Study On The Response Of Special Fiber Bragg Grating To Directional Bending

In order to meet the requirements of more precise geology information in the exploration and development of oil and gas fields,more and more scholars pay attention to the seismic vector information acquisition.As one of the common sensors for seismic detection,the fiber Bragg grating has better sensor performance compared with traditional electrical sensors in the aspect of vector detection.Existing FBG vector sensors employ one grating to detect one direction and several gratings to draw the vector information synergistically.With advancing of oil-gas exploration,the integration and miniaturization became a vital technical indicator in the new age.Recently,the vector sensor based on single fiber has been an important research direction in the optical fiber sensing field.Therefore,this thesis primarily studies the vector bending sensor by fiber grating.The main research findings are as follows:It has been developed a fiber grating inscribed in thin core fiber with off-axis inscription.A group of cladding resonances locates in the short wavelength of Bragg resonance.To understand the off-axis grating mechanism,this structure is simulated by COMSOL5.0 soft.The vector bending test shows that the sensor perform dependence on the orientations for bending measurement.These excited LP_0nn and LP_1nn modes intensities are increased in the side grating located,being decreased in the side grating deviated.The highest bending sensitivity of LP_1nn modes attain 1.93 dB/m^-11 in the positive and-1.95 dB/m^-11 in the negative.The highest bending sensitivity of LP_0nn modes reach 1.65 dB/m^-11 in the positive and-1.33 dB/m^-11 in the negative.This sensor meets the demand of seismic vector detection.By means of the off-axis inscription with the circular asymmetry from modified fiber wave guiding and the Michelson interference with higher bending sensitivity,we fabricates a2D vector bending sensor basing on off-axis FBG and MI.With the increased curvature,this parallel structure's cladding mode trough intensity located in 1566.535nm is decreasing and wavelength is shifting to short wavelength.With the curvature increasing,the highest bending sensitivity reach 5.5dB/m^-1 from the orientation near 0^o with the intensities decreasing,the others remain-1.5dB/m^-11 bending sensitivity with incline intensities,the bending sensitivity near 180^o is-1.31 dB/m^-1.This sensor meet the technical feature of simultaneous measurement curvature and bend orientation.Meanwhile,we also present a few-mode fiber grating vector bending sensor inscribed in seven core fiber.The LP₁₁1 mode also demonstrates the orientation dependence for bending measurement.LP₁₁1 mode intensities at majority of orientations perform transition from increase to decrease.Ranging 1.448 m^-1 to 1.454m^-1,with its intensity reincreasing,the bending sensitivities at 0^o and 180^o attain 1440±95dB/m^-11 and 1347±90dB/m^-1,respectively.Its intensity is decreasing persistently at 90^o and-90^o and the lower one still remain-253±7dB/m^-11 and-216±13dB/m^-1,respectively.With above feature,this sensor can realize high sensitivity detection of seismic vector.