Multi-parameter Measurement Of Fiber Coil Based On Stimulated Brillouin Scattering

Optical fiber sensing has developed rapidly during the past 30 years,and widely used in many fields of modern society.In the fiber optic gyroscopes and fiber optic hydrophones,the sensing fibers are wound into fiber coils to reduce volume and improve measurement accuracy.However,the performance of the fiber coil is limited by the Shupe effect in the fiber coils.Generally,the Shupe effect is induced by the uneven distribution of the temperature,strain and birefringence of the fiber coil with different fiber types and winding patterns.For different types of fiber and winding patterns,it is essential to test the performance of the fiber coils.Many direct and indirect methods can measure the strain or birefringence,while the measurement range is very short.This article focuses on the the realization of multi-parameter(strain,temperature and birefringence)measurement of fiber coil based on stimulated Brillouin scattering(SBS)in optica fiber.Besides,the distributed strain measurement of fiber Michelson hydrophones are disscussed,which improves the appliaction states of multi-parameter measurement technology.The main work of this article are listed below:(1)We propose two schemes for improving the performance of strain,temperature and birefringence measurement of fiber coil based on SBS.One is a5-cm spatial resolution differential pulse pair Brillouin optical time domain analysis system(DPP-BOTDA),which solves the problem of strain measurement accuracy of the fiber coil.We study the numerical solutions of DPP-BOTDA in transient state based on the SBS coupled wave equations,theoretically study the relationship between the strain measurement accuracy and the spatial resolution of D PP-BOTDA system.Based on the 5-cm DPP-BOTDA,the 17-??strain measurement accuracy has been achieved for the fiber coil with diameter greater than 7 cm.The other one is an enhanced Brillouin dynamic grating(BDG),which solves the problem of limited distance range of birefrinegnce measurement.We study the numerical solutions of the BDG based on the BDG coupled wave equations,and the influence of optical power and frequency setup on the BDG intensity distribution along the fiber.In the experiment,the BDG intensity increase along the fiber generated by the SBS between the pump pulse and frequency-upshift continuous wave.Based on the enhanced BDG,the birefringence measurement of a 3030-m fiber coil in fiber optic gyroscope has been realized with a spatial resolution of 40 cm.(2)We propose two schemes for multi-parameter(strain,temperature and birefringence)measurement of the fiber coil in fiber optic gyroscope based on SBS.One is for limited strain and temperature measurement based on DPP-BOTDA,and birefringence measurement based on BDG.We propose a multi-parameter measurement system based on SBS,which compromises the function s of DPP-BOTDA and BDG.The realization of DPP-BOTDA and BDG is in the fast axial of the polarization maintaining fiber(PMF),and the reading of the BDG is realized in the slow axial of the PMF.Based on the multi-parameter measurement system,a long-range,high-spatial and high-accuracy strain,temperature and birefringence measurement of fiber coil has been realized.The o ther one is for distinguishing the birefringence change of the fiber coil induced by axial-stress and transverse-stress.We propose a distributed measurement of strain and birefringence of the fiber coil by using DPP-BOTDA and enhanced BDG.Benefitting from the linear relationship between strain and birefringence,birefrinegcne induced by axial-stress can be achieved.In the experiment,a distributed measurement of axial-stress induced birefringence and transverse-stress induced birefringence of the fiber coil in fiber optic gyroscope has been realized.The results show that the birefringence coefficient of strain is 0.85×10^-8/??,the birefringence variation induced by transverse-stress is 10^-6.(3)We propose a DPP-BOTDA with a pulsed-probe wave,which can realize a online distributed strain measurement of the fiber coil of fiber Michelson hydrophones.In the pulsed-probe DPP-BOTDA system,the pulsed-probe is generated by external-modulation,and the width of the pulsed-probe is determined by the length difference between the sensor arm and the reference arm of the fiber Michelson hydrophone.Benefitting from the pulsed-probe,the interference between the reflected lights from the sensor arm and reference arm can be avoided.In the experiment,based on the pulsed-probe DPP-BOTDA,we obtain the strain distribution of the 64.59-m and 17.39-m arm-length-difference fiber Michelson hydrophones as the temperature increases from-40 to 80?,and the strain distribution of the 126.91-m arm-length-difference fiber Michelson hydrophone as the pressure increases from 0.1 to 10 MPa.In conclusion,this article is aiming at realizing and impro ving the overall performance of the multi-parameter(strain,temperature and birefringence)measurement of fiber coil by analyzing and solving the existed problems.Besides,a pulsed-probe DPP-BOTDA is proposed for enlarging its application.