**5. Conclusions**

In summary, a novel seawater salinity sensor based on the structure of optical microfiber coupler interferometer is proposed. The results show that the output spectra of OMCI red-shift as salinity increases, and the salinity response sensitivity reaches 303.7 pm/‰. The experiment is consistent with the theoretical analysis. In addition, we discussed the relationship between the OMCI arm difference and the wavelength drift error. When the arm difference is small, the interference peak of the output spectrum is relatively sparse. Moreover, there is an error between the dip wavelength in the output spectrum and the dip wavelength in the envelope (0 arm difference), and the smaller the arm difference, the greater the error. At the same time, when the sensitivity is low (that is, wavelength drift is much smaller than the interference peak spacing), the error is more obvious. Therefore, this paper proposes three improvement suggestions: firstly, controlling the OMCI arm difference to zero; secondly, controlling the OMCI arm difference to 5–30 mm; third, reducing the OM radius *r* to improve the sensor's signal-to-noise ratio. In practical applications, the latter two schemes are easier to implement. This OMCI salinity sensor meets the needs of salinity testing in most sea areas. It provides good research ideas and alternative technical solutions for the development of practical optical fiber salinity sensors.

**Author Contributions:** Conceptualization, L.Z. and Y.Y.; Validation, L.Z.; Data curation, Y.T. and G.L.; Writing—original draft, L.Z.; Writing—review and editing, K.W., Y.Y., H.H. and J.Y. Project administration, Z.Z.; Supervision, Y.Y.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Natural Science Foundation of China (Nos. 61805278, 61605249, and 61661004), Guangxi Science Foundation (Nos. 2017GXNSF and AA198227), the Equipment Pre-Research Field Foundation (61404140304), the China Postdoctoral Science Foundation (2018M633704) and Innovation Project of Guangxi Graduate Education (YCSW2020021)

**Acknowledgments:** The authors would like to thank the support of the laboratory and university.

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