Two-dimensional laser-induced phased arrays for remote volumetric ultrasonic imaging

Three-dimensional modality is critical for ultrasonic imaging to provide a representative volumetric view of objects, for better evaluation of the shape, size, and orientation of internal features. Conventional ultrasonic transducers are still used for the majority of phased-array ultrasonic measurements. However, transducer arrays have certain drawbacks and limitations: they use a contact technique, requiring couplants. They also have a considerable size/weight/footprint, preventing use in places with restricted access and/or extreme environments. Laser-generated and detected ultrasound presents a noncontact, remote, ultrasonic imaging method. Furthermore, laser-induced phased arrays (LIPAs) have been developed for ultrasonic imaging. Data captured with LIPAs have already been used in conjunction with the delay-and-sum imaging algorithm, the total focusing method (TFM), for high-quality, 2-D ultrasonic imaging. However, there have been very limited instances of 3-D laser ultrasonic imaging in the literature and no realization of the TFM yet, even though the TFM is considered the gold standard for ultrasonic imaging. This article presents a laser ultrasonic system capable of synthesizing 2-D LIPAs, which acquire all-optical data for 3-D TFM imaging. The potential and advantages of 2-D LIPAs for volumetric imaging are demonstrated first by comparing the use of 1-D and 2-D arrays to image a crack-like defect. Following this, 3-D TFM imaging using 2-D LIPAs is evaluated by imaging a sample with 4 bottom-drilled holes. The results are compared to those using three other 3-D laser ultrasonic techniques presented in the literature: monostatic SAFT, bistatic SAFT, and a method called fixed detector.