Sensing Characteristics Of A Novel Structural Long Period Fiber Grating

Long-period fiber grating?LPFG?has attracted widespread attention of many researchers since the last century.It has many advantages that other types of sensors do not have,and has good applications in many fields.In this paper,based on the ordinary LPFGs written by high frequency CO₂ lasers,two novel structural sensors based on LPFGs are proposed,and deeply studied the sensing characteristics of these new structural LPFGs.In this paper,we use micro perturbation LPFG?MP-LPFG?and near-helical LPFG?NH-LPFG?as the research object.The spectral principle and sensing characteristics of LPFG are briefly described.The spectral principles of MP-LPFG and near-helical LPFG are further studied.The main contents include:the basic principles of LPFG,characteristics and applications of LPFGs are briefly described;the recent developments of conventional and special LPFGs are summarized.Combining the mode coupling principle,transfer matrix analysis and phase theory,the spectral characteristics of MP-LPFG and NH-LPFG are analyzed.In this paper,a sensor based on a MP-LPFG is firstly proposed,which introduces a micro perturbation structure in the middle of the conventional grating region-a certain length of multi-mode structure.The micro perturbation structure makes the transmission peak of the original LPFG split into two resonance peaks.The fabrication method of MP-LPFG is described in detail.The axial strain and temperature sensing characteristics of the two transmission peaks are studied.The two resonant wavelengths vary linearly with the axial strain and temperature in a certain range,but the axial strain sensitivities of the two resonance wavelength is low.According to the axial strain and temperature sensitivities of these two resonant wavelengths,the dual parameter measurement of axial strain-temperature can be realized.Then,the bending and temperature sensing characteristics of the two transmission peaks are studied.The results show that the bending and temperature change linearly in a certain range.Therefore,the two-parameter measurement of bending and temperature can also be realized.A torsion sensor based on NH-LPFG is studied.The sensor is a LPFG with periodic,vertical and near-helical refractive index modulation.The cladding mode of NH-LPFG is higher order than that of ordinary LPFG.In this paper,the fabrication method of NH-LPFG is introduced,the torsion characteristics of NH-LPFG are studied,and the polarization dependence characteristics are experimentally studied.The results show that the resonant wavelength shifts linearly toward the direction of long-wave in the direction of clockwise and has a blue-shift linearly in the direction of counter-clockwise.It is insensitive to the polarization state of incident light.