Distributed Hydrostatic Pressure And Salinity Measurement Technology Based On Optical Fiber Brillouin Dynaminc Grating

Brillouin-based distributed fiber sensing technology has been widely used in structural health monitoring of large infrastructure such as roads,bridges,tunnels,and pipelines.However,limited by the sensing mechanism,traditional distributed Brillouin-based sensing technology can only measure temperature and strain.In order to meet the needs of multi-parameter measurement in practical applications,this paper proposes an optical fiber distributed measurement technique for hydrostatic pressure and salinity based on Brillouin dynamic grating(BDG).The technology is based on the sensitivity of the fiber phase birefringence to the hydrostatic pressure and salinity.The BDG is used to measure the fiber phase birefringence change to realize the distributed hydrostatic pressure and salinity measurement.Firstly,the theoretical research of BDG is carried out,and many factors affecting the sensing performance are analyzed,which provides theoretical support for optimizing experimental system parameters,improving measurement accuracy and sensing distance.The BDG is described by the Brillouin enhanced four-wave mixing model in this thesis.By solving the coupled wave equation,the influence of the probe power on the BDG reflection spectrum is theoretically analyzed which shows that the increasement of the probe will reduce the reflectivity,broaden the spectrum,reduce the measurement accuracy,and the influence of the pump power on the sensing range is also discussed.The results indicate that the continuous pump light will cause the pumping depletion effect and limit the sensing distance.In order to increase the sensing distance,the pulsed pump light should be used to excite the BDG.This content provides theoretical support for further optimizing the system and improving measurement accuracy.To settle the dispute that BDG can measure phase birefringence,the verification work on phase birefringence measured by BDG is carried out.Firstly,the birefringence of polarization-maintaining fiber(PMF)is theoretically calculated using finite element method(FEM),and the difference between phase birefringence and group birefringence in PMF is clarified.Then,the PMF group birefringence is measured by Sagnac interferometer.The measured group birefringence was consistent with the theoretical value,which proved the rationality of the analytical model.Then,BDG is used to measure the birefringence of the fiber.The measured results are in agreement with the calculated value.Therefore,the ability of BDG to measure the PMF phase birefringence is confirmed.This content provides a theoretical basis for the hydrostatic pressure and salinity distributed sensing by measuring the phase birefringence change using BDG.This thesis proposes a hydrostatic pressure distributed measurement technology based on BDG.Firstly,the theoretical model of the influence of hydrostatic pressure on phase birefringence is established.The relationship between hydrostatic pressure and birefringence-induced frequency shift(Bire FS)is analyzed,and the sensitivity of different PMFs to pressure is simulated.The results show the sensitivity of the Bire FS to static pressure is at least 100 times higher than that of the Brillouin frequency shift,and the pressure sensitivity of the PM-PCF is at least twice higher than that of Panda fiber.Therefore,PM-PCF is used as the sensing fiber to realize the distributed hydrostatic pressure sensing with a sensing range from 0 to 1.05 MPa,pressure sensitivity of 199 MHz/MPa,and measurement accuracy of 0.03 MPa.In order to compensate the temperature crosstalk in hydrostatic pressure measurement,a temperature crosstalk compensated method with BOTDA system is proposed to realize temperature-independent static pressure distributed sensing.The BOTDA is used to measure the BFS to demodulate the temperature change without increasing the complexity of the system.The relationship between temperature and Bire FS is used to compensate the temperature effect on the Bire FS.This method solves the temperature cross crosstalk problem of the distributed static pressure sensing system,and also can be applied to the distributed salinity sensing technology proposed below.Finally,this paper proposes a BDG-based distributed salinity measurement technique.The technology uses a polyimide-coated PM-PCF as a sensing fiber,and the polyimide coating is swollen based on the change in salinity,thereby modulating the fiber phase birefringence.Firstly,the theoretical model on the phase birefringence modulated by salinity is established,and the influence of salinity on the phase birefringence is obtained.Then,the BDG-based distributed salinity sensing experiment is carried out,which proves the distributed salinity measurement capability,good repeatability and temperature crosstalk compensation ability.The distributed salinity measurement is realized with a maximum salinity sensitivity of 139.6 MHz/(mol/L)and a maximum obtained sensing accuracy of 0.072 mol/L.In summary,this thesis proposes the distributed hydrostatic pressure and salinity measurement based on the fiber BDG,which effectively expands the application field of Brillouin fiber optic sensing technology and provide guidance for engineering applications.