Optical probing of pulsed, focused ultrasonic fields using a heterodyne interferometer

The pulsed acoustic field of a piezoelectric focused transducer transmitted in water has been investigated by using an optical heterodyne interferometer. The probe beam is reflected by a thin membrane that follows the motion of the fluid particle. Absolute measurements of the mechanical displacement have been performed in the frequency range 5–15 MHz with a spatial resolution better than 0.1 mm and a sensitivity of 0.1 nm with a 20 MHz detection bandwidth that corresponds to a minimum detectable acoustic energy density of 2 μJ/m2. The experimental results all agree quantitatively with diffraction theory predictions of impulse response at the focus of the transducer.