one type to work with rubber bands instead of springs if you choose, which in my experience simplifies the part search and assembly.

Overall, this isn't a difficult project but it does take some patience and willingness to assemble many parts. This works best if you're comfortable enough with CAD to adjust some of the sizes to adapt to the optical components you find. You will also need Fusion to open the F3Z file with all of the CAD files. The resulting project gives a fairly complete optical setup for basic prototyping optical systems like interferometers!

Tools:

1. 3D Printer (ideally SLA, but FDM should work)
2. Wire
3. Standard screwdrivers and basic equipment
4. Fusion 360 to open the F3Z file, which you can then use to modify individual components or just generate the STL files from there.

Optical bench

1. Large box
2. Sand (enough to cover a few inches of the bottom of the box)
3. Steel plate (must fit inside the box without touching the edges)
4. Optional but recommended: Towel, Spray Paint for steel)

Kinematic Mounts

1. 1/4-20 nuts (3 per mount)
2. 1/4-20 hex bolts (3 per mount)
3. Rubber bands (3 per mount) or small springs (ideally ~4mm diameter by 10mm, but many sizes would work)
4. Magnets with 0.75in diameter
5. 3D printed parts

Optics for Interferometer

1. 2 mirrors of 50mm diameter (these can be cheap, basic mirrors or mirror pieces)
2. 1 semi-silvered mirror (I found one for about $20 online)
3. 1 laser (Nothing too powerful, I found one for $8 online)
4. Laser safety glasses (Very important that you match the wavelength to your laser! Look for OD6)
5. Projection screen (could be a wall or piece of paper)

First you need a box, some sand, and a piece of steel for the optical bench. Just put the sand in the box, gently pack it down, and then place the steel on top. I built the box out of some scrap wood, but any box will do as long as your steel can lay flat inside of it. Once everything is set up, you can tape a piece of paper to one side as a screen to help the interference pattern show up.

You might want a towel or something between it to keep the sand from getting out, and it's always a good idea to spray paint your steel with something so it doesn't rust.

You can't use aluminum or another material for this project, because we use the magnetism of the steel to connect the kinematic mounts later on. I found a piece of steel that was 18 x 24 inches for about $12, but any size works as long as it is large enough to lay out your optical systems.

This is the 'head-on' kinematic mount, designed for optical elements that need to be used with an incident angle at or near 90 degrees. This is the mount I used for the laser and mirrors for the interferometer.

Once you print the pieces, you need to assemble them. Assembly is simple and can be done with rubber bands or small springs. If you're using springs, simply connect them in the place you would put the rubber bands and use a zip tie around the lip to hold each end in place.

1. Print all of the parts and post-process.
2. Insert three 1/4 -20 nuts into the sockets on the inside of the 'lens-side' piece. You may want to use super glue to hold them in place.
3. On the mount side, put the magnet in the circular socket at the bottom. You can add glue here to lock it in place if it isn't a friction fit.
4. Insert your optical element to the lens side. You might want the screws to be on the back (non-silvered) side if it is for a mirror. This isn't necessary but makes it easier to adjust.
5. Insert the lens guard over the lens, locking the lens in place. It should be tight with a friction fit, but you might want to add some glue or tape to make sure it doesn't come out.
6. Hold the mount side and the lens side together as shown in the image. Now, take a rubber band (or small spring) and feed it through the hole that doesn't have a nut behind it. You might want to use a small screwdriver to push it through.
7. Wrap each end of the rubber band around the lips on both sides, so that it is now holding the two parts together.
8. Repeat steps 6 and 7 for the other two holes with two more rubber bands. Make sure they are the same size and type of rubber band.
9. Put the knobs on the 1/4 -20 bolts, then screw them into the remaining holes on the lens side.
10. Now just adjust the knobs to change the tip and tilt of your optical elements!

This mount is for elements that will have light entering at a lower angle, for example 45 degrees. It is used for the half-silvered mirror in the Michelson interferometer.

The lips used in the rubber band - style interferometer get in the way of the laser, so this style has to use small springs instead. Look for springs with a 4mm diameter if possible, as they need to fit through the holes.

1. Follow step 2 (above) to assemble most of the mount the same way.
2. Instead of rubber bands, we have to use springs for this mount. Put your three springs around one of the rings so that they are all well-attached.
3. Now push them through the holes so they come out the other side. If they are too short to come out the other side, make a small tool from a piece of wire that is just a straight line with a small hook on the end. Then you can reach into the hole and pull the springs through.
4. Once each spring end is through the other side, connect them to the other ring to hold them in place.
5. Follow the rest of the kinematic mount steps!

If not all of your optical elements have a diameter of 50mm, don't worry! You can easily make adapters for any size below 50mm. I've included some files for adapters that I used for the interferometer, but you may need to check the sizes of your optical components.

There are adapters for a laser mount, a 'slider' to translate the whole mount in one axis, and a smaller lens adapter that works with a 1 inch diameter lens (the size of my semisilvered mirror). These should be inside the F3Z file that is attached to the Instructable.

Simply print out the adapters, insert the optical element, and close the other side with friction or glue. Now you have a simulated 50mm diameter!

Once you have three head-on mounts and one 45 mount, you're ready to build the interferometer. Make sure your optical elements and laser are all securely in their mounts, and then place them on your optical bench. the magnets should help them keep in place, and then you can fine-tune the laser light using the knobs on the mounts.

Set up the Michelson interferometer according to the diagram, and check out the interference pattern!

Make sure you're using laser safety glasses that are rated for the proper laser wavelength you have. When dealing with mirrors and DIY optical holders, it's always better to be safe.