Speckle Interferometer Setup
Optical Hardware
1x HeNe Laser
5x mirrors
1x plate beam splitter
1x non-polarizing beam splitter
1x neutral density filter
2x microscope objectives (magnifications may vary depending on your exact setup)
1x ground glass diffuser
2x irises
Setup
7) Place a beams splitter immediately after the ground glass. The advantage of having the splitter close to the ground glass is that it will increase the area of the ground glass in any images you take, which leads to you being able to see more fringes. Moving the ground glass further from the splitter so that the glass is within the depth of field of your camera (which you haven't placed yet) does not improve your results.
8) Measure the optical path length of the reference beam (from beam splitter to beam splitter). It is important that you know this path length to within roughly plus/minus 2cm.
Figure 2: A photo of the reference arm (excluding the ground glass).
The numbers next to the pieces of optical hardware indicate which
step of the "Reference Beam" section they correspond to.
6) Place a piece of ground glass diffuser such that its entire diameter is illuminated by the beam after it passes through the microscope objective (see Figure 3). With a 20x magnification objective, I placed the ground glass 38cm from the objective, though this distance will vary depending on what magnification you use. Note that the grit of the ground glass is less important than the diameter. Selecting as large of a ground glass diameter as possible will allow you to look for interference fringes over a larger area when all is said and done. Do not worry if the microscope objective appears dirty when illuminating the ground glass; this will have no effect on your results because the interferometer only cares about things that change from one picture to the next.
5) Place a microscope objective after the third mirror. The objective used in Figure 2 has a magnification of 20x, though an objective with any other magnification could be used instead, depending on what you have available. Again, the exact distance from the filter to the objective is irrelevant, but you will see shortly that it is wise to place the objective as close to the filter as you can.
4) Insert the neutral density filter. The exact distance from the third mirror to the filter is unimportant. However, in the interest of efficiently using the space on your table, you will likely want to put the filter as close to the third mirror as possible. This filter is important because it will allow you to adjust the intensity of the reference arm later. When setting up the interferometer, it is easiest to leave the beam unattenuated.
3) After the intersection point between the reference beam and the original beam, place a third mirror and ensure the reflected beam is parallel to the table. The distance between the second and third mirrors in Figure 2 is roughly 40cm.
2) Place a second mirror to reflect that same beam by 90 degrees yet again. In Figure 2, this mirror is 13cm from the first. Walk the mirrors. Again, it is important the the beam stays parallel to the table.
Figure 1: A bird's eye view of the setup. The red line denotes the path of the laser beam.
This interferometer design at first glance does not appear terribly difficult to set up. However, in practice it is quite a balancing act between many factors (speckle size, optical path length, magnification, etc). Following this guide should produce a setup capable of observing interference fringes. Though, you may later experiment with different microscope objectives, ground glass, camera settings, and optical hardware positions until you find a combination that you like even more than what will be presented in this guide. This design was found in the paper, "A simple design for an electronic speckle pattern interferometer", by Thomas Moore of Rollins College. A PDF of this paper is attached to this webpage.
First Steps
1) With the HeNe in place (top left of Figure 1), place a mirror some distance from the laser such that the beam reflects at a 90 degree angle. Use two irises to make certain that the beam is parallel to the table.
2) Place the non-polarizing beam splitter after the mirror and orient it so that 50% of the beam passes straight through without changing direction and the other 50% is perpendicular to the original beam. In Figure 1, the distance from the mirror to the beam splitter is roughly 23cm.
Reference Beam
1) Place a mirror so that the reference beam (the one that is made perpendicular to the original beam by the splitter) is reflected at a 90 degree angle back towards the laser. In Figure 2, this mirror is placed approximately 33cm from the beam splitter.
Figure 3: A photo of the microscope objective, ground glass, and
beam splitter in the reference arm. The numbers next to the
pieces of optical hardware indicate which step of the "Reference
Beam" section they correspond to.
