Research On Mechanical Parameters Sensing Characteristics Of Microscale Optical Fiber Mode Coipling Effect

Due to the positive effects of optical fiber sensors such as resistance to strong acid and alkaline environment,anti-electromagnetic interference,and fast response,it has demonstrated the importance in the fields of structural health monitoring,artificial intelligence and microscopic phenomenon.The modes of the actual fiber have the property of coupling with each other in two ways:transverse mode coupling between two fibers close to each other and longitudinal mode coupling with uneven geometric distribution in the longitudinal direction of a single fiber.In this thesis,by analyzing the shortcomings of current mechanical parameters sensors,we devote to the research to achieving sensitivity and reusability compatibility;fast identification of mechanical parameteres;stability enhancement in narrow environment and liquid surface tension sensors with enhanced sensitivity and quantified indexes.The main research contents are as follows:（1）Contrapose the problems of weak robustness and low sensitivity of strain sensing in transverse mode coupled structures,an improved Optical Microfiber Coupler（OMC）structure was proposed.Firstly,an OMC strain sensing model was established using silicone sealant,and the enhancement of OMC strain sensitivity due to encapsulation was demonstrated by theoretical analysis and numerical simulation.Then the strain response characteristics of OMC with different waist diameters were investigated,and the maximum strain sensitivity is 18.351 pm/μεin the strain range of 65.75-1315.4με.Finally,the simultaneous measurements are realized by the difference of wavelength response to temperature and strain using the matrix equation.（2）Aiming at the problems of the limitations in confined spaces and the differences in installation of transmissive MZIs in longitudinal mode coupled structure,the microsphere array MIs bending sensing structure with end face protection was proposed.Firstly,the sensing model was constructed by cutting and splicing to improve the practicality while realizing the bend measurability.Then the bending sensing response of MI structure with different number of microspheres was investigated,and the effect of different installation positions of the coupled excitation unit between stepper motors on the bending sensitivity is verified,and the maximum curvature measured is 7.726 m^-1.Finally,the ratio of intensity and wavelength response are used to achieve the identifiable strain and curvature.（3）To further explain the uncertainty of the coupling excitation unit installation and stimulate the cladding modes,a section of MMF was introduced into the MI sensing structure.Firstly,the MMF peanut-like MI bending sensing model was constructed,the end-face package scheme was optimized,and the influence of the presence or absence,length and numerical aperture of MMF on the mode coupling was analyzed.Then,the mechanism of bending turning point,a new phenomenon caused by the mode transformation,is analyzed.Finally,the effect of different lengths and numerical apertures of MMF and installation position on the sensitivity and bending turning point was investigated.The value of the bending turning point allows a quick estimation of the curvature.（4）Due to the ability of optical fiber to measure strain,refractive index,and bending,an LST sensor by reducing the stiffness of the single-mode fiber was proposed.Firstly,the tapered single-mode fiber was encapsulated by a rectangular semi-sealed cantilever beam to enhance the practicality and eliminate the error caused by vibration.Secondly,the spectral changes of the LST sensor during the processes of immersion and leaching the droplet were investigated,the visualization of microscopic phenomena is achieved according to the variation of the spectra.Then the spectral response of the LST sensor during the leaching out different refractive index solutions is investigated.Finally,the RI and LST of known water and isopropanol solutions are used as references to achieve the estimation of RI and LST of unknown solutions.In this paper,the ability to quickly identify the force state in which the mechanical sensor was achieved by demodulation of specific parametric quantities and identification of special points.In addition,the visualization of microscopic phenomena was solved by the measurability of mechanical parameters.The ablilty fast identification of mechanical states,robustness and high sensitivity co-occurrence were achieved,and some deficiencies in the measurement of microscopic were compensated,which is important for the optical fiber sensing structure to achieve a wide range of measurements.