## 5 Conclusions  
In summary, a novel seawater temperature sensor with two sensing regions based on the structure of optic microfiber coupler interferometer is proposed in this paper. The transmission characteristics of OMCI and the seawater temperature sensing characteristics are theoretically analyzed. Research results show that the device can perform temperature demodulation in 3 ways: Firstly, using a broad-spectrum light source as the input light, a large temperature dynamic range demodulation can be achieved by tracking the characteristic wavelength shift of the output spectra; secondly, the temperature demodulation can also be achieved by tracking the change of the interference peak spacing; lastly, using a narrow-line light source as the input light, a higher temperature resolution can be demodulated by monitoring the output intensity.  
The sensing performance of OMCI can be flexibly adjusted by optimizing the structure of OMC. For example, the number of characteristic wavelengths (dip and peak) in the OMCI output spectrum can be adjusted be changing the OMC uniform waist length, thereby, the multi-parameter sensing demodulation is achieved. Moreover, sensor sensitivity can be improved by decreasing the OMC waist radius. Making use of the response of the interferometer sensor arm to temperature, the temperature  
Figure 8: (**a**) Output spectra of OMCI sample 2 at different temperatures with \(l=7\) mm, \(r=1.45\) μm, \(l_{a}=6\) mm. (**b**) Relationships between the dip wavelengths drift and the temperatures.  
self-calibration function in multi-parameter sensing can be realized by designing specific package, which reduces the difficulty of multi-parameter demodulation.  
The seawater temperature sensing experiment is conducted to verify the sensing function and characteristics of OMCI. When the microfiber waist radius are 1.55 \(\upmu\)m and 1.45 \(\upmu\)m, the maximum temperature sensitivity reaches 860.5 pm/\({}^{\circ}\)C and 1007.4 pm/\({}^{\circ}\)C, respectively. The experimental results are in accordance with theoretical simulation. This study has laid the theoretical and technical foundation work for the development and practical development of marine environment monitoring sensors. It is foreseeable that marine environment parameter sensors based on OMCI will play an important application role in the fields of ocean environment monitoring and geophysical exploration.  
Conceptualization, L.Z. and Y.Y.; methodology, J.W.; validation, L.Z., Y.T. and L.C.; formal analysis, L.Z. and H.H.; investigation, Y.Y.; resources, J.W. and J.Y.; writing--original draft preparation, L.Z.; writing--review and editing, Y.Y. and H.H.; supervision, Z.Z. ([PERSON]); project administration, Z.Z. ([PERSON]); funding acquisition, Z.Z. ([PERSON]). All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Key R&D Program of China (2017 YFB0405503), the National Natural Science Foundation of China (No. 61805278, 61605249, 61661004), Guangxi Science Foundation (2017 GXNSFAA198227), the Equipment Pre-Research Field Foundation (61404140304) and the China Postdoctoral Science Foundation (2018M633704).  
The authors would like to thank the support of the laboratory and university.  
The authors declare no conflict of interest.