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.
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.
A method using fiber Bragg grating (FBG) array-based shape sensing is proposed for structure health monitoring of cylindrical marine structures in this paper. Because of the characteristics of FBG, any strain applied on the FBG is encoded as a wavelength shift of the light reflected by FBG. FBG sensors, with flexibility and small size, are bonded on the surface of the cylindrical shell structure to measure the strain of each fiber Bragg grating on the same certain points. And then we analyze the relationship of each parameter between FBG sensors and cylindrical structure. Experimental results prove that it is feasible for this method using FBG array-based shape sensing to monitor the deformation of the cylindrical structure. This method can be widely adopted in structure health monitoring of curved structures due to high precision, simple operation and low cost.
Ring resonators and Mach-Zehnder interference structures are promising candidates for compact optical filters and electro-optic modulators in the field of integrated photonics. Two types of ring-loaded Mach-Zehnder interferometers (MZIs) based on 3D direct laser writing are designed by finite-element simulation software. The model is composed of two Y-waveguides, Mach-Zehnder waveguide arm, and a micro-ring coupled with Mach-Zehnder waveguide arm side. The optimal radius of curvature of the two models and the spectral characteristics of the two models are calculated by numerical analysis. The optimal radii of curvature for the bend of Y-waveguides are 385 μm, the average free spectral ranges (FSRs) of are about 18 nm, and the average full widths at half maximum (FWHM) are about 1.4 nm and 3.2 nm, respectively, for the two MZI models. The numerical analysis results have practical reference value for the fabrication of resonator coupled Mach-Zehnder interferometer using 3D direct laser writing technology.
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