Research On Sensing Technology And Application Of Graphene Functionalized Long-period Fiber Gratings

In sensing technology,achieve high sensitivity,small size,low cost has always been a challenge.Fiber gratings,especially long period fiber gratings（LPFG）,have unique advantages for sensing and measurement because it can excite cladding mode transmission.In this paper,LPFG is prepared by periodically changing the physical structure of single-mode fiber after CO₂ laser heating to change the stress distribution that inside the fiber.LPFG can modulate the refractive index periodically,excite the cladding mode and sense the change of refractive index sensitively due to the elastic-optic effect.Compared with the laser exposure method for preparing LPFG,this method has the advantages of high stability,no annealing,lower transmission loss（no photosensitive fiber）,better reusability,et al.However,depends on the characteristics of LPFG itself are still difficult to meet the requirements of high sensing sensitivity,and how to avoid cross-sensitivity caused by other physical quantities（temperature,deformation,et al.）during measurement is also a problem that needs to be solved in LPFG sensing applications.In response to the above problems,this paper proposes to functionalize the prepared LPFG with graphene and its derivative graphene oxide（GO）to achieve high sensitivity to temperature,biological substances and chemical substances.The main research work and innovation achievements of this paper are as follows:（1）A dual-beam CO₂ laser was used as the heat source to fabricate a micro-tapered long period fiber grating（MTLPG）with a period of 680μm.Cone waist diameter 110μm.The number of cycles is 40.In addition,the chiral long-period fiber grating（CLPFG）was fabricated by introducing axial dislocation and torsion of two motors in the same direction after heating.The period of CLPFG is 648μm,and the numbers of cycles is 30.Compared with CLPFG prepared by traditional single-side twisting method or photonic crystal fiber,CLPFG prepared by this method has the advantages of lower cost and good repeatability of prepared parameters.The application in extreme conditions for a long time will not cause degradation of refractive index modulation.Two kinds of LPFGs prepared by this method have the advantages of no annealing,simple fabrication,low insertion loss and flexible parameters.（2）In view of the demand of micro deformation monitoring in engineering,the detection of mechanical shape variables such as bending,torsion and strain by CLPFG and MTLPG were studied,which show excellent sensing characteristics.The bending sensitivity of MTLPG is-2.13 nm/m,and the strain sensitivity is 0.77 d B/mε.Owing to the spiral structure of CLPFG,it can realize sensitive sensing of several micro deformation,its torsion sensing range reaches to±720°,amd the sensitivity is as high as-61.45nm?m m?rad^-1,which demonstrates larger measurement range and cheaper cost than the reported photonic crystal fiber.The sensitivity of bending sensor by CLPFG is-9.25 nm/m,and the strain sensitivity reaches 1.7 d B/mε.At the same time,the calibration parameters of LPFG to eliminate cross sensitivity of mechanical deformation are obtained.（3）Because the two kinds of LPFGs are not sensitive to temperature,based on the theory of thermal optical coefficient and thermal expansion coefficient and the high thermal optical coefficient of graphene,it is speculated that the graphene coating can effectively improve the sensing sensitivity.The graphene coating on the surface of MTLPG and CLPFG can achieve the temperature sensing sensitivity of 0.196 nm℃^-1and 0.113 nm℃^-1,respectively,which verified the theory.In addition,the sensitivity of MTLPG and CLPFG is 0.116 nm℃^-1 and 0.09 nm℃^-1 in the high temperature range of 100-1000℃,respectively.Both of them achieved high stability and repeatable test characteristics,which has a good application prospect for temperature sensing in high temperature and narrow space environment.（4）Based on the characteristics of GO that the surface and edge exist epoxy groups such as hydroxyl,carboxyl and carbonyl,GO-MTLPG was prepared through surface functionalization of MTLPG.The GO was deposited through chemical bonding technology and optical tweezers effect.The chemical bonding effect of the oxygen-containing group on the surface of GO and the amino group of the protein forms an amide bond,which realizes the sensing of human hemoglobin samples when urea and glucose are used as interfering substances,the sensing sensitivity reaches 2nm/（mg/m L）.The limit of detection is low to 0.02 mg/m L,which is far lower than the threshold of human anemia,and it has the characteristics of repeatable testing.At the same time,based on the characteristics of the oxygen-containing groups on the GO surface showing negative charge as a whole,the sensing research of Ni²⁺,Na⁺and Mn²⁺was done,respectively.The coexisting system by using the positive and negative electric attraction is proposed.Combined with the method of surface element content analysis,with the help of characteristic peak wavelength change and transmission intensity change,the concentration sensing and species differentiation of various metal ions are realized.