Ultrasound has strong penetrability in opaque media, which makes it one of the most important detection tools for structural safety monitoring. Compared with the traditional piezoelectric transducers(PZTs) used for ultrasonic detection, fiber-optic ultrasonic sensing system based on optical adaptivity of two-wave mixing (TWM) photorefractive crystal has strong characteristics of anti-electromagnetic interference, high sensitivity and strong multiplexing, as well as the adaptive detection for optical phase modulation and permanent dynamic recording of the interference pattern current position, which has a large quantity of applications in structural health monitoring. For the interferometric properties and ultrasonic detection technology of TWM in the field of structural safety monitoring, this paper introduces the principle of adaptive two-wave mixing interferometry, the sensing devices, and methods in various fiber Bragg grating (FBG) sensor systems and dynamic signal detection of Er-doped fiber gratings and discusses the application of fiber-optic ultrasonic sensing systems in ultrasonic detection based on adaptive TWM technology. Finally, the problems and solutions in the sensing system are analyzed. By miniaturizing the TWM interferometer and integrating the entire sensing system, it is expected to further improve the detection performance of the device and expand future research directions.
A fiber Fabry-Perot etalon interrogation system is proposed for dynamic signal measurement of fiber Bragg grating sensors. The system has a voltage feedback device and is designed to perform temperature control of the fiber Fabry-Perot etalon to increase the amplitude of the demodulated signal to a high level. Data acquisition system is used to form voltage feedback to control the temperature by adjusting the power of the thermoelectric cooler loaded under fiber Fabry-Perot etalon. By controlling the temperature change of the fiber Fabry-Perot etalon, it is possible to increase the amplitude of the demodulated signal and maintain it at a high threshold. The experimental results show that adaptive temperature control has good signal enhancement effect.
A photonic integrated adaptive two-wave mixing (TWM) interferometer is designed for demodulation of FBG acoustic emission sensor signals, whose various optical elements are integrated in a substrate material with photorefractive properties. A ridge waveguide based on InP:Fe is designed. Each component of the photonic integrated TWM interferometer is analyzed and optimized to minimize the loss of light in the transmission of the TWM interferometer and obtain the best demodulation performance. The feasibility of the optimized structure of the photonic integrated adaptive TWM interferometer is verified in theory, and the optimized structure will contribute to the miniaturization and integration of the TWM demodulation system based on InP:Fe.
Acoustic emission (AE) is an effective technology that can be used for structural health monitoring. One of the most attractive features is the ability to locate AE sources. Characteristic parameters of waveform importing Artificial Neural Network (ANN) model is proposed for acoustic emission source location. The waveform of AE signal is apperceived by sensors, and decreases dispersion effect by wavelet transform. Input of ANN includes characteristic parameters of AE signal, waveform data and characteristic quantities which have been preprocessed. Time difference of signals and other parameters acts as sample which can decrease the influence of wave speed. Based on the agreement that ANN has the ability approximate any nonlinear mapping, it is feasible to build a model of time difference of signals and other characteristics with AE source position. This locating method can be widely used in AE source location on account of high accuracy, practicality and reliability.
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