| Active ultrasonic detection is a powerful ultrasonic monitoring method used in the fields of structural health monitoring,non-destructive testing,material characterization,and medical diagnosis.Its core functional technologies include ultrasonic transducer and ultrasonic detection.Compared with traditional electrical methods,the optical fiber ultrasonic transducer/detector has the advantages of small size,light weight,easy integration,resistance to acid and alkali corrosion,and resistance to electromagnetic disturbance.In order to achieve health monitoring in specific and complex applications such as large buildings,bridges,and aircraft,it is often necessary to embed a large number of active ultrasonic detection points that are resistant to environmental disturbances in the structure.Simutaneously,in order to ensure stable and effective multi-point detection,each ultrasonic transducer needs to ensure a consistent signal strength and the number of energy conversion points is controllable.Therefore,how to obtain an energy-balanced multi-point ultrasonic transducer device in an optical fiber link,and how to obtain a multi-point,self-adaptive ultrasonic detection system,has become a technical problem to be solved urgently.In order to solve the above-mentioned problems,this thesis starts with the fiber-optic ultrasonic transducer and the fiber-optic ultrasonic detector respectively,and uses the method of combining theory and experiment to conduct research.The main research contents are as follows:First of all,in view of the insufficient balance of the energy conversion points in the existing optical fiber multi-point ultrasonic transducer technology,we innovatively proposed the use of peanut-shaped structure and single-mode-coreless-single-mode fiber micro-processing technology to achieve multi-point energy-controllable ultrasonic transducer solution.In theoretical research,we constructed a physical model through the finite-difference beam propagation method,analyzing the mode field distribution of the optical fiber microstructure,studying its specific energy coupling mechanism and the influence factors of energy extraction efficiency,to provide a theoretical basis for the program for obtaining a multi-point energy balanced ultrasonic transducer.In addition,we also analyzed the influence of the nonlinear effect in the optical fiber on the ultrasonic transducer,and studied how to control the specific parameters to ensure the effectiveness of the ultrasonic transducer.In the experimental research,by separately preparing the cascaded ultrasonic transducer units based on the above two optical fiber microstructures,a five-point energy-balanced ultrasonic transducer device was realized.By reasonably setting the relevant parameters of the optical fiber microstructure,the energy-balanced multi-point ultrasonic energy transducer is realized,which provides a new method for effectively controlling the number of ultrasonic energy conversion points in practical applications.Secondly,in order to be able to solve the multi-point ultrasonic detection problem in a single erbium-doped fiber laser,we carried out research on multi-point fiber lasers from the perspectives of time division multiplexing and wavelength division multiplexing.In this paper,theoretical models of multi-wavelength erbium-doped fiber lasers based on the mechanism of intensity-dependent loss,cavity loss modulation,polarization-dependent loss,and polarization hole-burning effect are established.Under this model,the mechanism of wavelength establishment under different mechanisms and the physical process of multi-wavelength realization are analyzed.The influence of factors such as cavity loss and erbium fiber gain on the multi-wavelength output characteristics under different working conditions is analyzed.Through theoretical analysis of the filtering characteristics of fiber Bragg gratings,a wavelength-switching laser by using grating scanning filtering and a wavelength-switching laser by using tunable optical filters is proposed,respectively.The above two wavelength switching mechanisms are based on the time division multiplexing feature,which effectively avoids the mode competition phenomenon of the laser resonator,and provides a new idea for the realization of multi-point switching ultrasonic detection.In addition,based on the conditions established by multi-wavelength lasers,this paper innovatively proposes a new method for achieving multi-wavelength laser oscillation through polarization-dependent loss modulation and polarization hole-burning effect,respectively.The multi-wavelength laser resonance mechanism based on wavelength division multiplexing will help to realize the ultrasonic detection of multiple points simultaneously.Finally,we separately studied the ultrasonic detection under the multi-wavelength mechanism based on the above time division multiplexing and wavelength division multiplexing.For the first time,the dynamic model of erbium-doped fiber laser with wavelength coherent gain was used to study the transient response to continuous ultrasound signal of laser resonator under different grating matching positions and different cavity losses,respectively;to study the multi-channel ultrasonic detection performance based on the mechanism of polarization dependent loss and polarization hole burning,respectively.Based on the above theoretical analysis,we innovatively proposed a adaptive multi-channel switching ultrasonic detection based on tunable optical filter and a adaptive multi-channel multiplexing ultrasonic detection based on polarization hole burning effect,respectively.Both of these schemes are completed in a single laser cavity,which overcomes the static background strain disturbances of >1400?? and >2700 ?? respectively,and both can withstand temperature changes of 20°C-50°C.This provides a solution for multi-point multiplexing in ultrasonic detection technology based on erbium-doped fiber lasers,and the anti-environmental disturbance characteristics also ensure the stable operation of the system.In summary,the thesis aims to provide new ideas and new methods for improving the performance of active multi-point ultrasonic detection by solving the problems of multi-point ultrasonic transducer/detection technology in terms of energy balance,multi-channel acquisition,and resistance to environmental disturbances.It has a good reference and guiding significance for the development trend of high efficiency,stability,and small embedded of the future all-fiber active ultrasonic monitoring system. |
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