Image Beam
1) When putting together the image arm, the most important thing to remember is that the optical path lengths for both the reference and image beams need to be the same. Start by placing a mirror some distance from the non-polarizing beam splitter (see Figure 4). Take note of the distance from the beam splitter to the mirror. In my setup, this distance is ~64.5cm.
2) Place the metal plate (or whatever object you are observing) in front of the second beam splitter and far enough away so that the path lengths of the image and reference arm are identical. This distance in my setup is roughly 42.5cm. Placing posts on both the left and right sides of the plate will help you bend it later. TIP: when dogging down the posts on either side of the plate, it is helpful to position the table clamps right next to the sides of the plate to restrict the plate's movement.
3) Insert a microscope objective after the mirror placed in step 1) to diverge the beam. The more divergent the beam, the larger the area you will be able to see with the interferometer. If you are not satisfied with how divergent the beam is even with the strongest objective available to you, you can add a diverging lens after the objective. For this step, I used a 40x magnification objective and a diverging lens. These two combined were able to illuminate most of an 8in x 8in metal plate. Again, do not worry if the objective is dirty.
Figure 4: The image arm. The numbers next to the
pieces of optical hardware indicate which step of the
"Image Beam" section they correspond to.
Translation Stage
The translation stage is only needed if you plan on observing the interference patterns produced by bending a metal plate (which is recommended because it is a good test of whether or not your setup works).
1) Screw the translation stage down to the table.
2) Put together a series of posts so that the end of one post can point directly into the plate (see Figure 5). Put a screw into the end of the post that points into the plate. Mount these posts to the top of the translation stage so that the screw the horizontal post lines up with the slit down the middle of the plate.
3) You will want to find the position of the translation stage when the post first makes contact with the plate. Turning the knob on the translation stage so that it moves forward toward the plate, wiggle the plate. When you are no longer able to wiggle it, the post has made contact with the plate and you can stop turning the knob.
Figure 5: The translation stage.
Camera
1) Place the camera behind the second beam splitter so that it sees the speckle patterns from both the ground glass and metal plate completely overlapping.
2) Focus the camera on the metal plate.
3) Three of the most important camera settings (aside from focus) for this interferometer are the f-number, exposure time, and zoom. The settings I used are: 5 for the f-number, 0.5s exposure time, and the camera all the way zoomed out.
Testing the Setup
To test your setup, do the following:
1) Adjust the neutral density filter so that the intensity of the reference beam and image beam are the same. An easy way to do this is to look at the beam splitter from the opposite side of the ground glass and repeatedly block either the reference beam or the image beam (this is much quicker than taking photos).
2) Use Adobe Lightroom to take a photo, via tethered capture, of the plate without any deformation due to the translation stage. Photos are best taken with the room lights off and the door closed, but you should still get decent results even with the lights on.
3) Turn the knob on the stage so that it increases by 1 tick, which corresponds to the stage moving forward by ~2.5 microns. Take another photo.
4) Turn both photos into black & white and export them as TIFF files.
5) Use ImageJ to subtract the two photos. If everything is done correctly, you should see interference fringes.
6) If your fringes have low contrast (or if you could not see any fringes at all) try the following to improve your results:
a) Make sure the reference and image beams are parallel to the table.
b) Make sure the optical path lengths are the same in both arms. If the two differ by more than ~2cm, your results will have a reduction in quality.
c) Adjust the angles of the ground glass and the beam splitter in front of the camera.
d) Adjust the neutral density filter. If the reference arm beam is either too intense or not intense enough, you will find low contrast interference patterns.
Example Results
Figure 6: The interference pattern produced when a metal plate is bent by 0.5 ticks on the translation stage (~1.27 microns).
Figure 7: The interference pattern produced when a metal plate is bent by 1 tick on the translation stage (~2.54 microns).
Figure 8: The interference pattern produce when a metal plate is bent by 2 ticks on the translation stage (~5.08 microns).