| The ocean occupies about 70%of the earth’s surface area.As the most basic parameters of Marine science,the temperature and depth(pressure)in the ocean always affect the seawater environment,the movement of the sea body and the economic benefits of human maritime activities.In the temperature range of-5～35℃ and the depth range of 0～100m,most of the methods used in the past are the combination of physical and chemical methods,which have many problems and limitations.Fiber optic sensors play an increasingly important role in the field of ocean sensing because of their small size,anti-electromagnetic interference,corrosion resistance and good chemical stability.In this thesis,a variety of sensing structures which can be used to measure the depth of water temperature are designed and studied based on the sensing characteristics of optical fiber interferometric sensors.The main research work includes the following parts:(1)In this thesis,a misalignment fiber optic Mach-Zehnder Interferometer(MZI)sensing structure based on the principle of optical fiber Mach-Zehnder Interferometer(M-Z)is designed.The response of structural parameters and environmental parameters to its transmission spectrum is simulated by beam propagation method.The dislocation welding system was built and the large dislocation MZI sensing structure was fabricated by the dislocation welding technology.Then,the refractive index(salinity)sensing characteristics were explored.The sensitivity is-16602.81nm/RIU in the refractive index range of 1.333～1.341.According to the conversion relationship between seawater salinity and refractive index,its sensitivity is about 3.32nm‰ in the salinity range of 0～40‰.At the same time,the temperature sensing characteristics of the structure are studied.The structure is insensitive to temperature measurement.In order to improve the sensitivity of temperature measurement,a temperature sensitive material,Polydimethylsiloxane(PDMS),was partially coated in the misplaced fused sensing area for sensitization.The experimental results show that the sensitivity of the sensitized temperature sensing structure can reach 9.60nm/℃ in the temperature range of 5～40℃.(2)Aiming at the problem that the measurement range of the highly sensitive sensor structure is too small,this thesis proposes a method to extend the large dynamic measurement range of the highly sensitive sensor structure,which effectively combines the characteristics of the two sensing structures with the contradiction between the sensitivity and the measurement range.In the process of the research,the misplaced MZI sensor structure has good temperature and salt sensing characteristics,FBG sensor and photonic crystal fiber(PCF)based on the mode Interference(MI)sensor structure as the reference sensor,combined with the temperature and salinity sensing characteristics of the two.The measurement method was validated and analyzed.The results show that the maximum absolute error of temperature parameter is about 0.31℃,and the maximum absolute error of salinity parameter is about 0.55‰.The error sources are analyzed.The measurement method was then applied with salinity simulation data,and the maximum salinity error was reduced to about 0.0054‰,which was reduced by about two orders of magnitude compared with the experimental data.(3)In this thesis,two optical fiber pressure sensing structures based on FabryPerot(FP)interference principle are designed and studied:The Fabry Perot Interferometer(FPI)based on Hollow core fiber(HSF),the optical fiber Fabry-Perot Interferometer based on PDMS and their improved structures are analyzed.After setting up the experimental system and making the sensor structure,the temperature and pressure sensing characteristics of the two are studied.Among them,the experimental results of temperature and pressure measurement of micro-spherical air cavity FPI based on HCF show that the sensitivity is 2.28 PM/℃ and-2.82nm/MPa respectively,which has a great advantage in the measurement range while the sensitivity is lower.The sensitivity of the cavity fiber FPI pressure sensing structure based on PDMS is-1.25nm/℃ in the temperature range of 5～39℃ and 2444.70nm/MPa in the pressure range of 0.5～0.61MPa.Due to the high sensitivity and pressure sensitivity,a cavity free fiber FPI pressure sensing structure based on PDMS was further improved.It has a temperature sensitivity of about 0.98nm/℃ in the range of 30～31 ℃,and a pressure sensitivity of about-13.82nm/MPa in the range of 0～1MPa.An integrated and chained design is proposed to better adapt the sensor structure to the seawater multi-point measurement environment.In this thesis,based on the principle of interferometric sensing,the sensing structure is designed and manufactured,and the temperature-depth correlation characteristics of various sensing structures are explored.In this thesis,the large dynamic measurement range of the high sensitivity sensor structure is studied,and a method of extending the measurement range of the high sensitivity sensor structure is proposed,which is innovative and widely applicable,and can be used to measure a variety of parameters(such as temperature and salinity).And according to the difference of sensitivity response of different order,it has the potential of simultaneous measurement of multiple parameters. |
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