3H-5 Photoacoustic Tomographic Characterization of Surface and Undersurface Simulated Defects with a Line-focus Laser Beam

In this paper, the analyses of both surface and undersurface photoacoustic (PA) tomographic imaging with a line-focus laser beam were formulated. Since PA signal in solid specimen is dominantly proportional to the local absorption coefficient of the specimen alpha (x,y) times laser beam intensity divided by the modulation frequency, illumination of the specimen with a line-focus laser beam in PAT corresponds to the summation of the absorption coefficient along some inclined direction thetas measured from some axis (x-axis). A second harmonic green laser beam of a LD-pumped YAG laser was expanded and focused on a specimen by concave and cylindrical lenses, respectively. The CT scanning (rotation and translation) of a laser beam on the specimen surface was achieved by a mechanical rotating stage and a mechanical stepping stage controlled by a computer. The size of a laser beam on a specimen was 25 mm (length) times 1 mm (width). The measured area was 26 mm times 26 mm, while the reconstructed area was 18 mm times 18 mm. Rotation and translation steps were 2pi/64 and 26 mm/64, respectively. The experiments for the simulated surface defect with shapes of a cross and a semicircle with a diameter of 13 mm resulted in the reasonable reconstruction of the original images