The acousto-optic technique presented by Wevers, Devolder, Leroy, and
De Meester [Appl. Phys. Lett. 66, 1466 (1995)] was based on the inform
ation concerning the phase shift between the incident and the reflecte
d ultrasound at critical angles. Two laser beams were used, one being
diffracted by the incident ultrasound, the other being diffracted by t
he reflected ultrasound. Information concerning the geometrical shape
of the sample as well as information concerning the thickness of a thi
n layer could be obtained from measurements in the near-field of the l
ight diffraction. In this letter it is shown that using only one laser
beam, traveling through the intersection region of both ultrasounds,
exact thickness information of the layer can be obtained. Theoretical
calculations are derived and compared with practical measurements for
a 3 mu m copper layer on a stainless-steel plate. (C) 1996 American In
stitute of Physics.