Extracting Rayleigh Waves from Noise Using a Differential Optical Interferometer

This article introduces a new strategy for extracting Rayleigh waves from a noisy environment using a differential optical interferometer (DOI). The DOI consists of a bidirectional coupler; one arm, positioned over a test specimen, senses an acoustic signal immersed in noise while the other arm, positioned over an acoustic isolator, senses the noise alone. Analytical arguments show that when the coupler is designed properly, the phase shifts between the signals returning to a photodetector produce an intensity modulation that is directly proportional to the pure acoustic signal. The analysis is verified when the DOI is used to track the surface displacement of a piezoelectric transducer. Then, random noise is introduced into an aluminum test specimen using a motor equipped with a rotating eccentric cam. The DOI is used to subtract the relatively high-amplitude but low-frequency mechanical vibrations of the test surface from a combined signal that includes the desired Rayleigh wave. Finally, an experimental configuration is demonstrated which allows an acoustic signal to be extracted from noise of a similar frequency.