Research On Vector Mechanical-strain Sensors Based On Novel Fiber Bragg Gratings

In the field of production log and oil-gas development and exploitation,the vector information of seismic waves signal and pipeline deflection contributes to the description of stratum structure,the evaluation of drilling state and the monitoring of engineering safety for the processes of resource exploration and pipeline transportation.Fiber Bragg grating(FBG)based sensors attract great interest from sensing application of petroleum industry owing to their advantages such as high sensitivity,small size,corrosion resistance,electromagnetic immunity,and multiplex capacity.However,the conventional FBG sensor cannot directly measure the vector vibration or deflection due to its own symmetric geometry and cross-sensitivity.In this dissertation,the fabrication of asymmetric-geometric FBG was proposed and experimentally demonstrated.The fabricated FBGs that have more superior sensing property and reserves classic advantages of conventional FBG can provide available solutions for vector sensing.The research contents are mainly as follows:1.The vector mechanical based on fiber interferometers and conventional fiber gratings were reviewed in the background section.And the mode-coupling characteristics of conventional FBG,cladding FBG and titled FBG were analyzed in the theoretical section.The classic fabrications of FBG were introduced,including the inscription methods and formation mechanism of FBG.The main problems of FBG writing and the precisely position control of FBG were also discussed.2.The novel eccentric FBG was inscribed on depressed cladding fiber based on femtosecond laser and precise laser-focusing control.The light-retaining capacity of the depressed cladding fiber used is weak,resulting in serious bending loss of Bragg mode.And the asymmetric configuration introduced the bending orientation-dependence for eccentric FBG.So the eccentric FBG realized a three-dimensional shape sensing via integrating the radial bending responses with axial stretch responses.3.The FBG were respectively inscribed on the core and cladding of thin core fiber(TCF)and multi clad fiber(MCF,single core and four cladding layers)based on the high-intensity femtosecond laser.Both of them excited a core mode resonance and a cladding mode resonance due to the core-mismatch induced cladding mode coupling and FBG absorption based on phase matching conditio.The cladding FBG on TCF showed great sensitivity and certain direction dependence for the static fiber-bending measurement.The cladding FBG on MCF presented significant fiber-bending sensitivity and direction dependence,which was applied to measure the static and dynamic(acceleration)vector fiber-bending.4.Based on the bending induced transient pressure inside fiber,we proposed the orthogonal-FBG configuration.(1)The orthogonal titiled fiber Bragg grating(TFBG)was demonstrated.The information of vector bending can be obtained via integrating with the bending responses of normal TFBG and the orthogonal TFBG.(2)Two orthogonal cladding FBGs were in the mentioned multi clad fiber.They were used to measure static bending and dynamic acceleration,and their direction response show trigonometric distribution and have phase difference of ?/2.The results demonstrated that their bending sensitivities versus direction were changed as sine and cosine,respectively.And their acceleration sensitivities versus direction also coincided with the regular trigonometry function.The recovery of vector information was realized with recording their responses and simple trigonometric function numeration.The intensity-based interrogation of proposed FBG breaks through the limitation of conventional FBG,improving the stability and reducing facility cost.The presented sensors show a promising application prospect for oil-industry.