**2. Optical fiber sensors: theoretical approaches**

Manfred Börner, a German physicist, developed, in 1965, the first fiber optic patent related to a working fiber-optic data transmission system [60, 61]. Years later, in 1978, the concept of wavelength division multiplexing, where several optic signal chargers are multiplexed into a single optical fiber through different wavelengths was firstly published [62]. Since then, the optical fiber community has expanded and the use of optical fibers as sensing elements attracted a lot of attention. **Figure 3** summarizes the different types of fiber optic sensors developed in the last years [63]. Regarding the monitorization of physical and chemical parameters in LIBs, just some of them were already tested. The fiber Bragg grating sensors (FBG) and tilted FBG sensors (TFBGs) were used to track temperature, strain, refractive index, and SoX, inside and outside of batteries, correlating these signals with electrochemical events during their operation. From the interferometric sensors, Fabry-Perot interferometric (FPI) and Mach-Zehnder interferometric (MZI) sensors were tested to monitor and decouple temperature, strain, and SoC signals. Evanescent wave sensors based on surface plasmon resonance and evanescent field fluorescence were also already used to monitor temperature shifts and SoC values in batteries. OFS based on Rayleigh scattering distributed sensing was also already used. However, due to their instrumental complexity, elevated interrogation costs, and low experimental use relative to the other methods, this type of sensor will not be approached in this chapter.
