## 3 Sensing Characteristics and Demodulation  
### Analysis of Sensing Characteristics  
According to Equation (4), different structural parameters of OMC will cause different sensing characteristics of OMCI. The mainly structural parameters of OMC are the length \(L\) of the uniform waist region and the radius \(r\) of the microfiber at waist region, and its transmission characteristics are mainly determined by the two parameters (the OMC used in this paper is drawn by a volume conservation model, the coupling length and uniform waist length satisfy a specific function relationship [19; 22]). From an intuitive analysis of the evanescent transmission characteristics of OMC, the thinner radius r of the microfiber at waist region will cause the larger proportion of the evanescent field of the transmitted light field, so it is more sensitive to environmental changes. In view of this, this paper first analyzes the sensing characteristics of OMCI with different OMC waist radius. During the simulation, the seawater salinity was set to 30%, and the arm difference is 0 mm (an equal arm interferometer is selected for sensor characteristic analysis), and we suppose input light intensity \(P_{0}\) = 1. Besides, the length of OMC uniform waist is set 3 mm, when the radius of the microfiber at waist region are selected as 1, 3, and 5 um respectively, the calculation results of output spectra are shown in Figure 2a. It can be seen from the figure that the thinner of the waist region, the more the number of characteristic wavelengths (dip and peak). In addition, it can be seen from Figure 2a that the transmission spectra of OMCI blue-shift with the seawater temperature increasing. For the three OMCs with different waist diameters, the characteristic wavelengths (three peak points with similar wavelengths) were selected to calculate the respective temperature sensing response characteristics. The results are shown in Figure 2b, for the OMC with waist radius of 1, 3, and 5 \(\upmu\)m, the temperature sensitivity of OMCI are 1204.9 pm/\({}^{\circ}\)C, 266.9 pm/\({}^{\circ}\)C, 145.7 pm/\({}^{\circ}\)C respectively. The calculation results show that the sensor got higher temperature sensitivity as the thinner OMC waist radius, which is consistent with the previous analysis. In practical applications, higher sensitivity can be obtained by reducing the waist radius of OMC. However, the thinner OMC waist radius will reduce the mechanical strength of the device and increase the insertion loss of the device. In addition, it is required higher manufacturing process, so it should be comprehensively considered. In order to study the effect of the uniform waist length \(L\) on the OMCI sensing characteristics, we used OMCs with different \(L\) to simulate the temperature sensing characteristics. Fix the waist radius \(r=2\)\(\upmu\)m, \(L=1,3\), and 5 mm were set respectively. The other parameters are consistent with the previous simulation. The OMCI output spectra calculation results are shown in Figure 3a. From the results that the more the characteristic wavelengths (dip and peak) of the OMCI output spectrum with the longer uniform waist length \(L\). For OMCIs with different uniform waist lengths, we also calculated the sensor response characteristics, and the results are shown in Figure 3b. From the simulation results, it can be seen that the waist length of OMC has less influence on the temperature sensitivity of OMCI. Because of the wavelength dependence of OMC, the sensing sensitivity is varied at different wavelengths [5]. Therefore, by properly selecting the \(L\) and \(r\) parameters of the device, multiple dip or peak characteristic wavelengths in the OMCI output spectrum can be achieved, and then multiple parameter sensing can be realized by tracking multiple characteristic wavelengths. The number of characteristic wavelengths determines the number of parameters that can be monitored. Besides, it also determines the dynamic range of the monitored parameters.