**5. Conclusions**

A new design of an ultrasensitive Sagnac-interferometer-based optical fiber temperature sensor was theoretically and numerically proposed here. Using the finite element method, the modal birefringence of the fiber under different ambient temperatures and the birefringence sensitivity of the

fiber were numerically investigated while one of the airholes was infiltrated with liquid chloroform. A large birefringence sensitivity of almost 29% to a 10 ◦C temperature variation was reported for the optimized fiber design. The performance of the proposed optical fiber in a temperature sensor Sagnac interferometer is studied in detail, and an average linear temperature sensitivity of 17.53 nm/ ◦C with an average resolution of 5.7 × 10−<sup>4</sup> ◦C was achieved over a temperature range of 20 ◦C (15 ◦C to 45 ◦C).

**Author Contributions:** Conceptualization, Y.E.M.; Methodology, Y.E.M. and C.L.; Software, A.A.; Validation, V.D.; Formal Analysis, Y.E.M. and A.A.; Data Curation, Y.E.M. and V.D.; Writing-Original Draft Preparation, Y.E.M.; Writing-Review & Editing, Y.E.M and A.A. and C.L.; Visualization, Y.E.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** Y.E.M. acknowledges the support of Dalhousie University for providing the resources to perform this research, and C.L. acknowledges the support of the National Natural Science Foundation of China and the National Key Research and Development Program of China.

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