Interferometer - How We Measure Tiny Physical Changes

A Fiber Optic Interferometer is a highly coherent laser light that's launched into a single mode optical fiber and then split equally into two paths to form the two legs of an interferometer.
One leg of the fiber is wound around a piezoelectric cylinder while the other leg lies on the table. A voltage is applied to the piezoelectric cylinder to excite it into radial vibrations.
The light passing through the leg on the cylinder travels different distances as the cylinder vibrates. When the light from two legs is recombined, the intensity of the combined light measured by a photodetector changes rapidly.
Very small path length differences are detected by constructive and destructive interference as the light in the two paths changes phase relative to each other.