By Patten Priestley and Andrew Schoewe
We studied the characteristics of a HeNe laser and a sodium lamp through a process called interferometry. The motivation behind our experiment was to understand the Michelson-Morley interferometer, learn about optics, and differentiate interference patterns of different light sources.
An interferometer is a device used to measure the interference phenomena between two waves. The most important aspect of an interferometer is the path that the beam travels. It begins at the light source and is split into two waves by an angled partially silvered mirror. One part of the beam (red) travels to a mirror that can move back and forth, while the other (blue) travels to a stationary mirror. The path the red beam travels is equal to twice the distance between the partially silvered mirror and the movable mirror because the beam hits the movable mirror after traveling L1 and then returns to the partially silvered mirror after traveling L1. (Similar logic shows that path 2 is equal to twice L2.)
When the beams are reflected back to the partially silvered mirror, they recombine and are sent to the detector. This detector senses whether or not the beams interfere destructively or constructively. Complete destructive interference occurs when path 1 = path 2 +/- (n+.5)*wavelength of source light. This implies that the blue wave is completely out of phase with the red wave. Complete constructive interference occurs when path 1 = path 2 +/- (n)*wavelength of source light. This implies that the blue wave is completely in phase with the red wave.
The hardest part of our experiment was trying to distinguish a clear fringe pattern. The following is a picture of a fringe pattern of a HeNe laser. In the very center is destructive interference, surrounded by a bright red fringe of constructive interference, and so on. The fringe pattern is due to the different parts of the beam traveling different distances. In other words, the combined wave in the middle of the pattern traveled a different distance from the combined wave further away from the middle.
To perform this experiment we changed the length of path 1 by moving the movable mirror at a constant rate using a step motor (see setup page). We used a photo multiplier connected to a high voltage source and an ammeter to calculate the intensity of the beam that was shown on the detector. Once we calibrated the rate at which the mirror moved, we could find the experimental wavelength of the light source by looking at the intensity over distance. We could then do an FFT of this data and find how many frequencies were emitted by the light source. We could also look at the beat pattern of the light to determine the distance between the multiple wavelengths and the average wavelength of the light.