Research On High-precision Strain Sensing Technology Based On Novel Fiber Bragg Grating

During the development of modern civilization,oil and gas resources are the important energy.In order to get the better exploration,efficient development and production process,the accurate measurement of the strain information has the very important and significant role.Fiber Bragg grating（FBG）based strain sensing technology,comparing with the traditional electrical strain sensors,have great development potential and widely application future owing to the advantages such as electromagnetic immunity,corrosion resistance,high temperature and pressure resistance,good electrical insulation and multiplex capacity.In this dissertation,based on the high peak power femtosecond laser and phase mask technique,novel FBGs are fabricated and the problems of high precision,direction dependence and multi-parameter measurement are studied.Two fiber bending（displacement）sensors are proposed by using the novel FBGs inscription technique over few mode fiber（FMF）.A simultaneous strain and high-temperature measurement sensor is demonstrated based on dual FBGs written in single mode fiber（SMF）and thin-core fiber（TCF）.An early diaphragm pressure sensor construction is designed and built based on FBG.The research contents are mainly as follows:1.The developments of the optical fiber sensor technology and fiber grating are reviewed.The research status of the fiber strain sensor and some representative and novel vector strain sensors based on the fiber grating are introduced.The characteristics of the FBG are analyzed in the theoretical section.The mechanism,system construction and inscription methods of FBG are written by femtosecond laser.The experimental system of FBG fabrication by femtosecond laser in laboratory is described in detail.2.Based on the index profiles of the FMFs,two inscription methods,at center and off-axis of the fiber core,are proposed to inscribe FBG by femtosecond laser and phase mask technique.The coupling characteristics and bending（displacement）mechanism of the FBG written in the FMF（FM-FBG）are researched.Two kinds of bending（displacement）sensing devices are demonstrated.They both are based on the FM-FBG and use the reflection intensity of the coupling mode(LP_01-11)for bending（displacement）sensing.（1）A fiber bending（displacement）sensor based on the center FBG,which is inscribed in a four-mode fiber（four-mode FBG）is proposed.In this device,the FBG is used as a“mirror”,but a section of four-mode fiber is used as a sensing element.By analyzing the reflection spectrum,a suitable length of transmission path of four-mode fiber is selected.The reflection intensity of the coupling mode is measured for different bending（displacement）,which is highly orientation-dependent（0°-360°）to fiber bending（displacement）.And the wavelength information of coupling mode can monitor the temperature change.（2）An orientation-dependent fiber bending（displacement）sensor based on the off-axis FBG over a two-mode fiber（two-mode FBG）is demonstrated.Compare with the four-mode FBG,this sensor has a higher bending sensitivity and better response of the orientation-dependent.The wavelength-referenced temperature information could also be demonstrated together with the fiber bend.3.A compact simultaneous strain and high-temperature measurement scheme using dual FBGs written in welding section between the SMF and TCF by the femtosecond laser and phase mask technique.The total length of this dual FBGs is only 4 mm.The different effective refractive indexes between the cores of SMF and TCF are the key point because such FBGs support two separate resonances in reflection spectrum for sensing measurement.Etching process is utilized on the grating of TCF to increase the sensitivity differences of the dual FBGs.High annealing process is introduced in the dual FBGs to improve the thermal reliability and stability.When the diameter of the grating of TCF is 70μm,high strain sensitivity of 3.25 pm/μ?and high temperature up to 800°C.4.Based on the combination of the diaphragm and FBG,an early strain sensor prototype sample structure is designed and experimentally demonstrated.The methods such as the designing of the whole mechanical structure,the selection of the material and the designing of the thickness and diameter of the diaphragm and mechanics simulation are carried on for accomplishing the design.The pressure measurement range of this strain sensor prototype sample structure is 0-40 Mpa with the measurement accuracy of 0.073%.The significance of this work is to provide some good experiences and basis for the applied research,and most importantly the further practical application of fiber strain sensor of our laboratory.