Research On Magnetic Field Sensor Based On Optical Fiber Microcavity Filled With Magnetic Fluid

Fiber-optics Fabry-Perot(F-P)interferometer sensor not only has the excellent characteristics of optical fiber sensor,but also has the characteristics of high sensitivity,high stability,good anti-interference ability and remote sensing,which is widely used in the measurement of many fields such as temperature,strain,refractive index and other physical parameters.Magnetic fluid,as a new kind of functional material,has both the magnetism of magnetic material and the fluidity of liquid,as well as the unique optical properties such as thermal lens effect,light transmission characteristic,birefringence effect and magnetorefractive characteristic,which has attracted wide attention of researchers.Based on fiber-optics F-P cavity filled with magnetic fluid,the magnetic field sensor combines the magnetorefractive characteristics of magnetic fluid with the interference characteristics of F-P cavity,which solves the problem that the optical fiber Verdet constant is too low to directly measure the magnetic field.In addition,compared with the traditional magnetic field sensor,the magnetic field sensor has the advantages of high measurement accuracy,fast response speed,strong antielectromagnetic interference ability,and suitable for various harsh environments.However,the sensitivity of the magnetic field sensor is usually still low,it is difficult to meet the actual needs.In order to further improve the sensitivity of the optical fiber F-P sensor,firstly,the sensitivity-enhanced fiber-optics F-P sensor based on Vernier effect was designed and fabricated in this paper.Then,the gas and liquid refractive index sensing characteristics of the sensor were studied.Finally,the magnetic field sensing characteristics of the sensor were studied by filling magnetic fluid in the F-P opencavity.The specific contents are as follows:The structure of four-mirror-reflection cascaded F-P double-cavity was designed according to the characteristics of gas refractive index sensor.The conditions under which the Vernier effect was generated and the relationship between the amplification factor and the double-cavity parameters were analyzed.According to the design and simulation results,the sensing open-cavity and the closed reference cavity were prepared by lateral offset splicing using single mode fiber and splicing single mode fiber-hollow fiber-single mode fiber,in turn,with cavity length of 165.2?m and 151.4?m,respectively.Experimental results showed that the reference cavity and the sensing cavity after cascade produced obvious Vernier effect;When the refractive index of the measured gas increases in the range of 1.000027?1.000266,gradually,the interference spectrum of the cascaded double-cavity gradually redshifted and presented a linear change,with a sensitivity of 18623nm/RIU,about 12.3 times that of a single sensing cavity(1558nm/RIU),which was consistent with the theoretical analysis.In view of the high loss of F-P sensor filled with liquid,a liquid refractive index sensor based on parallel double-cavity structure was designed,and the conditions for Vernier effect of parallel F-P double-cavity structure and the relationship between amplification factor and parameters of the double-cavity structure were analyzed theoretically.According to the design and simulation results,the sensing cavity and the reference cavity with matching cavity length were fabricated by fusion.The sensing cavity was a liquid injectable open cavity with a length of 166.8?m,which is fabricated by lateral offset splicing of single mode fiber.The reference cavity is a closed cavity with a length of 196.9?m,which is fabricated by fusing single mode fiber-hollow fiber-single mode fiber.The experimental results showed that the interference spectrum envelope was obvious after the sensor cavity and the reference cavity were connected in parallel.When the refractive index increases from 1.3330 to 1.3381,gradually,the envelope redshift of the interference spectrum of the parallel double-cavity was linear,and its sensitivity was 9048.78nm/RIU,about 8 times of that of a single sensing cavity(1121.53 nm /RIU).The experimental results were consistent with the simulation results.Based on the above research,the magnetic field sensor of optical fiber microcavity filled with magnetic fluid was studied.Firstly,the F-P cavity filled with magnetic fluid was encapsulated by a quartz tube,and then a reference cavity matching the optical path of the cavity filled with magnetic fluid was prepared.Finally,a Vernier effect was generated by parallel connection of the cavity filled with magnetic fluid and reference cavity.The experimental results showed that.The linearity of magnetic field sensing characteristic curve is good in the range of 30Oe-60 Oe.The maximum magnetic field sensitivity of the single cavity filled with magnetic fluid was 0.18nm/Oe.The maximum magnetic field sensitivity of the parallel double-cavity was 1.44nm/Oe,about 8 times that of the single cavity.When the intensity of the magnetic field reaches saturation,the refractive index of the magnetic fluid will not change and the reflection spectrum will not shift.