In-fiber Fabry–Perot interferometry (FOFPI) is an accurate and well-established sensing technology that is used to monitor a wide range of physical and chemical parameters, such as strain, temperature, vibration, pressure, concentration, and refractive index. They can be categorized into four types: Fabry-Perot, Mach-Zehnder, Michelson, and Sagnac. In this post we will explain the basics of interferometry and the different types of existing interferometers. The basic operation of an interferometer. Radiation absorption excites an orbital electron to a higher energy level.