Some sensors measure at a defined point, others detect several discrete measuring points along a fiber, while so-called distributed methods enable a continuous profile over many kilometers. These are based on established optical principles such as interference . A fiber-optic sensor is a sensor that uses optical fiber either as the sensing element ("intrinsic sensors"), or as a means of relaying signals from a remote sensor to the electronics that process the signals ("extrinsic sensors"). Fibers have many uses in remote sensing. The distributed type uses technology making the entire optical fiber function as a sensor. It includes OTDR, which measures the presence and location of. Radiation absorption excites an orbital electron to a higher energy level. Heating the material enables the trapped states to interact with phonons and decay into lower-energy. A fiber optic sensor measures a physical quantity by modulating the intensity, spectrum, phase, or polarization of light traveling through the optical fiber system. Optical signals are transmitted through a glass fiber. If external influences such as temperature, strain, pressure, or vibration change along the fiber or at its end, the measurable properties of the.