Lower detection limit of the acousto-optic effect using Optical Feedback Interferometry

Abstract

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.

Self-Mixing Interferometer for Acoustic Measurements through Vibrometric Calibration

“…Disturbances of reference beams or installation effects such as sound wave diffraction on optical parts may also occur. In contrast, the Self-Mixing Interferometer (SMI) [ 3 , 10 , 11 , 12 ], also known as Optical Feedback Interferometer (OFI) [ 12 , 13 , 14 ], is a self-aligned interferometer with only two elements in a single component: a laser diode (LD) and a photodiode (PD). The LD targets a retro-reflective surface which back-scatters part of the emitted photons into the LD cavity, where interferences occur from this optical feedback.…”

“…The latter can be estimated by measuring the LD power using the embedded PD, the current of which is converted into voltage to form the SMI signal. Thus, it is possible to use the SMI both as a vibrometer [ 3 , 15 , 16 , 17 ] or as an acousto-optical sensor [ 12 , 13 , 14 , 18 , 19 ].…”

“…However, few studies were focused on its use for Sound Pressure Level (SPL) measurements. A proportional relationship between the amplitude of the SMI signal and the acoustic pressure of a plane wave measured by a microphone has been observed experimentally [ 13 ]. The limits of this relationship are not yet clearly defined [ 14 ].…”

“…The limits of this relationship are not yet clearly defined [ 14 ]. In the previous studies, a reference microphone was mandatory to perform the SMI calibration in situ [ 13 , 14 ]. This limits the interest of such a device compared with the use of a microphone alone.…”

“…In fact, acoustic waves in air cause the refractive index to change at a rate of Pa −1 . For instance, if the distance between the LD and the retro-reflective surface is about 20 cm, an acoustic wave at 112 dB SPL induces an optical path variation of 1.2 nanometers [ 13 ], which is three orders of magnitude below mechanical vibrations of a few micrometers [ 3 ]. For the measurement of C and in strong feedback regime, the methods can be divided into two categories: phase unwrapping methods [ 29 , 30 , 31 , 33 ] and direct SMI signal analysis methods [ 32 , 34 , 35 ].…”