Fabrication Of Micro-fiber Microcavity Structure With Integrated Microtips And Study On Curvature Sensing

The optical fiber curvature sensor has been widely researched and applied especially in the fields of transportation,building structure detection,etc.The main principle of sensing is that when the optical fiber is bent,the loss of the output light intensity increases,and the change in the output light intensity or the shift of the spectral wavelength through the sensor structure realizes the sensing of the curvature.According to the interference type of the sensor,there are currently four types of optical fiber curvature sensors.One is based on grating type fiber curvature sensor,which is mainly divided into Bragg fiber grating and long period fiber grating type curvature sensor,and the demodulation method is mostly wavelength demodulation.The remaining three are classified based on the principle of interference,mainly including Fabry-Perot interferometric structure,Sagnac interferometric structure and Mach-Zehnder interferometric sensing structure.The types of demodulation mainly include wavelength or intensity solution Tune.among them.The Mach-Zehnder interferometer based on the all-fiber structure has the advantages of simple structure and fusion method,and a wide wavelength measurement range.It is an important research direction in the field of optical fiber sensing and has achieved many results.This paper proposes an all-fiber curvature sensing structure based on integrated microlenses in the fiber.Theoretically,the optical coupling theory of fiber microlenses is firstly summarized,the structure model of integrated microlenses in the fiber is established,and spherical and D-shaped fiber microlens structures are prepared.The structure is simple to prepare and has good repeatability.Theoretically calculated and compared the divergence angles of the output light of the microlens structure and flat-ended single-mode fiber,and experimentally verified that the prepared microlens structure has collimating effect.The Mach-Zehnder interference spectrum was obtained using simulation software.Then by comparing the divergence angle of the single-mode fiber at the input end and the acceptance angle of the interference structure,it is demonstrated that the optical coupling of the core in the interference area is increased due to the existence of the fiber microlens structure.From this,the light intensity coupled to the hollow fiber core and the tube wall and the coupling efficiency are calculated.According to the Mach-Zehnder interference light intensity formula,the function of the total interference light intensity with the bending angle is finally obtained.The relationship between sensitivity and various physical parameters is analyzed through a functional relationship,and the method of improving sensitivity is discussed.In terms of experiments,we built a precise cutting platform for optical fibers to achieve precise control of the length of the optical fiber interference cavity.Using the arc discharge method,a Mach-Zehnder interferometric sensing structure was prepared,and finally demodulated by interference spectroscopy to achieve a sensitive measurement of curvature.According to the interference principle of the Mach-Zehnder interferometer,the influence of the bending of the fiber structure on the interference line is analyzed.The theory shows that when the bending curvature of the sensor structure gradually increases,the difference between the light intensity of the core and the tube wall through the interference region will gradually decrease,resulting in an increase in the contrast of the interference spectrum;The change is so small that the wavelength of the interference line hardly drifts.The variation of the interference spectrum measured by the experiment is consistent with the theory.Experimentally,the maximum sensitivity of the sensor structure is-0.41 d B/m^-1 in the large curvature range of 0-15.31m^-1.It can be used to detect large curvature.Finally,we studied the influence of the interference cavity length of the sensing structure on the curvature sensitivity of the sensing structure,and found that the larger the cavity length in the interference region,the more obvious the change in the interference spectrum contrast and the greater the curvature sensitivity.