Research On Multipoint Fiber-optic Laser-ultrasound Generation Based On Core-cladding Mode Coupling Mechanism

All-fiber optic ultrasonic sensors have attracted much attention because of their small size,light weight and electromagnetic interference immunity.Fiber-optic laser-ultrasound transducer is a key part of all fiber optic ultrasonic testing system.Up to now,most currently used fiber-optic ultrasonic transducers allow effective ultrasound generation at only a single location,namely,at the fiber tip,although there have been a few limited proposals for achieving multipoint ultrasound generation along the length of a fiber,such as unbalanced strength ultrasound signal,limitation of generated points.The challenge so far has been to find a means to effectively extract the light signal from specific locations on the optical fiber sidewall for ultrasound generation.In this dissertation,the potential applications of multi point fiber laser ultrasonic transducers in the health monitoring of all fiber structures are analyzed by investigating the research status of fiber laser ultrasonic excitation.Aiming at the disadvantages of current multi point fiber-optic laser-ultrasonic transducers,we proposed a method of multi point fiber optic laser-ultrasound generation based on core-cladding mode coupling mechanism,and analyzed 5 kinds of fiber micro structure for achieving core-cladding mode coupling.Then these 5 kinds of structures were used for the preparation of multi point optical fiber laser ultrasonic excitation transducer.The contents of the proposed and implemented are as follows:In simulation,by using the finite element beam propagation method and Rsoft software,we established the model of fiber core-offset structure,core-opened taper,waist-enlarged taper,and completed the simulation of light field distribution and coupling ratio analysis.It is demonstrated that the three structures can effectively derive the laser to the cladding in the fiber core of the single-mode fiber.The coupling ratio of the corresponding structure can be controlled respectively by controlling the misalignment of the fiber core-offset structure,taper length of core-opened taper structure,and diameter of waist-enlarged taper at the waist region.The relationship between the radiation energy and the bending radius of the fiber bending structure is analyzed,and the feasibility of the fiber 90 degree bending structure for multi-point laser ultrasonic excitation is demonstrated.In experiment,in manual mode of fiber fusion splicer,fiber core-offset structures,core-opened tapers and waist-enlarged tapers of special coupling ratios are prepared.Multi-point fiber laser ultrasonic transducers based on these three structures are proposed and fabricated.A multi-point laser ultrasonic transducer based on fiber 90 degree bending structure was fabricated by effective experimental method.A multi-point laser ultrasonic transducer based on the different coupling depth of ghost modes of tilted fiber Prague fiber is proposed and fabricated.3-point excitation on a single optical fiber is realized,and the excitation point can be increased by 3 times by combining with the previous method based on different phantom wavelength.In summary,this dissertation proposes to use the 5 kinds of fiber micro structure for achieving core-cladding mode coupling,and solved key problem that the fiber core energy is quantitatively exported to cladding.The proposed transducers can generate balanced ultrasonic signal,improve the number of excitation points.They also have the advantages of simple preparation method,low equipment requirement and low preparation cost.The research content has important application value in the field of all fiber structure health monitoring.