This article explains the principle of Fiber Bragg Grating (FBG) sensors based on the fundamental concept of "reflection and interference of light waves," including the principles of temperature measurement, stress measurement, and strain measurement using FBGs. It is known that the index variation along the major axis of the fiber can induce the coupling of counter-propagating modes at the Bragg wavelength (. Optical fiber sensors (OFS) appeared just after the invention of the practical optical fiber by Corning Glass Works in 1970, now Corning Incorporated, that produced the first fiber with losses below 20 dB/km.