Procedure

4.1 Alignment

1. Mount the diode laser in the light source holder. Be sure that the white pads on the laser (thermoelectric coolers) are in contact with the mount structure.
2. Use the toggle switch on the control box to turn on the diode laser. Pull the toggle switch out, then switch it to the appropriate light source.
3. There are two mirrors between the light source and the beam splitter of the interferometer. You’ll use these two mirrors to do most of the alignment. Each mirror IN the interferometer can make small adjustments; one mirror can adjust vertically, the other horizontally.
4. There are two white paddles that can be mounted to the beam splitter. Use the two mirrors to align the interferometer beams through the center hole of these paddles. Adjust the mirrors IN the interferometer if needed. Be sure that the beam is overlapping near-perfectly in the interferometer.
5. Look at the output of your interferometer. You should see either two beams very close together, or see one beam with some striations. Make adjustments to the mirrors IN the interferometer to get the beams overlapped and to see only a few striations – ideally just one or two.
6. Turn on the motor that moves one of the interferometer mirrors. You should see the striation pattern sweep through the image of the output beam.

4.2 Electronic Detection

1. Mount the detector so that it can monitor the output of the interferometer. With some adjustments, you should be able to see your detection signal on the oscilloscope. Check that your detector is working by blocking the laser beam { you should see the signal on the oscilloscope drastically change.
2. Turn on the motor that moves one of the interferometer mirrors. The signal on the oscilloscope should change sinusoidally as the intensity of the output beam of the interferometer changes with the constructive and destructive in the interferometer.
3. Use the DC offset knob to compensate for the DC portion of the detector signal. The signal on the scope should oscillate about zero volts. You should notice a square wave showing up on the other channel of the oscilloscope. This square wave is the output of the comparator in the control box.
4. Along with the square wave, you should see the counter on the control box start to tick upward with each square wave.
5. Now would be a great time to explore the low-pass filter settings on the control box. Set the low-pass filter to a very short timescale and tap the table. Note the increase in number of counts from the comparator. Change the low-pass filter to a higher time constant and tap again. Repeat until your taps or foot stomps don’t have a strong effect, but that the oscillation from the changing arm-length of the interferometer is still in tact.

4.3 Taking Data

1. Reset the counts on the control box to zero and note the position of the caliper. Start the motor. Ideally, you’ll measure at least 100 counts before stopping the motor. Then note the new position of the caliper.
2. Repeat the previous step at least four times for increasing number of counts.  The current record is written on the whiteboard beside the apparatus.  You will be fitting this data to determine the wavelength of the light source, so be sure to take an appropriate amount of data.

4.4 Helium Neon Laser

1. Turn off the diode laser and remove it from the light source mount. Install the Helium-Neon laser.
2. You’ll have to make some small adjustments to re-align the interferometer, but hopefully not as much as with the first light source.
3. Take measurements for a series of total lengths (and therefore total counts) as you did with the diode laser.  You will be fitting this data, too.

4.5 Thermal Expansion

1. Unscrew the bolts holding the motor and remove it – thermal expansion will now be the cause of the change.
2. Move the micrometer away from the stage so that it is roughly where the motor was.  Unwind the micrometer so that it’s near halfway of its range.
3. Insert one of the samples between the rods from the micrometer and the stage.  Wind up the micrometer so that the sample is held firmly between the micrometer and the stage.
4. Plug the thermocouple connector into the thermocouple reader.  Check that it really is near room temperature.
5. Connect the leads of the resistive heater to the positive and negative terminals of the power supply (it doesn’t matter which way you connect those)
6. Double check the alignment of your interferometer and make sure you still see fringes. Turn the micrometer a little and make sure the pattern changes.
7. Record the current temperature of the sample, then turn the current on the power supply up to 0.5 A.
8. The signal on the oscilloscope should be slowly oscillating about zero and generating counts on the control box.  If not, make some quick adjustments to the DC offset to center the signal around zero volts.
9. Heat the sample by 10-15 degrees, but keep the maximum temperature below 40 degrees.  Be sure to record the initial and final temperatures and the number of wavelengths that have been traversed.
9. Take data for all three samples:  copper, invar (36% nickel, 63% iron), and alumina (Al₂O₃)