Underwater surface profiling using laser generated ultrasound.

This study develops underwater surface profiling utilizing a hybrid technique with laser excitation and ultrasonic detection, in which a laser beam is scanned over the object surface and ultrasonic waves radiating from the surface are measured by an array of transducers. Using a measurement system employing a 532 nm wavelength pulsed laser, which transmits well through water, and a 1-MHz, 64-element ultrasonic linear array probe, we conducted experiments of surface profiling on an aluminum extrusion frame. The results demonstrated that this measurement technique can reconstruct several geometrical features of the cross section of the frame. However, the accuracy of the reconstruction can be affected by uncertainty in the position of the array relative to the path of the laser beam. This issue is addressed by observing that to form an image of the medium it is sufficient to use the waveforms recorded by a single array element. Moreover, the images from different elements align with each other only if the correct array position is used when forming the images. Therefore, we proposed an optimization method that finds the true position of the array by imposing that all the element-generated images align with each other.