Theoretical And Experimental Researches On LPFG Fiber Sensor Based On Nano Materials

Long period fiber grating（LPFG）is a kind of fiber grating with a period between tens to hundreds of micrometers.When a band of transmitted light satisfy the phase matching condition,the energy in the fundamental mode will be coupled to the cladding modes,thereby forming a band-stop filter.Fiber gratings have lots of strengths,for its high sensitivity,small size,strong anti-electromagnetic interference etc.,therefore have been widely used in the desert,marine,chemical and other high-risk environment.In recent years,with the upgrading of the production process and the emergence of special fibers,LPFGs get a rapid development.Compared with traditional sensors,LPFGs have great advantages and potential in the gas sensing and biosensing fields.In this thesis,a special FMF-LPFG fiber sensor based on graphene oxide materials was designed and fabricated,and its sensing performance was analyzed by a series of numerical simulations and experiments.The main contents and innovation points of this thesis were summarized as follows:（1）This thesis introduced the origin,principle and applications of LPFGs.The current states and progress of few mode fiber and graphene were introduced.（2）The three-layer foundation model and four-layer coating model of LPFGs were introduced in detail.Through the numerical simulation,we explored the model reorganization of the coated fiber and mode conversion of the few mode fiber.Also,the existence of the mode conversion was verified by our experiments.Besides,we compared and analyzed the FMF-LPFG and SMF-LPFG in temperature,strain and refractive index of the sensing characteristics.（3）We fabricated three kinds of LPFGs with sing mode,two mode and four mode by CO2 laser direct writing method.All of them had good phenomena in transmission spectrum,and their resonant wavelengths were all around 1550nm and the transmission depths can reach more than 20 dB.（4）We compared the curvature and refractive index of two different LPFGs with sing mode and two mode by experiments.It was found that the TMF-LPFG had a better sensitivity characteristic both in curvature and refractive index sensing.The curvature slope of the TMF-LPFG was 32.80nm/m^-1,which was 1.69 times of that of SMF-LPFG.The refractive index sensitivity was 231.5nm/RIU,which was 46.3%higher than that of SMF-LPFG.（5）We successfully plated the graphene oxide on the surface of the LPFGs using APTES solution.Compared with the sensing characteristics before and after coating,it was found that the TMF-LPFG coated with graphene oxide had a wider measurement range and a higher sensitivity response in refractive index sensing.When the external refractive index was 1.4558,the resonant wavelength blue shifted10.46nm,and the refractive index sensitivity can reach 583.6nm/RIU,2.52 times of that before the coating.