Bragg imaging of light diffracted by ultrasonic waves and its application to flaw detection

Diffraction of light by ultrasonic waves in water has been studied in the range of frequencies 4 MHz and 28 MHz. In the lower frequency of range Raman-Nath diffraction is observed. As the frequency increases the diffraction becomes more and more analogous to x-ray diffraction by crystalline lattices and hence is referred to as Bragg diffraction. In the Raman-Nath range diffraction is optimum at normal incidence. In the Bragg range diffraction is optimum for nonnormal incidence. For light incident at the angle of optimum Bragg diffraction images of the ultrasonic wavefront appear in the diffraction orders. One image is observed in the first orders and two images appear in the second orders. The theory of Blomme and Leroy (Acustica 59, 182-192, 1986) is used to analyze the Bragg diffraction. It is adequate to explain imaging in both the first and second orders.

A practical application to flaw detection is made by using Bragg diffraction to image irregularities inside parallel plates of metal or glass.