| Magnetic field detection is essential in many fields,such as military,industrial and electric power transmission.Fiber-optic devices have attracted a variety of research interests due to their outstanding advantages over conventional electromagnetism sensors,including small size,high sensitivity,and immunity to electromagnetic interference.Photonic crystal fiber(PCF)exhibits unique optical properties owing to its flexible air hole structure,which are more natural for sensing purposes by incorporating with other sensitive materials.As a new kind of functional medium,magnetic fluid(MF)has versatile magneto-optic properties,for example,birefringence,Faraday Effect,field-dependent transmission,tunable refractive index and so on.In recent years,a wide range of optic-magnetic sensors based on magnetic fluid has emerged.In this thesis,the photonic crystal fiber as the research object,and the new functional materials magnetic fluid as sensing media were selected.The sensing characteristics and technologies of the magnetic field sensors based on PCF and MF were researched.The primary job and the achievement of the thesis include six aspects as follows:(1)Theoretical analysis shows that the refractive index of MF can be modulated by the temperature and the external magnetic field.In this thesis,a method based on Fresnel reflection at the fiber end face was presented.Experimental measurements were carried out to investigate the magnetic field(intensity and direction)and temperature-dependent refractive index of the MF.The effect of temperature on is relatively insignificant with the sensitivity of-8×10-5/?.With the increase of the magnetic field intensity,the nMF increases gradually when H//Light,while decreases when H // Light.The effect of magnetic field on nMF is 4×10-5/Gs in the range of 100 to 600 Gs.Besides,the mechanism of refractive index modification was analyzed from the point of the microstructure by the Monte Carlo method.This feature provides a good theoretical basis for the application of magnetic fluid in fiber-optic sensors.(2)The transmission model of PCF was established by finite element method,and the effect of duty ratio on transmission characteristics was analyzed theoretically.The transmission model of HB-PCF with liquid filled was developed to analyze the birefringence of HB-PCF in different filling modes,in this thesis was selective filling and full filling.The results showed that it has a higher sensitivity of birefringence in full filling case than selective filling.(3)Two technical difficulties existed in the experimental study on magneto-optical devices based on magnetic fluid and PCF have been overcome.The first one is to realize the low loss fusion of SMF and the PCF filled with liquid.Several main parameters(arc_time,arc_power,arc center_offset and push_dist),which influence the welding quality were analyzed.After a large number of experiments,a set of optimal welding parameters was determined by control variable method.Note that,a little bit of air should continue to pump after the liquid was filled in the air holes.This avoids direct contact between liquid and PCF ends.The second is the realization of the PCF taper by the secondary development of a fusion machine.Two kinds of taper schemes are proposed for SMF and PCF,respectively.Test results indicated the program has significant stability.The solution of these two problems provided the technological guarantee for the establishment of the experimental system.(4)Two devices were produced during the experiment.According to the Helmholtz coils,a pair of coils with a circulating liquid cooling loop was made to generate the magnetic field.The magnetic field can be uniformly changed by changing the internal current of the coil.In addition,a PDMS detection cell was specially designed to protect the weak taper and increase the stability and robustness of the device.(5)A magnetic field sensor was proposed merging the advantages of both HB-PCF and magnetic fluid.A section of MF-filled HB-PCF was inserted in the Sagnac loop as the sensing element.The transmission function of the Sagnac interferometer was deduced theoretically.The refractive index of MF is changed by a tunable magnetic field,and the resonant dips wavelength produced by the Sagnac FLM drift correspondingly.Therefore,the magnetic field intensity is obtained by the wavelength drift of the resonant dip in the Sagnac interferometer.In the refractive index verification experiment,ethanol was filled in the air holes of HB-PCF.The resonant wavelength shifts when the external temperature changes.The RI sensitivity of 0.8833nm/? was obtained.Through the experiment,the sensitivity of the structure was proved.In the magnetic field measurement,the sensitivity was as high as 107.3 pm/Gs.The sensor in preference to other sensors has small volume,anti-electromagnetic interference,and high sensitivity,thus enjoying a broad prospect of market application.(6)A compact and robust refractive index(RI)sensor based on a tapered photonic crystal fiber(PCF)Mach-Zehnder interferometer was proposed.It consists of a section of tapered PCF which is spliced between two single-mode fibers(SMF).A strong evanescent field was formed near the tapered region and made it susceptible to external RI variations.The interference was utilized between core modes and cladding modes of the PCF.Experimentally,the sensor exhibits highly RI sensitivity which is found to be 51.902nm/RIU within a range from 1.3411 to 1.3737.A resolution of 1.93×10-5 RIU was achieved since the OSA has a resolution of 1 pm.The temperature effect of the sensor is also analyzed.The magnetic fluid is filled in the capillary to coat the PCF taper and sealed by UV glue at both sides to avoid the magnetic fluid from leaking out.Based on the theoretical and experimental results,the sensitivity of magnetic field intensity 3.32 pm/Gs was proved.The Resolution reached 0.3Gs. |
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