Grating-lobe Suppression through Angular Weighting for Laser Induced Phased Arrays

Abstract

Laser Induced Phased Arrays (LIPAs) use lasers to generate and detect ultrasound, synthesizing the array in post processing. Data acquisition is done remotely, without couplant, addressing NDE challenges of inspection on complex structures and under extreme environments. Previously highresolution ultrasonic imaging of components using LIPAs have been demonstrated by capturing the Full Matrix and employing the Total Focusing Method (TFM) as imaging algorithm. However, the Full Matrix Capture data acquisition method requires long acquisition time due to the mechanical scanning of lasers, compromising the application potential of LIPAs. It is possible to increase the data acquisition speed by reducing the number of array elements. Nevertheless, this can lead to the generation of grating lobes (GL), when the Nyquist sampling limit is exceeded for the interelement spacing of the array. In transducer phased arrays, GL can be suppressed by applying an angular limit in the TFM algorithm. This implementation is ideal for omnidirectional directivity centered around the surface normal, which is not the case for LIPA elements. In this work we are using an adaptive angular weighting factor that is suitable for LIPAs considering LU characteristics.