Improvement in contactless generation of ultrasound with an array of thermoelastic sources

High focusing and wide angular scanning of ultrasonic beams have been achieved by using an array of photothermal strip sources. The phased array was implemented from a multiple beam, YAG laser providing 16 time-delayed optical pulses. Ultrasonic pulses are focused in any direction in the solid or at its surface by applying the appropriate time-delay law between the laser pulses. For longitudinal waves, acoustic signals and directivity patterns have been probed on duraluminum sample by using either an optical probe or a piezoelectric transducer. Experimental results were compared with computations taking into account the diffraction effects due to the thermoelastic source shape. Then, it is shown that bulk defects can be detected by using longitudinal waves, the amplitudes of which were 24 dB enhanced by focusing