Narrowband laser generation/air-coupled detection: ultrasonic system for on-line process control of composites

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.