Single-mode guided wave technique using ring-arrayed laser beam for thin-tube inspection

The laser ultrasonic technique has been considered as a useful, non-contact method to launch guided waves in thin tubes. Then, in practical applications of the guided wave technique, it is very important, though often difficult, to identify propagating modes. Thus, the guided wave technique is preferred when it comes to launching and detecting a single mode. In this paper, we propose a method using a ring-arrayed laser beam that enables the generation of only the specific axisymmetric mode uniformly in the circumferential direction. This method uses an air-coupled transducer as a detector, which is able to detect only a single mode by tuning its detection angle. Experimental results for tube specimens proved the mode selectivity, uniformity of wave intensity in the circumferential direction, and less amplitude drop along the propagation distance compared with the conventional line array beam method.