In our prototype system, we built in 2 channels with two pulsed YAG lasers for generating, and 5 channels with a CW green YAG laser, an optical switch, and a Fabry-Perot interferometer for detecting ultrasound. This multi-channel generation and detection can make only the required signal emphasized by synchronous summation. That system achieved improving SNR of ultrasound and controlling the refractive angle within 10deg. Moreover, we applied optical fiber transmission of laser light on both the transmitter and receiver sides and minimized a sensor head up to 35m x 35mm x 55mm. We applied the system to detect artificial slit defects in SUS pipes with 10mm thickness.
Test specimens consist of flat, weld, and tapered parts.
We generated a longitudinal wave and detected a share vertical wave after mode conversion at the defects with 60 deg. Moreover, the system could make the noise level from a weld zone and a bottom surface reduced and contributed to clearly distinguish defect signal. We could confirm applicability of the laser UT system with phased array technique to thin-wall pipes using test pieces. Applying the phased array had great effect to control the sound field and reduce ultrasound noise.