An optical method for the detection of in-plane motion of acoustic waves propagating in solids

A new interference-based optical technique called in-plane displacement interferometer (IPDI) is introduced for the detection of in-plane (IP) motion of ultrasonic waves. The IPDI employs a tiny square indentation of about 30 microns in width on the sample surface and a relatively simple optical arrangement for the detection of IP motion of ultrasonics waves. With the IPDI, measurement of IP motions of Lamb waves propagating along a thin brass foil is demonstrated. Dispersion relations for the So mode dominated by in-plane motion in the low fd (frequency times thickness) regime is obtained with the IPDI.