**4. Discussion**

The waist region of OMCI can respond to salinity and temperature simultaneously. There is a proven solution to eliminate the effects of cross-sensitivity; that is, tracking two sensing dips and constructing a salinity and temperature sensitivity matrix, then performing inversion calculations [12,22,24], as shown in Equation (6). In addition, the two arms of the interferometer only respond to temperature; tracking the distance of two adjacent interference peaks can realize this sensitivity to temperature. In view of this, the temperature sensitivity of the interferometer can compensate for the temperature sensitivity of the OMCI's waist region; we have analyzed this feature in detail in previous work [31].

$$
\begin{bmatrix}
\Delta\lambda\_{dip1} \\
\Delta\lambda\_{dip2}
\end{bmatrix} = \begin{bmatrix}
\S\_{S1}S\_{T1} \\
\S\_{S2}S\_{T2}
\end{bmatrix} \begin{bmatrix}
\Delta S \\
\Delta T
\end{bmatrix} \tag{6}
$$

According to the above results, it shows excellent salinity sensing performance by reasonably setting the parameters of OMCI sensor. Comparing other salinity sensors, for example, the salinity sensors based on optical reflection have high sensitivity, but their prism systems are complicated and difficult to apply to harsh marine environments. The structure of the fiber grating salinity sensor is relatively simple, but its sensitivity is generally low. There is a sensitivity of around 50–200 pm/‰ with the SPR sensor, However, what matters is that the metal film should be coated effectively. As far as we know, most interference structures are complex although some of them can reach high sensitivity. As we all know, fiber optic sensors are immune to electromagnetic interference, but most of them are sensitive to temperature; in practical application, ways of eliminating the effects of cross-sensitivity should be considered. The sensor designed in this paper not only has high sensitivity, a compact structure to encapsulate it, and ease of manufacturing, but also has special advantages for temperature self-compensation.
