| Salinity is one of the most important physical parameters of the ocean.It is closely related to climate.precipitation,sea surface evaporation and other factors.It is an important indicator of climate change and affects the hydrological evolutions.Its accurate measurement is of great significance to marine research.Traditionally,salinity measurements primarily include specific gravity method,conductivity measurement and optical measurement techniques.The specific gravity method refers to the measurement of the ratio of the seawater to the standard seawater under certain conditions,which can not be realized in situ.It is used in laboratory research.Conductivity measurement method refers to the calculation of seawater salinity by measuring seawater conductivity.Seabird is the market leader,followed and imitated by domestic and foreign scientific research institutions.Optical measurement method refers to the calculation of seawater salinity by obtaining seawater refractive index by optical means.It has become a research hotspot of marine sensors at home and abroad because of its high precision and easy miniaturization.The commonly used optical measurement methods mainly include optical refraction method,total internal reflection method,fiber grating method,optical interference method,surface plasmon method,photonic crystal and optical beat frequency.The optical refraction method uses the refraction effect of the V-block to obtain the salinity through the position change of the refracted beam.This method has a simple structure,but is limited by the accuracy’ of the position sensor.The accuracy of salinity measurement is±0.005 PSU,and the measurement range is only 15 PSU-42 PSU.The total internal reflection method obtains salinity by recording the angle of total reflection through cameras and other equipment.Due to the limitation of pixels of camera equipment,the accuracy is low.Fiber Bragg grating method uses optical fiber as the sensing component,but the optical fiber is fragile.In the measurement of salinity,the optical fiber needs to contact with seawater,so it is easy to be damaged.The measurement range of optical interferometry is limited by the wavelength period of light,and the measurement range is small.At present,the precision of surface plasmon method and photonic crystal method is low,and the technical maturity has been in the laboratory stage.The optical beat frequency method is limited by the counting range of the frequency meter,so the measurement range is small.The above optical measurement methods are difficult to meet the accuracy requirements of salinity measurement in marine climate research,especially in deep-sea climate research.In order to solve the above problems,an optical measurement method based on FP cavity is proposed for the first time to improve the measurement resolution.Using FP cavity as the core component of the sensor and frequency locking principle,a frequency locked salinity measurement system based on FP cavity is established by combining salinity and feedback voltage.Among them,the FP cavity combines the cavity mirror and cavity by hydroxyl bonding,which can achieve the purpose of system stability.The cavity length of FP cavity is determined to be 40 mm through mode matching simulation and optical geometry simulation,The FP cavity adopts a flat concave cavity structure.The reflectivity of the concave mirror is 97.6%,the radius of curvature is 70 mm.the matching length between the collimator and the plane mirror is 49.5 mm,and the focal length of the lens for pattern matching is 60 mm.A 633 nm circulator is used to collect the reflected light of FP cavity,and a Moku integrated control module is used to modulate,demodulate and feedback control the system.The measurement system uses dither frequency locking method to generate error signal,and locks the frequency of 633 nm DBR laser to the resonant frequency of hollow FP cavity with seawater,which provides a novel method for salinity measurement.In addition,this dissertation also proposes a new type of salinity sensor using in-situ ocean salinity sensing technology based on small angle measurement.The salinity sensor uses V-block to modulate the light deflection angle,uses FP cavity to demodulate the deflection angle,and obtains the change of seawater salinity through the change of light intensity.Theoretical simulation shows that the scheme can achieve the accuracy of 10-8 RIU,and the scheme can measure the seawater salinity with high accuracy.Focusing on the above directions,the main research results are as follows:1)The geometric parameters,reflectivity,selection of stable cavity and curvature radius of concave mirror of FP cavity are designed.The stress of FP cavity and its influence on the stability of FP cavity are analyzed by finite element mnethod.The coupling between the collimator of the measuring head and the FP cav ity is simulated.At the same time,the optical path is designed and the matching lens is selected.The FP cavity is made of glass ceramics,and the FP cavity is made by bonding and hydroxyl bonding.2)The development of frequency-locked salinity measurement system based on FP cavity,the coupling and debugging of 633 nm DBR laser,and the all fiber connection from coupler to FP cavity collimator is established.Investigate and study the bonding mode of FP cavity,and test the fabricated FP cavity.The performance of the laser in the system is tested,mainly in the range of no mode hopping.The factors affecting the propagation of light in FP cavity are analyzed,and the scattering of light in FP cavity is simulated by Monte Carlo method.3)The developed feedback system is tested.The experiments show that the performance of the salinity measurement system of rectangular FP cavity and cylindrical FP cavity can meet the requirements.Through the analysis of Allan variance and power spectral density of rectangular FP cavity,the zero bias instability range of salinity is 1.24×10-4 PSU-2.73×10-4 PSU,the zero bias instability range of salinity in cylindrical FP cavity is 4.46×10-5 PSU-1.14×10-4 PSU.4)A V-block salinity sensor enhanced by FP cavity is proposed.Through FP cavity,the laser deflection angle in V-block is converted into light intensity change,which improves the resolution of the measurement system by an order of magnitude compared with the traditional V-block refractometer. |
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