A fiber optic sensor is a sensing device that uses fiber optic technology for measuring physical quantities such as temperature, pressure, strain, voltages and acceleration, to name a few. In particular, it uses an optical fiber as the sensing element, called an intrinsic sensor, or uses it to transport signals from the remote sensor to the signal processing module (extrinsic sensor). Fiber optic sensors are immune to electromagnetic interference and can handle extreme conditions, so they are gaining popularity as the sensor of choice for many industries.
Techopedia explains Fiber Optic Sensor
Sensors use optical fibers in different ways, depending on the application of the former. Primarily, an optical fiber is used because of its small size or because it has little to no power requirement at the remote location. Another reason, however, is that many sensors can be multiplexed along the length of the fiber by using a different light wavelength shift for each sensor or by sensing the time it takes for light to travel along the fiber through each different sensor.
If wavelength shift is used, this can be sensed and calculated with the use of an instrument that implements optical frequency domain reflectometry. Sensing for time delay, on the other hand, requires a device that uses an optical time-domain reflectometer.
To create a temperature sensor, an optical fiber with an evanescent loss property that varies along with temperature is used or the Raman scattering produced in the fiber by the temperature is analyzed. To measure electrical voltages, nonlinear optical effects are sensed in specially doped fiber that alters its light polarization as a function of the electrical field.