The generation of narrow‐band and directed ultrasound using spatially and temporally modulated arrays

Abstract

Owing to their generally poor sensitivity relative to conventional contact ultrasonic methods, laser‐based ultrasonic systems have proven to be effective for only a limited range of applications, and only then with very careful and specific designs. Recent investigations at The Johns Hopkins University have been directed at the use of spatial arrays and temporal laser beam modulation to generate ultrasonic signals which are both narrow band and may be directed over some angle within the test piece. The degree to which sensitivity may be enhanced by these methods, however, is a strong function of the temporal and spatial nature of the laser array source as well as the acoustic mode one wishes to excite. For example, variations in the dimension of each element in an array, array spacing, and array source rise time may all affect dramatically the degree to which one is able to generate narrow‐band signals for high‐sensitivity detection. Ultrasonic directivity issues are also somewhat more complicated than they are represented in much of the current literature. In fact, the degree to which the ultrasonic energy may be directed by laser array sources of any type is a strong function of the wave shape generated by each element in the array. Consequently, individual features such as pulse height or pulse repetition, which may be derived by superposition of the signals from elements of an array, may be directed over a range of angles in a solid material while total far‐field energy directivity may remain unchanged.