Research On Optical Fiber Sensing Key Technology For Seawater Temperature Salt And Depth Measurement

Seawater temperature salt and depth profile measurement is an important content of physical ocean research and an important element of ocean hydrological observation.It is significant in the development of marine resources,prediction of marine disasters,research on global climate change,safe operation of marine equipment,and research on marine life.The traditional electronic thermohaline depth meter is a conductivity temperature depth(CTD)measurement system based on electronic devices,which is widely used in marine environmental parameter monitoring.However,it has certain limitations in instrument miniaturization and distributed measurement.Optical fiber sensors integrate sensing and transmission,and has the advantages of resisting electromagnetic interference,being suitable for on-site and having remote sensing monitoring,which compensate for the shortcomings of electrical sensors.They are considered as a new generation of marine environmental monitoring sensors.The optical fiber temperature salt and depth profile measurement system uses optical fiber sensors as measuring elements and optical fiber for data transmission to achieve basic marine data acquisition.This article conducts a series of studies on the key technologies of fiber optic sensing in seawater temperature and salinity monitoring:1.A expendable bathy thermographt(XBT)sensor based on Mach-Zehnder interferometer(MZI)sensing principle was proposed and designed.MZI adopts a single mode fiber(SMF)-coreless fiber(CLF)-few-mode fiber(FMF)-CLF-SMF structure;Using the Rsoft BeamPROP BPM tool,the numerical simulation of the excitation process of higher order modes of CLF in structures was carried out;The relationship between higher order modes and the refractive index of the external environment is simulated and calculated.The fabrication of the sensor structure has been completed,and its temperature and refractive index response characteristics have been tested and evaluated.The sensor’s temperature measurement sensitivity is 0.072 nm/℃,it has been demonstrated that it can be used as an optical fiber seawater XBT sensor for temperature depth profile measurement.2.A expendable conductivity-temperature-depth(XCTD)sensor with a hybrid structure of fiber Bragg grating(FBG)and MZI was proposed and designed.The MZI adopts an SMF-FMF-CLF-FMF-SMF structure;The process of FMF excitation of higher order modes is simulated and analyzed;The fabrication of FBG and MZI hybrid sensors was completed,and their temperature and refractive index response characteristics were tested and evaluated;The FBG temperature measurement sensitivity is not high,but it has strong anti-interference ability and good stability.Due to the coaxial fusion connection method,the MZI sensor has good structural robustness,and the resonance wave valley is little affected by temperature,with high salinity measurement sensitivity.This sensor is expected to be applied to sea areas with relatively poor seawater environment.A balloon shaped seawater XCTD sensor based on twin-core and twin-hole fiber(TCTHF)is proposed and designed;The relationship between the effective refractive index of the side core fundamental mode and the thickness of the side core cladding of the TCTHF,the relationship between the effective refractive index of the side core fundamental mode and the refractive index of the external environment,and the effect of bending on the evanescent field of fiber mode transmission are analyzed by simulation;The sensor has been fabricated,and its temperature and refractive index response characteristics have been tested and evaluated.The sensor’s temperature and salinity measurement sensitivities are 0.050 nm/℃ and 58.185 pm/‰,respectively.It can be used as an optical fiber XCTD sensor for high-precision temperature,salinity,and depth profile measurement.3.A dual FBG temperature and depth measurement sensor combined with a diaphragm and a cantilever beam was fabricated for use in a quasi-distributed seawater temperature and salinity profile measurement system.The pressure measurement sensitivity of dual FBG temperature depth sensing structure is analyzed theoretically;Starting from the small deflection theory of elasticity,the maximum pressure measurement values that can be achieved by pressure sensitive film sheets with different thicknesses are analyzed.The relationship between the pressure measurement sensitivity of sensors made of poly tetra fluoroethylene(PTFE)film and the thickness of the PTFE film is simulated and calculated.The relationship between the pressure measurement sensitivity and the width and thickness of the stainless steel cantilever beam is calculated under the same size conditions.The maximum pressure measurement value and pressure measurement sensitivity that can be achieved when PTFE membrane and stainless steel membrane are used as pressure sensing membrane sheets are compared;The fabrication method of the sensing structure is described,and the sensor is completed.The temperature and pressure response characteristics of the sensor are tested and evaluated.The experimental results show that the pressure measurement sensitivity of the dual FBG sensor designed is twice that of a single FBG sensor.It can achieve synchronous temperature and pressure measurement and serial multiplexing of the sensing structure.and is expected to be applied to the quasi-distributed measurement of seawater temperature,salinity,and depth profiles.4.A quasi-distributed measurement system for seawater temperature and depth was designed,which combines FBG temperature sensors with Fabry-Perot(F-P)pressure sensors.Based on the multi beam interference theory,the intensity distribution curves of the reflection spectrum and transmission spectrum of the F-P cavity with different fineness are simulated and analyzed;The relationship between the variation of the reflected spectral trough value of a high precision extrinsic Fabry-Perot interferometer(EFPI)and the variation of the cavity length is derived;On this basis,the characteristics of the reflective spectra of FBG temperature sensors and F-P pressure sensors are described.Both sensors use the change of central wavelength to reflect the change of physical quantities and demodulation methods;Based on the design of the structural model of seawater temperature and depth sensing array,the FBG temperature sensor and F-P pressure sensor used in the temperature and depth sensing array were calibrated and data processed in the laboratory;Using the designed temperature depth array sensing system,a preliminary exploration experiment of seawater temperature depth profile was conducted,and a comparative analysis of temperature depth sensing array data and seabird SEB17CTD measurement data was completed.