**5. Conclusions**

In conclusion, we proposed an in-fiber Mach–Zehnder temperature sensor with a structure of SMF–MMF–DCF–MMF–SMF. The sensor operating principle is based on the interference of the two propagating modes in the dual cores. The eccentric core serves as a reference arm, while the central core adjacent to the fluidic channel filled with glycerol– water solution works as a sensing arm. Thus, the interfered transmitted spectrum of the sensor shifts as a function of temperature. Both experiments and numerical simulations were carried out to characterize the temperature sensor. For the 20-cm-long DCF infiltrated with 15-cm-long glycerol–water solution, the experimental sensitivity of 2.77 nm/◦C was obtained in the range of 25 to 40 ◦C. For the 36.5-cm-long DCF infiltrated with 29.5-cm-long solution, a higher sensitivity of 5.69 nm/◦C was achieved in the range of 25 to 40 ◦C. The experimental results coincided well with the simulated counterparts. The proposed sensor has good robustness and stability. We believe that this sensing regime may be potentially used for high precision temperature measurements as well as detection of other parameters, such as liquid or gas refractive index or magnetic field, thanks to the presence of the fluidic channel.

**Author Contributions:** Conceptualization, X.H. and H.Q. (Hang Qu); methodology, H.Q. (Haiming Qiu) and C.Z.; investigation, H.Q. (Haiming Qiu), C.Z., H.C., and Q.Y.; writing—original draft preparation, H.Q. (Hang Qu) and X.H.; writing—review and editing, H.Q. (Hang Qu) and Z.L.; supervision, X.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project is supported by the Specialized Project Fund in Science and Technology of Guangdong Province (no. 190827105565385 and 190827105545434), Special projects in key fields of colleges and universities in Guangdong Province (no. 2020ZDZX3035 and 2020ZDZX3037), the Startup fund (no. NTF19023 and NTF18016) from Shantou University, and the Optics and Photoelectronics Project (no. 2018KCXTD011).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data sharing is not applicable.

**Conflicts of Interest:** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
