Optical detection of weak spatial modulation in acoustic pulses by applying 2-dimensional Fourier transforms

摘要

This paper deals with the application of schlieren techniques to the optical diagnostics of ultrasonic pulses, passing a grid target. We used bright field schlieren method with the image post-processing by 2-dimensional Fourier transform (2D FT). In case the basic wavelength of the pulse is larger than the grid spatial period, the ultrasonic wave does not resolve the grid and the corresponding modulation is absent in the schlieren images. However the pulse contains also the super-harmonics of its basic frequency, while their wavelengths, which are smaller than the grid period and capable therefore to resolve the target. The resulting modulation is hardly visible in the schlieren images, but could be easily revealed with the 2D FT. The quantitative interpretation of the schlieren images and their FT are presented in the paper as well.