Measurement of leaky Lamb wave velocity using Raman-Nath diffraction

A method of measuring the phase velocity of a leaky Lamb wave using Raman-Nath diffraction is investigated. Each mode of the wave with the different phase velocity in a substrate is radiated at each radiation angle into a load medium that is in contact with the substrate. An incident optical wave into the load medium can be diffracted by the radiated acoustic wave, and the diffraction image is formed in parallel to a wave vector of the radiated acoustic wave, reflecting the phase velocity of the leaky Lamb wave. By observing and processing the diffraction image stored in the form of digital matrix data, the phase velocity can be estimated. This method can be used to observe the dispersion of the phase velocity, the separation of plural modes excited at the same frequency, and the generation of the third harmonics of the wave number. This technique has the merit of measuring the leaky Lamb wave phase velocity separately, simply and precisely.