To find the wavelength of the laser source

• The laser beam must strike at the center of the movable mirror and should be reflected directly back into the laser aperture.
• Adjust the position of the beam splitter so that the beam is reflected to the fixed mirror.
• Adjust the angle of beam splitter to be 45 degrees. There will be two sets of bright spots on the screen, one set from the fixed mirror and another from the movable mirror.
• Adjust the angle of the beam splitter to make the two sets of spots as close together as possible.
• With the screws on the back of the adjustable mirror, adjust the mirror’s tilt until the two sets of spots on the screen coincide.
• Expand the laser beam slowly by rotating the collimating lens in front of the laser.
• Align the laser with the interferometer and make certain that the fringes are moving when the micrometer screw is turned.
• Mark a point on the screen and note the micrometer reading.
• As the screw is moved, the fringes begin to displace. Count the number of fringes N that move past the mark (either inward or outward). To avoid the effects of backlash in the micrometer screw, turn the micrometer handle one full turn before starting the count.
• Note the micrometer readings at the beginning and end of the count. Calculate the distance d' the mirror is moved, according to the beginning and ending micrometer readings. Repeat the procedure several times. Average the readings.
• With a known wavelength laser, use d = Nλ/2 to calculate the actual distance moved. The calibration constant of the interferometer is then k= d/d'. All subsequent distance measurements with the micrometer should be multiplied by the calibration constant k. Ideally, k would be exactly 1, but factors such as wear and thermal expansion can cause it to vary.
• Once the calibration constant is known, if the laser source has an unknown wavelength, it can be calculated with the same equation.