The Mach–Zehnder interferometer is a device used to determine the relative phase shift

Phase (waves)

Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...

 between two collimated beams from a coherent

Coherence (physics)

In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....

 light source. The interferometer has been used, amongst other things, to measure small phase shifts in one of the two beams caused by a small sample or the change in length of one of the paths. The apparatus is named after the physicists Ludwig Mach (the son of Ernst Mach

Ernst Mach

Ernst Mach was an Austrian physicist and philosopher, noted for his contributions to physics such as the Mach number and the study of shock waves...

) and Ludwig Zehnder

Ludwig Zehnder

Ludwig Louis Albert Zehnder , Swiss physicist, inventor of an interferometer ....

.

The Michelson interferometer

Michelson interferometer

The Michelson interferometer is the most common configuration for optical interferometry and was invented by Albert Abraham Michelson. An interference pattern is produced by splitting a beam of light into two paths, bouncing the beams back and recombining them...

 is a Mach–Zehnder interferometer that has been folded back upon itself.