**4. Conclusions**

With this work, we present a new, precise and cost-effective approach to read-out FBG utilizing a novel AWG interrogation system. The polymeric AWG are simple and cost-efficient to produce by using direct laser lithography, a technique that allows rapid development of integrated optical systems and enables short times from simulation to an operational prototype. For the application as a status monitoring device of lithium-ion batteries, the designed AWG interrogation system shows good optical performance. With a comparison to an optical spectrum analyzer, we calculated the approximate accuracy of our new measurement system to ±7.59 × 10−<sup>2</sup> nm. The resolution is defined by the S-functions to 1 × 10−<sup>3</sup> nm. The obtained accuracy is sufficient to observe the typical strain behavior (e.g., as in ref. [25,26]) of a single cell during one full charge cycle and was maintained for 25 cycles over one month. Nevertheless, the influence of the relative air humidity is not negligible and has to be investigated further. Although we are able to minimize the error with simple methods, the systems robustness has to be improved in order to become competitive to state-of-the-art electrical BMS. Besides this, future work will focus on AWG with at least 16 output channels to distinguish up to 8 FBG and to enable the usage of a reference FBG for temperature compensation in order to monitor a battery cell under non standardized conditions and extend the observations to a multitude of cells. The herein presented interrogation unit can furthermore be used to evaluate a multitude of FBG by using standard optical accessories e.g., an optical switch, or by using a multilayer design with several stacked AWG in combination with a two-dimensional CMOS image sensor to evaluate the individual output channel intensities. In future studies, the acquired optical information will be investigated in more detail for use in advanced algorithms that are able to exactly determine the SOC and SOH of a battery system. Furthermore, it is desirable to develop an all polymeric sensor system that consists not only of a polymeric AWG, but also takes advantage of other polymeric components, like a polymeric coupler and FBG.

**Author Contributions:** For the presented research work the authors are contributed in the following manner: J.M., A.N. and W.S. are responsible for the conceptualization and funding acquisition; J.M. and A.N. are responsible for data curation, investigation, methodology, visualization and writing; J.M. is responsible for formal analysis and software; A.N. and C.K. are responsible for project administration; E.P., C.K., A.N. and J.M. are responsible for resources; C.K. is responsible for validation; C.K., A.N. and J.M. are responsible for writing—review and editing; W.S is responsible for supervision.

**Funding:** This research was funded by the German Federal Ministry for Economic Affairs and Energy, gran<sup>t</sup> number 03ET6105A.

**Acknowledgments:** Financial support for the conduction of the experiments from the Federal Ministry for Economic Affairs and Energy is gratefully acknowledged.

**Conflicts of Interest:** The authors declare no conflict of interest.
