Michelson interferometer

The Michelson interferometer is the classic setup for optical interferometry and was invented by Albert Abraham Michelson and used for the famous Michelson-Morley experiment. See there for a detailed discussion of its principle.

But Michelson had already used it for other purposes of interferometry, and it still has applications, e.g. for the detection of gravitational waves. But for most purposes, the geometry of the Mach-Zehnder interferometer is more useful.