Differential two-wave mixing interferometer for crack detection in metallic structures based on laser-induced ultrasound

Abstract

To meet the need of high-sensitivity crack detection in metallic structures with a contactless method, a differential two-wave mixing interferometer based on two nonlinear Bi12SiO20 photorefractive crystals is proposed for laser-induced ultrasound signal measurement. The two Bi12SiO20 crystals are applied with the electric fields of opposite polarities to obtain the dual-path differential signals corresponding to the laser-induced ultrasound under detection, in which the noise is not affected by the polarity of the applied electric field. The signal noise can be effectively suppressed and the sensitivity is doubled by the differential processing of the dual-path signals, by which the high-sensitivity detection of laser-induced ultrasound can be achieved to characterize the crack in metallic structures. The further optimization of system parameters is performed to obtain the optimal detection sensitivity. Both the numerical analysis and experiments have been carried out to demonstrate the feasibility of the proposed method. The proposed system provides a powerful and reliable tool for contactless detection of laser-induced ultrasound and characterization of metal cracks.