## 3 Sensing Characteristics and Demodulation
### Temperature Self-Calibration and Demodulation Optimization  
It can be seen from the foregoing analysis that OMCI is a hybrid device. After the optimized design, both the OMC and the interferometer part can be used as a sensing unit to respond to change of ambient temperature. For the majority of fiber optic sensors, there is a cross-sensitive problem (the same sensor can be sensitive to different parameters). How to realize cross-sensitive demodulation becomes an urgent problem to be solved in the practical design of optical fiber sensors. It can be seen from the research results [15] that OMC can respond to changes in seawater temperature, salinity, and depth at the same time. To achieve the synchronous and accurate demodulation of these three parameters, the inversion operation of the sensitivity matrix is needed to be solved. That is, getting three or more characteristic wavelengths (peak and dip) in the output spectrum by optimizing the design of the OMC structure, then tracking the three characteristic wavelengths to calibrate the response sensitivity of salinity, temperature, and pressure, finally, the cross sensitivity matrix is obtained:  
\[\left[\begin{array}{c}\Delta\lambda_{dip1}\\ \Delta\lambda_{dip2}\\ \Delta\lambda_{dip3}\end{array}\right]=\left[\begin{array}{c}S_{S1}S_{T1}S_ {P1}\\ S_{S2}S_{T2}S_{P2}\\ S_{S3}S_{T3}S_{P3}\end{array}\right]\left[\begin{array}{c}\Delta S\\ \Delta T\\ \Delta P\end{array}\right]. \tag{6}\]  
It can be seen from the above decoupling scheme of cross-sensitivity that the key to realizing high precision three-parameter demodulation lies in the construction of the sensitivity matrix database, and realization of three dimensional matrix high-precision operations. In practical applications, there are problems such as difficulty in calibration, and the large amount of calculation data.  
The OMCI designed and manufactured in this article is provided with two temperature sensing response units: One is waist region, and the other is two arms of interferometer. If the two arms of OMCI are sealed in a thermally conductive metal cabin, and waist region is placed in a seawater compartment, the two arms of interferometer will only respond to changes of seawater temperature, while waist region can simultaneously respond to changes of seawater temperature, salinity, and depth. The foregoing analysis shows that the temperature effect of the OMCI interferometer can be achieved  
Figure 5: (**a**) Detail of the improved sensor output spectra. (**b**) Relationship between interference peak distance and temperature after improved device.  
by tracking the interference peak distance d, and d is only controlled by the phase difference of the interferometer (only be adjusted to the optical path difference of the two arms of the interferometer). Therefore, by making full use of the temperature response capability of the two arms of interferometer, the temperature sensing sensitivity can be settled separately, that is, the \(S_{T}\) can be calibrated by \(S_{T\text{-}\Delta d}\) so as to achieve self-calibration. If the seawater temperature response is used as a known parameter, the three-parameter cross-sensitivity inversion operation reduced from three dimensional to two dimensional, as shown in Equation (7).  
\[\left[\begin{array}{c}\Delta\lambda_{dip1}\\ \Delta\lambda_{dip2}\\ \Delta\lambda_{dip3}\end{array}\right]=\left[\begin{array}{c}S_{S1}S_{P1}\\ S_{S2}S_{P2}\\ S_{S3}S_{P3}\end{array}\right]\left[\begin{array}{c}\Delta S\\ \Delta P\end{array}\right] \tag{7}\]  
Therefore, by means of the above sensor design ideas and demodulation schemes, it will greatly simplify the calculation and data processing difficulty of OMCI salinity-temperature-depth three parameter sensing demodulation, and also reduce the aliasing error of wavelength drift in multi-parameter sensing.