**3. Results and Discussion**

To demonstrate the performance of the new polymeric AWG interrogator, long-term experiments with the presented FBG sensor were conducted. The optical sensor, in this particular case, serves predominately as a strain sensor since it is fixed to the surface of a lithium-ion pouch cell by means of instant adhesive. Although, FBG sensors are sensitive to both temperature and strain, the reader should note that for the cyclization experiments presented in this paper, the shift in reflected wavelength is almost exclusively due to strain because the current rate is low and the resulting heat generation of the cell minimal. Furthermore, the cell is placed in a temperature regulated environment, first to investigate the shift of the reflected wavelength due to changes of the temperature (part A) and

second to ensure a constant temperature during the cycling experiments (part B). The AWG interrogator is placed along with the light source and the coupler in a separate temperature chamber that is set to a constant temperature of 16 ◦C. The intensity signals transmitted from the microcontroller are processed by a personal computer in the analysis software in which the previously obtained relationship between intensities and FBG wavelength is calculated, displayed and finally stored. The overall experimental setup is schematically shown in Figure 4.

**Figure 4.** General setup for the temperature (part A) and cyclic experiments (part B). The AWG interrogator, along with the superluminescent diode (SLED) light source, and the fiber-coupler are put into a temperature chamber with a constant temperature of 16 ◦C and a constant relative humidity of 43% (only for part B). The investigated lithium-ion cell is put in a temperature chamber with a variable temperature for experimental part A and a constant temperature of 20 ◦C for experimental part B, respectively. The cell is electrically connected to a battery cyclization unit and optically connected to the AWG as well as to the optical spectrum analyzer (OSA). The AWG evaluates the FBG wavelength through the reflected spectrum and the OSA evaluates it through the transmitted spectrum.
