**5. Conclusions**

In this paper, a high-sensitivity MZI was designed and fabricated. The single mode fiber and the multimode fiber were directly spliced by a commercial fusion splicer to form a coupling point. The multimode fiber was in the middle of the sensing arm, and a waist-enlarged bitaper was fused in the middle of the sensing arm, used to realize beam adjustment. Finally, the cladding of the sensing arm was etched with hydrofluoric acid to reduce its diameter, which in turn achieved improved response sensitivity. Fourier transform analysis shows that both the waist-enlarged bitaper and erosion excite higherorder modes, resulting in a larger optical path difference for light transmitted in the optical fiber core and cladding. The refractive index sensitivity responses of these three sensors were experimentally investigated, and the refractive index sensitivity of SMS MZI is 57.623 nm/RIU, with a linearity of 0.9795; the refractive index sensitivity of SMBMS MZI is 61.607 nm/RIU, with a linearity of 0.9545; and the refractive index sensitivity of ESMBMS MZI is 287.65 nm/RIU, with a linearity of 0.9843. It is shown that ESMBMS MZI has a very high refractive index sensitivity and good linearity. As the new ESMBMS MZI, designed in this paper, possesses the advantages of high sensitivity, good linearity, low cost, and simple fabrication, it has a high practical value and good application prospects for future production.

**Author Contributions:** Conceptualization, Q.L.; data curation, N.Z., Z.W. and F.Z.; formal analysis, Z.Z.; resources, L.Z., B.T. and Z.J.; software, K.Y. and Y.J.; writing—original draft, N.Z.; writing review & editing, Q.L. and P.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Postdoctoral Innovative Talent Support Program (Grant No. BX20200274), the National Natural Science Foundation of China (Grant Nos. 51720105016, 52105560), the Post-doctoral Science Foundation (Grant No. 2021M702596), Shaanxi Province Surface Project (Grant No. S2022-JC-YB-1996), and the National Science and Technology Major Project (Grant No. J2019-V-0006-0100).

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