Research On Ocean Temperature-Salinity-Depth Sensor Based On Optical Microfiber Coupler Michelson Interferometer

Seawater temperature,salinity and depth are the three most important parameters of the ocean,which are closely related to global climate change,offshore equipment operation,marine resource development,and marine biological research.The CTD(Conductivity-Temperature-Depth)system is traditionally used to measure ocean temperature,salinity and depth,but it is bulky,expensive,susceptible to electromagnetic interference and relies on imports.The development of a new generation of domestic marine monitoring equipment can provide important support for the realization of a maritime power.Optical fiber sensing technology has the characteristics of high precision and anti-electromagnetic interference,which can realize real-time,high-efficiency,rapid,in-situ,large-scale and continuous monitoring of marine environmental parameters.According to actual engineering application requirements,this paper designs a new type of optical fiber Temperature-Salinity-Depth sensor structure.The main research contents are as follows:(1)First,the research progress of optical fiber sensing technology in seawater temperature,salinity and depth is introduced in detail,and the advantages and disadvantages of optical prism type,fiber surface plasmon resonance type,fiber grating type,special fiber type and other sensing structures are compared and summarized.(2)An optical path structure based on Optical Microfiber Coupler-Michelson interferometer(OMC-MI)is proposed.The structure has two sensing units,which are the waist region with large-scale evanescent field transmission and the interference arm that affects the interference characteristics.The optical power output formula of the sensor is theoretically deduced,and the anti-polarization fading characteristics of the Faraday rotator mirror are analyzed,which provides theoretical guidance for selecting suitable optical components to form OMC-MI.(3)Numerical calculation method is used to simulate and analyze the influence of sensor structure parameters such as waist radius,length and arm difference on temperature,salinity and pressure.The simulation results show that the reflection spectrum blue shifts with increasing temperature and red shifts with increasing salinity or pressure.The three sensor structure parameters all affect the output spectrum characteristics of the sensor.The envelope of the reflection spectrum is determined by the coupling filter characteristics of the OMC,and the interference peaks in the envelope are determined by the arm difference of the Michelson interferometer.By tracking the drift of the characteristic wavelength and the interference peak spacing,the demodulation of the waist sensor unit and the interference arm sensor unit can be realized respectively.At the same time,the paper also analyzes the sensing error and proposes two improvements.(4)The temperature sensing and pressure sensing sensitization structures are proposed,and the sensitization structure is simulated and calculated.A three-parameter synchronous sensing method for temperature,salinity and depth is proposed,moreover,the optimized demodulation schemes for temperature compensation and pressure compensation are designed for the special dual-sensing unit structure of OMC-MI.(5)The experiment verified the content of the simulation analysis.The highest sensitivity of temperature,salinity and depth(pressure)tested by OMC-MI reached-1007.4pm/?,303.7pm/‰ and 58.1pm/MPa,respectively.Its temperature and salinity performance has reached the second level of the national standard,which can meet the temperature and salinity measurement requirements of most sea areas in the world.(6)Package design of sensor probe according to application environment,which solves the problems of the sensor's deep sea high pressure resistance,shallow sea sediment,and plankton crosstalk to the sensor.At the same time,the package design enables the interference arm sensing unit to only respond to temperature but not to pressure,and realize multi-parameter sensing optimization based on temperature compensation.The sensor designed in this paper has the advantages of high sensitivity,compact structure,easy production,easy integration and packaging,and has important potential for practical applications in marine environment monitoring.