Quantitative measurement of ultrasonic pressures with an optical heterodyne interferometer

A novel approach of acoustic pressure measurement using an optical heterodyne interferometer is described. The optical phase shift of the probe beam crossing the acoustic field is proportional to the acoustic pressure integrated along the light path. By using broadband electronic processing, one can detect quantitatively the acoustic pressure of short pulses with a sensitivity of the order of 4×10^-3 (Pa/σHz) in water and 3×10^-4 (Pa/σHz) in air. The experimental results obtained show that this optical heterodyne method offers a sensitive way of measuring the low ultrasonic pressures with a rather uniform response over a broad frequency range in the low megahertz regime. The Fourier decomposition of the probed signal provides the absorption spectrum and an example is given for air