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A hybrid laser generation/air-coupled detection ultrasonic system is presented which capitalizes on matching the source and receiver bandwidth. Laser generation of narrowband (approximately 1.2 MHz) ultrasound is achieved through a spatial modulation technique. The frequency of the generated ultrasound is chosen to match the response characteristics of a micromachined air-coupled capacitance transducer for good signal throughput. The system has proven sensitive to surface defects in composite materials, and is well suited for on-line process control applications, with standoff distance for the transmitter and receiver greater than 2 cm, while retaining single-shot operation. The system provides a significant reduction in cost and complexity relative to laser-in/laser-out systems.

This paper demonstrates the use of a fiberoptic bundle for flexible, compact, remote, and noncontact laser generation of surface ultrasonic waves in materials. The bundle is able to deliver Nd:YAG pulsed light with a 60% delivery efficiency up to an average energy of 55 mJ/pulse for a pulse duration on the order of 10 ns and a pulse repetition rate of 20 Hz without signs of fiber damage. Details of the bundle construction and surface preparation are given, and pulsed light delivery tests performed with single tapered fibers are discussed. The high-power light delivery capabilities of the bundle are demonstrated for the generation of narrow-band surface waves in a Carbon/PEEK composite laminate by a spatial modulation technique that employs a periodic transmission mask. Single laser pulse ultrasonic tonebursts are clearly detectable using a small aperture piezoelectric transducer while ensuring thermoelastic generation conditions. The theory of narrow-band generation of surface acoustic waves is improved by accounting for the strength nonuniformity of the illumination sources. In addition, the effect of the number of illumination sources on the bandwidth of the generated surface wave is assessed experimentally, and excellent agreement is shown with the theoretical results predicted by the improved model.