Study On Refractive Index Sensing And Beam Splitter Based On Tapered Multicore Fiber

Owing to numerous advantages over electronic based sensors,such as immunity to electromagnetic interference,compact size,low cost and rapid response,optical fiber sensors have attracted considerable interests for its applications involving environmental monitoring,bio-molecules detection and clinical examination.The topic in our research is based on the tapered multicore fiber sensors.We analyzed the sensing property and the related influence factors of the proposed optical fiber sensors in terms of the evanescent field and polarization dependence property.The refractive index sensor and optical beam splitter based on tapered multicore fiber were fabricated and demonstrated by the hydrogen flame heating method.The smaller waist diameter and higher environment refractive index induce stronger intensity of the evanescent field,which had a directly impact on the sensitivity of the RI sensor.To further enhance the interaction between the light and the surrounding matter,the graphene was covered on the surface of the fiber.Experiments reveal that the sensitivity of the hybird waveguide enhances obiviouly compared with the optical fiber waveguide.The finite element method was utilized to stimulate the optical field distribution of the fiber.The experimental data has good agreement with the stimulated results.In addition,comparing the circular symmetry structure with the asymmetry structure of the tapered MCF,the polarization dependence property of the optical beam splitter was proved.Because of the PDMS substrate effect,there exists four supermodes in the asymmetric structure,namely s-polarization state center core mode,p-polarization state center core mode,s-polarization state outer core modes and p-polarization state outer core modes.The four supermodes result in two sets of interferences,which generate two periodic transmission spectrums.The mainstream of our research are shown as follow:1.The graphene-coated MCF based refractive index sensor(1)Due to the special characteristics of the MCF,we proposed and fabricated a tapered MCF based refractive index sensor.The device was easy to fabricate,great stability and repeatability,fast response.(2)Focusing on the evanescent field of the MCF: hydrogen frame stretching method was utilized to fabricate the in-line interferometer based on the MCF with different waist diameter.Experiments reveal that smaller waist diameter and higher environment refractive index induce more evanescent field,and further enhance the sensitivity of the in-line interferometer.(3)As for the evanescent field of the tapered graphene-coated MCF interferometer: there existed sensitivity difference between the MCF and the graphene-coated MCF.In conclusion,graphene enhanced the evanescent field.(4)The finite element method was utilized to simulate the enhancement of the evanescent field caused by the graphene: the simulated results were listed in different SRI or waist diameter.The experiment were in accord with the simulations.2.Polarization dependence optical beam splitter based on tapered MCF(1)Owing to the intermodal coupling of the seven-core fiber,the in-line interferometer based on MCF with waist diameter of 10?m was fabricated with hydrogen frame stretching method.The coupling and interference between the center core mode and the side core modes strengthened as the core diameter and core spacing decreased.The transmission spectra of the center core and the six side cores were monitored by splicing the MCF with the fan-out device.(2)Duing to the circular symmetry structure of the MCF,the polarized light have nothing to do with the coupling and interference between the two supermodes.Comparing the transmission spectrum of the center core with the side cores,we can draw a conclusion: the spectrums of the side cores are substantially overlap,whereas,phase difference of the transmission oscillations of the center core and the side core modes is estimated to be ?/2.(3)The circular symmetry structure of the proposed in-line interferometer was broken because of the PDMS substrate effect.The polarization dependence of the optical beam splitter based on MCF was obviously observed.The transmission spectrums of the center core and the side cores changed with different polarized light.(4)Analyzing the spectra of the center core and the side cores,we can control the intensity distribution either in the center core or the side cores by adjusting the linear polarized light.