Blog post: swansont: This Isn't Your Grandfather's Michelson Interferometer

In its simplest form, a Michelson interferometer is used with monochromatic light. However, this limits its effectiveness because, before a measurement is made, the length of the distance to be measured must be known to within one half of the wavelength (?/2) of the light used – typically less than 500 nm. The problem is that the distance being measured can be expressed as an integer multiple of ?/2 plus a fraction of ?/2 – but this integer multiple cannot be determined from the interference data.

Physicists have found two ways round this problem. One is to use several lasers at different colours to gain more information about the system. The other is to use a light source with a range of wavelengths and then look for phase differences in the interfering light, which can be related to distance. These techniques have their own problems, not least that thousands of different lasers would ease a measurement but installing them all in a lab would be impractical.