Studies On Birefringence Characteristics Of Spun Optical Fibers And Polarimetric Fiber Optic Current Sensor

The fiber optic current sensor(FOCS)has the advantages of excellent conductor insulation ability,anti electromagnetic interference ability,small volume and large detection dynamic range.It meets the development trend of sensor intelligence and digitization.It is one of the important directions of current sensing technology.As the main sensing element of FOCS,the polarization characteristics of spun fiber have a great influence on the performance of FOCS.Firstly,based on the theory of polarization optics,the birefringence characteristics of spun fiber affected by current are analyzed and studied,and the terms such as current induced linear birefringence and effective Verdet constant of spun fiber are proposed and measured accurately for the first time;Secondly,the influence of spun fiber’s linear birefringence and current induced linear birefringence on FOCS’s ratio error is analyzed and studied;Finally,based on a polarization fiber optic current sensor(PFOCS)structure,a new scheme and signal processing algorithm for simultaneously measuring instantaneous temperature and power current are proposed for the first time,which is used for temperature error compensation of power current measurement,and its performance is analyzed and studied by using the algorithm.The content of this paper mainly includes the following aspects:1.On the basis of the Jones theoretical model of spun fiber,the relationship between the fabrication parameters of spun fiber and current induced birefringence in spun fiber is studied.The terms of current induced linear birefringence and effective Verdet constant of a spun fiber are proposed for the first time.It is proved theoretically and experimentally that the linear relationship between Faraday polarization rotation angle and current is still approximately held even if there is with birefringence in spun fiber(its scale factor is the effective Verde constant),which lays a theoretical foundation for current and magnetic field measurement and sensing in the irrational case of linear birefringence.Based on the fully automatic binary magneto-optic polarization analysis system,we propose a time difference method and complete the accurate measurement of current induced linear and circular birefringence in spun fiber.The relationship between the effective Verdet constant of spun fiber and the fabrication parameters of spun fiber is verified experimentally.The method and the experimental results presented in this dissertation shall prove beneficial for scientists and engineers working in the field of current sensing.2.Based on the theory of wave optics,the polarization evolution process in the optical path of reflective interferometric FOCS is analyzed and studied,and we establish a numerical analytical model of the relationship between reflective interferometric fiber optic current sensor’s ratio error and quarter wave plate and birefringence.Then,we prove analytically and confirm experimentally for the first time that the quadratic dependency on electric current of FOCS’s measurement inaccuracy mainly originates from the residual linear birefringence of the spun fiber used in the FOCS and the influence of current induced birefringence on the FOCS’s ratio error is analyzed in high Faraday phase-shift measurement.Finally,by first measuring residual linear birefringence,together with the circular birefringence of the spun fiber,we propose and demonstrate for the first time that the ratio error deterioration of the FOCS can be significantly reduced from 1.22% to 0.15% at equivalent current 200 k A by software compensation.Further reduction of nonlinear ratio error deterioration to 0.02% at equivalent current 200 k A can be obtained if the the method of curve fitting is used,and the quadratic relation between the ratio error and the current under test(CUT)is experimentally obtained.3.Based on a PFOCS structure with linear birefringence automatic compensation function,a new scheme and signal processing algorithm for simultaneously measuring instantaneous temperature and power current are proposed.There is no need to introduce additional temperature senser to obtain the temperature error compensation coefficient.In the range of-10℃～ 70℃,the temperature deviation is less than ±0.5℃and the current measurement accuracy is better than±0.5% in the range of 30A～720A,meeting the requirements of class 0.5 electronic current transformer for measurement specified in national standards.