The Michelson Interferometer

The aim of this practical is to make the students familiar with the simplest type of interferometers, one in which only two (plane or spherical) waves interfere. Interferences are sensitive to differences in the phase of waves, which varies over very small distances (less than one micrometer for visible light), corresponding to very short times (period of about 2 fs - a femtosecond is 10^-15 s). Therefore interferometry is an extremely sensitive method, used to detect and measure small deformations, displacements, etc.

Besides its historical importance in the origin of special relativity, the Michelson interferometer is a very convenient tool, featuring large free spaces in its two arms, between the beam-splitter and the mirrors. Using a Michelson interferometer, you can detect minute changes, for example in the temperature or composition of the atmosphere in one of the arms, the other one serving as a reference. A two-wave interferometer has many other possible uses. A very important one is as a Fourier-transform spectrometer, to analyze the spectrum of a light source, or the absorption spectrum of a sample.

The Michelson interferometer of this practical is built from standard optical and optomechanical components on an optical breadboard (60 cm x 90 cm). There are no "black boxes" involved; all parts are accessible and their function can be observed and understood directly. The interferometer provides an introduction to the elementary tools which are present in every optics laboratory, and to the basic tricks used in optical alignment.

The first part of the practical is aimed at giving the students a "feeling" for the sensitivity of a Michelson interferometer and the different types of interference patterns which can be observed (planar waves, spherical waves, monochromatic light, white light). That way a direct understanding of important concepts in wave optics, such as the coherence length of a light source or the quality of a wavefront, can be gained in an almost playful manner. The precision of the interferometer optics is high enough to allow white light interferometry and the observation of color fringes.

The second part of the practical is centered on Fourier transform spectroscopy. The students build a fully functional Fourier transform spectrometer based on the Michelson interferometer: A light source is coupled into the interferometer and its interference signal is monitored while the length of one arm of the interferometer is varied using a motorized translation stage. The emission spectrum of the light source can then be obtained from a Fourier transformation of the interference signal. The fringes of a helium-neon laser (recorded simultaneously) are used to calibrate the data and correct for the non-ideal behavior of the motorized stage. An example of the accuracy and resolution which is achievable can be seen below in the emission spectrum of a sodium lamp, as recorded with the scanning Michelson interferometer:

This was of course only a short overview of the 'Michelson Interferometer' student experiment; a detailed description can be found in the manual (PDF, 274 KByte). For questions related to the organization of the second year lab practical, please refer to the physics education pages of our institute. The Michelson experiment is currently supervised by Ted Xia.

If you have further questions or suggestions for improvement, you can also contact Florian Kulzer or Michel Orrit.