Research On Vector Vibration Acceleration Sensing Technology Based On Cladding Fiber Bragg Gratings

For the difficult problems in oil and gas exploration such as poor high temperature resistance,large volume and limited series of seismic wave exploration vector detection technology,the cladding fiber Bragg grating（FBG）vector acceleration sensing technology suitable for three-dimensional（3D）borehole seismic exploration with direction recognition ability,miniaturization and reusability was studied.The 3D vector acceleration sensing based on single mode fiber（SMF）was realized by cascading z-component accelerometer.It laid the foundation and accumulated experience for the further development of the fiber optic 3D geophone array system in the borehole.The research contents of this paper are mainly as follows:1.The research status of optical fiber vector sensing technology was discussed.The mechanism and method of FBG vector sensing were compared and analyzed.The experimental platform and experimental scheme of FBG preparation in our laboratory was introduced.2.Based on the FBG vector sensing principle of cladding mode excitation,three vector bending and acceleration sensors were proposed.（1）An eccentric FBG（EFBG）was inscribed point-by-point（Pb P）in a SMF using a femtosecond laser,and the cladding mode（>40 d B）was recoupled to excite the ghost mode through an abrupt taper.Owing to the asymmetry caused by the lateral offset of the EFBG,the ghost mode showed a significant directional response to acceleration.The vector acceleration had the acceleration sensitivities of 35.62 m V/g and-35.92 m V/g in the forward and reverse directions.（2）The FBG was inscribed over the core and inner cladding of the side-hole fiber（SHF）by femtosecond laser combined with phase mask method.The transmission intensities of both resonances were highly sensitive to bending of the fiber,and the bending response showed orientation dependence.The 1D vector bending measurement with a maximum bending sensitivity of-3.41 d B/m^-1 in the core mode and-4.05 d B/m^-1 in the cladding mode was achieved.（3）The FBG was inscribed over the core and single-side inner cladding of the multi-clad fiber（MCLF）by femtosecond laser combined with phase mask method.The core mismatch excites independent core and cladding modes,and the cladding modes had excellent bending direction correlation.The dual-off-axis fiber Bragg grating（FBG）inscription over both the fiber core and cladding.The 2D vector bending measurement was realized.The maximum errors in measuring the magnitude and angular of curvature were 20.3%and 8.79°.3.Based on the cladding FBG vector sensing principle,two 2D vector bending and acceleration sensors were proposed.（1）The cladding FBG was inscribed in the cladding of standard SMF with a laser direct writing system.The reflected energy of the cladding FBG comes from the evanescent coupling of the fundamental core-guided mode.The bending also changed the coupling coefficient.The E-field of the fundamental mode of bent fiber generated a departure from the fiber center and further induced a measurable change in the reflected power.Therefore,the reflected energy of the cladding FBG responded sensitively to bending of the fiber.The cladding FBG is located on one side of the fiber cladding and rotated relative to the direction of the fiber bending,which breaks the cylindrical symmetry of the fiber.The 2D vector bending and acceleration sensing were realized based on orthogonal cladding FBGs inscribed in SMF.The maximum errors in measuring the magnitude and angular of curvature were 5.0%and 5.0°,and the minimum errors were 0.1%and 0.1°.When the resonance frequency was 30 Hz,the maximum acceleration sensitivity of the x and y components were 21.3 m V/g and 20.3 m V/g,respectively.The maximum angular deviation of the measured acceleration is 4.1°and the minimum deviation is 0.1°.（2）A 2D vector bending sensor that was proposed and demonstrated for the first time,based on an edge-core cladding-type fiber Bragg grating（ECLFBG）inscribed in an edge-core.The ECLFBG was written parallel to the edge-core using a femtosecond laser point-by-point technique.Combining the trend and sensitivity of the wavelength shift and reflection intensity variations of the ECLFBG,the bending magnitude and direction can be measured simultaneously.4.The mechanical encapsulation and sensitization structure of the fiber optic vector accelerometer were studied.A 3D vector accelerometer based on the combination of SMF orthogonal cladding FBG and z component was developed,and the 3D acceleration measurement was realized.The maximum acceleration sensitivity of the three components are 151.8 m V/g,146.3 m V/g,and 201.9 m V/g,respectively.The resonant frequencies of three components were 151 Hz,151 Hz,and 137 Hz,respectively.The sensor sensitivity and resonant frequency were simultaneously improved through mechanical structure packaging.