Producing deep depth of field and depth-independent resolution in NDE with limited diffraction beams

Limited diffraction beams, such as Durnin's J0 Bessel beam, are a class of nonspreading solutions to the isotropic/homogeneous scalar wave equation. These beams can be approximately produced with finite aperture and energy over a deep depth of field. In this paper, we report the application of a broadband J0 Bessel beam to nondestructive evaluation (NDE) of materials. Pulse-echo images of a stainless steel block phantom were obtained with both the J0 Bessel beam and a conventional focused Gaussian beam. Results show that uniformly high resolutions were obtained with the J0 Bessel beam over a large distance. In addition, the lateral resolution of the J0 Bessel beam is almost independent of the speed of sound of the materials inspected. In contrast, the lateral resolution of images obtained with the conventional focused Gaussian beam changes dramatically with the distance and the focal length of the beam in water is greatly reduced by the steel block. Therefore, limited diffraction beams could be useful for nondestructive evaluation of materials of different speeds of sound. Restoration of pulse-echo images obtained with these beams could be simplified.