Research On Magnetic Field Sensor Based On Magnetic Fluid And Singlemode-No-Core-Singlemode Fiber Structure

Magnetic fluid is a stable colloidal system.It consists of magnetic nanoparticles,surfactant and liquid carriers,and presents solid-magnetic-material-like magnetism and liquid-like fluidity.The microstructures within MF are diverse under the external applied magnetic field.This will lead to many unique optical properties of MF under an applied magnetic field,such as tunable refractive index and tunable birefringence.The tunable properties of magnetic fluid make it have great application prospects in the field of new optical devices,such as magnetic field sensor and tunable optical buffer.Fiber optic sensor has many advantages,such as compact structure,strong to electromagnetic interference,corrosion resistance and high temperature resistance.Combining the magnetic fluid with the diverse fiber structure,the magnetic field sensor with high sensitivity and stable performance can be produced.This paper introduces the principle of multimode interference based on singlemode-no-core-singlemode structure and its application in sensing field,we also present the key points of fabrication and encapsulation of the structure.A magnetic field sensor based on fiber Bragg grating and singlemode-no-core-singlemode structure has been proposed.We use magnetic fluid to wrap the outside of the no-core fiber.Due to the filtering effect of the no-core fiber,the spectrum will drift when the external magnetic field changes,and the wavelength of the Bragg reflection will not be affected.As a result,the intensity of the reflection peak will change.The purpose of detecting magnetic field intensity can be achieved by monitoring the power variation at the reflection peak.We obtained the magnetic sensitivity of 0.204dBm/mT(0?20mT)through experiments.We also propose a high-sensitivity magnetic field sensor which is made by using the singlemode-no-core-singlemode structure and the principle of refractive index matching.Unlike the traditional sensor with the same structure,which based on the principle of mode interference,we obtained a higher sensitivity.We use a magnetic fluid with proper refractive index to wrap the no-core fiber part.When the broadband light passes the structure,if the effective refractive index of the no-core fiber is equal to the effective refractive index of the magnetic fluid corresponding to a particular wavelength(refractive index matching),the light at this wavelength will leak a lot of no-core fiber.In this case,we can observe a coupling wavelength dip in the transmission spectrum.The dip is extremely sensitive to the ambient environment.When the refractive index of the Magnetic fluid changes due to the change of the external magnetic field,the coupling dip will produce significant drift,by monitoring its drift position,we can achieve the purpose of detecting the magnetic field.We first studied the position of the coupling wavelength dip with different magnetic fluid concentrations,and obtained a refractive index sensitivity of 116.681 ?m/RIU.Then the influence of different length of no-core fiber on sensitivity is studied.Under low magnetic field intensity(?6mT),the sensitivity of 5.49 and 5.62nm/mT is obtained respectively,and the sensitivity of 1.51 and 1.42nm/mT is obtained respectively under high intensity magnetic field(6?26mT).The result shows that the length of the no-core fiber has little effect on the sensitivity of the sensor.Finally,we studied the effect of magnetic fluid with different concentration on the sensitivity of the sensor,when the initial refractive index of the magnetic fluid sample was 1.45691,we obtained the highest sensitivity of 6.33(?6m T)and 1.83nm/mT(6?26mT).