## 4 Experiment Set up and Results  
In order to fully verify the foregoing theoretical analysis results, this paper have accomplished an experimental test of the seawater temperature sensing performance of OMCI. The experiment measurement system is shown in Figure 6a, in concluding ASE broadband source (1528-1603 nm), optical spectrum analyzer (OSA, 600-1700 nm, resolution 0.01 nm, Q8384, ADVANTEST, Tokyo, Japan), conductivity meter (HS-15 TW, Leici, Shanghai, China), temperature control water tank (KQ2200 DE, Kunshan Ultrasonic Instruments, Kunshan, Jiangsu, China), and signal processing computer. The ASE light source is connected to the port1 of OMCI, and the spectrum analyzer is connected to the port2 of OMCI. In this work, the OMCI sample 1 was selected first, whose structural parameters are waist length L = 7 mm, the uniform waist radius \(r\) = 1.55 \(\upmu\)m and the initial arm difference of interferometer \(l_{a}\approx 6\) mm. From the foregoing theoretical analysis, it can be known that the temperature response capability of the interferometer sensing unit is expected to be limited, because the interferometer is fabricated by conventional silica glass optical fiber. With this structural parameter, OMC will become the main sensing unit of OMCI. The practical temperature measuring experimental setup is shown in Figure 6b. In the temperature experiment, seawater sample was made from pure water and NaCl, the initial salinity of the seawater sample was 12.2%, and the water tank capacity was about 2L. In order to avoid the error caused by the uneven heating of seawater during the experiment, the temperatur  
Figure 6: (**a**) Schematic of the experiment setup for measurement of temperature in seawater. (**b**) The temperature measuring experimental setup. was carried out in the way of cooling. The seawater sample was heated to 35.3 \({}^{\circ}\)C, then cool it naturally to 22.3 \({}^{\circ}\)C. Meanwhile, the temperature and the salinity of the seawater sample is measured by a conductivity meter in real time. The effect of test errors caused by changes in seawater temperature that may cause the salinity vary has been eliminated. The output spectra at different temperatures are shown in Figure 7a. It can be clearly seen that the spectrum shift to short wavelength significantly with the increasing temperature and the insertion loss of simple1 is approximately 3 db. The temperature response sensitivity of OMCI is obtained by tracking the characteristic wavelength dip at different temperatures. The calculation and fitting results are shown in Figure 7b, when the microfiber waist radius \(r\) = 1.55 \(\upmu\)m, the temperature sensitivity response of the four characteristic wavelengths are \(\text{S}_{\text{T-dip1}}\) = \(-\)811.2 pm/\({}^{\circ}\)C, \(\text{S}_{\text{T-dip2}}\) = \(-\)860.5 pm/\({}^{\circ}\)C, \(\text{S}_{\text{T-dip3}}\) = \(-\)815.0 pm/\({}^{\circ}\)C, \(\text{S}_{\text{T-dip4}}\) = \(-\)678.0 pm/\({}^{\circ}\)C. The minimum dynamic range of dip3 wavelength is 14.3 nm, and the allowable temperature variation range is 17.6 \({}^{\circ}\)C.  
The previous simulation results show that the OMC is the main sensing unit, and the main parameter that affects the temperature sensitivity of OMCI is the radius of the microfiber at the waist region. In order to verify this characteristic, an OMCI sample (arm difference \(l_{a}\)\(\approx\) 5 mm) was re-made to experiment the temperature sensing response. A new OMC sample with a relatively thinner waist radius was used, \(r\) = 1.45 \(\upmu\)m, and keeping the waist length \(L\) = 7 mm unchanged, the output spectra at different temperatures are shown in Figure 8a. It can be found that the insertion loss of simple 2 is approximately 2 db and there are more dips than the previous sample in the output spectrum. The temperature response sensitivity of OMCI is obtained by tracking the characteristic wavelength dip at different temperatures. The calculation and fitting results are shown in Figure 8b.