Simulation of the Array Signals Processing Based on Automatic Gain Control for Two-Wave Mixing Interferometer

Laser ultrasonic testing represents an attractive method for non-contact measurements with high sensitivity and broad detection bandwidth, which make it possible to measure ultrasound field in some offensive environments. Because of the noise caused by serious scattering, the conventional optical interferometer cannot work properly on rough surfaces. The environmental noise is also a big uncertainty budget for the measuring results. In this paper, an array signals processing method for a new type of two-wave mixing interferometer is introduced. In order to calibrate the displacement of the target, a mirror driven by the piezoelectric transducer introduces a known displacement at a low frequency on the reference beam. In addition, the wavelet neural network(WNN) is applied to denoise the input signals. Moreover, an automatic gain control (AGC) scheme is employed to the output of the photodiode array which can calibrate the output sensitivity. Then according to the output sensitivity, the displacement of the target measured by the diode array can be demodulated. Both the out-plane and in-plane displacements can be measured simultaneously, which may result in a wide usage in non-destructive areas.