Measurement and 3D imaging of acoustic waves through the acousto-optic effect has recently been demonstrated by means of Optical Feedback Interferometry (OFI). In this paper we study experimentally the lower limits of detection of an acoustic wave using an OFI sensor. We show that the OFI sensor exhibits a linear response to acoustic power variations, and we obtain a lower limit of detection of 83 dB rms for a planar acoustic wave at 3 kHz. We also determine the equivalent displacement, that is seen by the OFI sensor at this pressure level, to be 96 pm. A deeper understanding of the limits of the technology and the quantification of the acousto-optic effect shall help improve the applications already created for the measurement of acoustic pressure waves using OFI.

It is established in the Optical Feedback Interferometry (OFI) theory that the shape of the interferometric fringe has an impact on the detector's response to very small displacement measurements. In this paper we validate -for the first time based on experimental results -this statement by comparing experiments to an established model implementation. Through these experiments we show that the amplitude of the signals induced by sub-lambda/2 optical path variations is linearly dependent on the slope of the underlying fringe. Thus, careful control of the phase allows us to maximize the detection amplitude of very small displacements by positioning the phase where the fringe slope is the steepest. These results are directly applicable to established OFI applications that measure sub-lambda/2 optical path variations, such as OFI vibrometers or acoustic imaging though the acousto-optic effect.