Research On Curvature Sensing Characteristics Of Substrate Encapsulated And Self-assembled Optical Fiber Structures

Optical fiber sensing has the advantages of anti-electromagnetic interference,corrosion resistance,good biocompatibility,and remote monitoring,so it has developed rapidly in many application fields such as harsh environments and biomedicine.In optical fiber sensing,the high sensitivity and vector measurement of curvature are of great significance in the fields of artificial intelligence and invasive medicine.However,the existing optical fiber curvature sensors have shortcomings such as low measurement sensitivity,unfavorable miniaturization,and measurement blind spots,which are difficult to meet practical application requirements.In this paper,theoretical and experimental researches on substrate-packaged and self-assembled optical fiber sensors are carried out,aiming to provide a simple preparation and high-sensitivity method for vector measurement for fiber curvature sensing technology.The main research contents of this paper are as follows:（1）Aiming at the problem that the axis-symmetrical distribution of the sensing fiber is not sensitive to the bending orientation,a curvature sensing structure of substrate package is proposed.Firstly,the sensing structure model of the substrate package is established,and the theoretical analysis and numerical simulation are carried out.Secondly,the sensing sensitivity expressions of the packaged Fabry-Perot interferometer（FPI）structure and the packaged fiber Bragg grating（FBG）structure are derived.Then,the vector curvature sensing characteristics and the axial stress cross sensing characteristics of the two fiber structures are experimentally studied,and the effects of FPI cavity length and packaging operation on the curvature sensitivity are explored.（2）To solve the problem of measurement blind spots in multiple bending orientations or specific bending orientations in dual-fiber sensing structures and substrate packaging structures,a triangularly distributed vector curvature sensing structure is prepared by self-assembly.The theoretical model and mechanical properties of the sensing structure are analyzed,and the vector curvature sensing characteristics of the sensing structure are studied.In the bending sensing experiments with orientations of 0-300o,the curvature sensitivities of the two fiber FPIs have different trends and do not tend to 0 at the same time,so the sensing structure has the potential to realize bend sensing in any orientation.（3）To solve the contradiction between high sensitivity and miniaturization of the sensing structure,a compact sensing structure with suspended core fiber and hollow core fiber in series is proposed.Firstly,the beam transmission and spectral characteristics in the sensing fiber when light is incident from both ends of the cascaded FPI are analyzed.Secondly,the influence of the incident direction of the beam and the Vernier effect on the sensitivity of curvature sensing is studied.When the beam is incident on the sensing fiber from different directions,its spectral characteristics and sensing sensitivity are different,but the curvature response characteristics are the same.In addition,the vector curvature sensing characteristics of the sensing structure are explored.When the sensing structure is bent in the opposite direction,the spectral response of the sensing fiber is opposite,but the curvature sensitivity is similar.The Vernier effect improves the curvature sensitivity by 7.61 times without increasing the volume of the sensing structure.Then,the effect of compact tandem and self-assembly on miniaturization is explored.Finally,by analyzing the spectrum and response characteristics of FBG and cascaded FPI,it is proved that the sensing structure can realize the simultaneous measurement of temperature and curvature.（4）Aiming at the problem that the intra-cavity refractive index change of hollow-core fiber FPI structure is usually ignored in the curvature sensing experiment,a sensitivity enhancement scheme of the photosensitive glue-filled fiber sensing structure is proposed.Firstly,the effects of elastic-optical effect and Vernier effect on improving the sensitivity of curvature sensing are theoretically analyzed.Secondly,the structure model of the photosensitive adhesive-filled sensing fiber is constructed,and its beam transmission and spectral characteristics are discussed.The curvature sensitivity of the Vernier effect envelope is-43.014 nm/m^-1 through the curvature sensing experiment,which is 2 orders of magnitude higher than that of the air cavity FPI.Finally,experiments show that the sensing structure also has a sensitizing effect in temperature detection.In this paper,substrate-packaged and self-assembled curvature sensing structures are proposed,which simplifies the preparation process and realizes vector curvature sensing.The sensitivity of the sensing structure is improved by using the elastic-optic effect and the Vernier effect,which solves the contradiction between high sensitivity and miniaturization.To a certain extent,it makes up for some shortcomings of the fiber curvature sensing structure,which is of great significance for the application of the sensing structure in the fields of artificial intelligence and medical detection.