Research On Method Of Measuring Seawater Salinity By Distributed Optical Fiber

More than 70%of the earth's surface is the ocean,and seawater salinity is one of the important indicators of the marine environment.The salinity affects important aspects such as seafood farming,submarine operations,and marine environmental protection.Ocean salinity measurement methods include conductivity method,ultrasonic method,and satellite detection method.However,these methods are only for the single point of seawater or the salinity of the sea surface.Distributed seawater measurement requires the ability to measure salinity at multiple points in a section of seawater,which is difficult to traditional methods.Optical fiber has the advantages of anti-electromagnetic interference,corrosion resistance,small size and easy cascading.Therefore,optical fiber sensing plays a very important role in the sensing field.This paper is based on fiber optic technology to achieve distributed measurement of seawater salinity,mainly for different marine salinity measurement requirements including fiber grating cascade,nocore fiber cascade,SPR cascade method.This article mainly completes the following three parts:(1)A theoretical analysis of the measurement of seawater salinity based on a polyimide(PI)coated fiber grating was completed.The center wavelength of the fiber grating has the characteristic of being drifted by stress,and the polyimide material expands or contracts with changing in the external salinity.Applying PI to the grating,the center wavelength will drift according to the change in salinity.In the salinity of 0?40‰,the sensitivity of single point to salinity is about 9.7pm/‰.Theoretically,ten different center wavelength sensors can be cascaded in the 1520?1610nm band.In order to improve its sensitivity to salinity,the cladding of the grating region is corroded,which reduces the thickness of the cladding and makes the grating region more sensitive to stress,thus improving the sensitivity to salinity.Sensor sensitivity has tripled,which is approximately 28.6 pm/‰.But the valley 3dB bandwidth of the single-point output spectrum is broadened,resulting in the number of cascades within the finite source bandwidth to three.(2)Nocore fibers are more sensitive to the salinity of external solutions due to their absence of cladding properties.This paper designs a cascaded singlemode-nocore-singlemode(SNS)sensing structure to achieve distributed seawater salinity.The measurement principle of SNS structure is analyzed,and the factors affecting the output spectrum of SNS structure are analyzed,including the length,diameter and environmental salinity of nocore fiber.Through Rsoft simulation,the final experiment used a 125?m diameter nocore fiber as the sensing area of the SNS structure,achieving a 3-point cascade in the 1520-1610nm band,and the nocore fiber length of each cascade point is 19mm,22mm,38mm length respectively.In the salinity range of 0?40‰,the sensitivity to salinity is about 102pm/‰.(3)SPR fiber sensor can improve salinity sensitivity more effectively.In this paper,the center position of the resonance trough of the surface plasmons of different metals is obtained through theoretical analysis.Gold and silver were selected to coat the surface of the fiber.Because the resonance troughs of gold and silver are different in position,the cascades can be used to separate the respective measurement points.In order to improve the stability and the sensitivity to salinity,several sets of experiments were carried out on different metal film thicknesses.The experiment finally chose 48nm thickness of gold and 39nm thicknesss of silver film.Its sensitivity to salinity is respectively 481pm/‰,542pm/‰.Due to 3dB bandwidth of the resonance valley was wide,only two points of cascading measurement can be realized in the 300?1000 nm band,but the sensitivity is greatly improved,so the fiber SPR cascaded sensor can be used for high sensitivity.The salinity sensors studied in this paper have the advantages of good repeatability and long-term monitoring.The error caused by the change of ocean temperature can be effectively reduced by means of temperature compensation.Moreover,the sensors can be monitored in real time over long distances,which can provide reference to realization of a wider range of marine salinity monitoring in the future.