Study On Fabrication And Sensing Characteristics Of Long-period Fiber Gratings Based On Structural Modulation

Optical fiber sensor has attracted great interest for researchers due to the advantages of anti-electromagnetic interference,long life,and high reusability compared with the sensors based on electricity and magnetism.With the complexity of the measurement environment and the urgency of the measurement demand,optical fiber sensors are requested to be higher sensitivity,compact structure and flexible preparation methods.Therefore,various types of optical fiber sensors have been proposed and fabricated.In order to overcome the shortcomings of large backward reflection and low sensitivity of fiber Bragg gratings,a long-period fiber grating coupled with core mode and cladding mode is proposed.The LPFG has great potential applications due to its advantages of easy fabrication,high sensitivity,and compact structure.When the external environment changes,the physical parameters such as torsion,strain and bending can be accurately measured by monitoring the spectral changes of LPFG.Numerous studies have shown that the LPFG is more sensitive to changes in the external environment after special design.In this thesis,to improve the sensing performance of LPFG in torsion,bending,strain and others,LPFGs based on structural modulation with flexible structure and high sensitivity are fabricated.The sensing characteristics of the near-helical modulation LPFG(NHM-LPFG),the defective-type LPFG(DT-LPFG),and the special-shape core LPFG(SC-LPFG)have been studied experimentally.Meanwhile,to solve the problem of temperature cross-sensitivity,LPFG composite structures with bimodal spectral characteristics are proposed to realize the dual parameters measurement.The research contents of this thesis are as follows:1.Based on the helical refractive index modulation,we propose three NHM-LPFGs including the segmented-LPFG,the dislocation-orthogonal-type LPFG(DOT-LPFG),and the helical polished LPFG(HPLPFG).These sensors have overcome the problem of concentric rotation during fabrication.The gratings have the advantages of stable fabrication method and flexible periodic structure.By optimizing the grating parameters and fabrication method,the segmented-LPFG and HPLPFG have higher torsion sensitivities,the DOT-LPFG measures the torsion with polarization-independent.2.The DT-LPFGs realize the measurements of bending,torsion,and temperature,which are fabricated by embedding different defects into the center of the LPFG.The defects including multimode fiber and pre-twisted structure are embedded in LPFGs introduce the change in phase.The stable two parameter measurement is realized by monitoring the change of two dips in the spectrum.At the same time,the multimode fiber embedded in LPFG has higher curvature sensitivity.The pre-twisted structure embedded in LPFG has ability to improve the torsion sensitivity.3.A novel method for the fabrication of grating is proposed.Firstly,a fiber sheet was formed by polishing.Secondly,the fiber sheet was periodically heated using the filament heating method.Lastly,the SC-LPFG was finished.The SC-LPFG has realized the measurement of strain,torsion,vector bending with high sensitivities.By optimizing the sensor parameters,the sensing performances of the grating are improved.In particular,the strain sensitivity of the grating can be as high as-18.51 pm/??.4.In order to solve temperature cross-sensitivity,three composite structures based on LPFGs are proposed.These dual-peak sensors achieved simultaneous measurement of torsion/strain and temperature,simultaneously,which formed by segmented-LPFG and a fiber bitaper cascades,S-type LPFG and common LPFG cascades,and a pair of DOT-LPFGs with opposite torsion direction.The composite structures have high resolution of torsion,strain and temperature.In this thesis,we propose some methods to fabricate LPFGs based on structural modulation to improve the sensitivities of torsion,curvature and strain.At the same time,the DT-LPFGs and composite structures can avoid the temperature crosstalk problem in the measurement process,and the stability of double parameters measurements has been verified.We have done great efforts in the novel fabrication technology and sensing mechanism of the LPFG.