A simplified flash-lamp pumped high-average-power Nd:YAG Q-switched laser system based on a master oscillator power amplifier platform was developed toward outside laser remote sensing. The performance of the laser system was demonstrated, obtaining 4.7 J output pulse energy with a 50 Hz operating frequency on the optical breadboard of 1.8 m x 0.7 m size. The pulse energy from master oscillator was approximately 250 mJ with 14 ns pulse duration that was amplified by first Nd:YAG rod crystals with double pass amplification. Then, output laser pulse from first YAG rod was amplified by second and third Nd:YAG rod crystals. The beam pattern was image relayed using lens pair between all Nd:YAG rods to maintain the good beam spatial profile in rod amplifiers to avoid the optical damages induced by non-uniform beam profile. The focal lengths of thermal lens effect in each Nd:YAG rod crystal was about 2 m that were compensated by an adjustment of lens pairs. The amplified pulse laser was focused using focusing lens pair on the concrete surface to generate panel vibrations by laser ablation and/or thermal stress, acting thus as a hammer. The focal length of lens pair was approximately 7 m that is assumed the typical a tunnel roof in Japan. The energy transfer efficiency from final amplifier to concrete surface was approximately 87%, its main reason of reduction of efficiency was beam quality of master oscillator. That efficiency was 89% with only oscillator beam.
High-speed laser remote sensing of defects inside a concrete specimen was demonstrated. In the proposed measurement setup, high-power laser pulses irradiated a concrete surface to generate vibration that can be detected by an optical interferometer, which was constructed using photorefractive crystal. The laser-based remote sensing system achieved inspection speeds of 25 Hz. The predominant frequency of a mock-up defect that was embedded in a concrete specimen was measured. The inspection result was identical to that obtained using a conventional hammering method.