Ultrasound vibration measurements based on laser optical feedback imaging

Abstract

This paper is devoted to the detection of ultrasound vibrations with nanometric amplitude by using a LOFI (Laser Optical Feedback Imaging) setup. By means of numerical simulations, we show typical examples of ultrasound vibrations having different temporal shapes (harmonic and transient) extracted from the laser output power modulation induced by the frequency shifted optical feedback. Considering the laser quantum noise dynamic and the detection noise separately, we show that the simulated vibration noise is in good agreement with the theoretical prediction. Also, we demonstrate that Ultra High Frequencies (in the GHz range) can be detected by using a usual LOFI setup with a low power laser (few mW) and a conventional detection with a usual white noise level. Then we show how the noise of a short transient vibration can be reduced by the reconstruction of its wide vibration spectrum by concatenation. Finally, the experimental detection of transient-harmonics ultrasound vibrations propagating in water and detected at the air/water interface is presented.

Photo-acoustic tomography based on laser optical feedback imaging of surface displacements

“…For PAT, the images are reconstructed from the optical measurements of surface displacements made at several locations just after the optical irradiation of the absorbing sample with a pulsed laser. We show that the LOFI vibrometer based on a CW microchip Nd:YAG laser with a relaxation frequency in the megahertz range, provides surface displacement measurements a displacement sensitivity (at the quantum limit) of 1 nm for a single shot measurement in a bandwidth of 1.3 MHz [14]. Finally, a 2D (x, z) experimental image of a phantom composed of two polyamide tubes with a an internal diameter of 800 µm filled with red ink and submerged at -3.5 mm depth in a tank filled with water has been reconstructed from a 1D LOFI temporal acquisition along the water surface.…”

“… [14]. Experimentally, a LOFI movie of the surface displacement can be obtained pixel by pixel (i.e.…”

“…The main objective of the present paper is to demonstrate how a Laser Optical Feedback Imaging (LOFI) setup (which is an optical free space technic [8]) can be used for optical detection of ultrasound in Photo-Acoustic Tomography (PAT). Nanometer measurements based on SMI (Self Mixing Interferometry) have already been realized, either for acoustic perturbations or for nm-sized vibrations [9][10][11][12][13][14]. Here, the use of a LOFI setup is motivated by the combination of the four following reasons: i) The LOFI interferometer is always self-aligned because the laser simultaneously fulfils the functions of the source (i.e.…”

“…1). The green laser used to generated the PA absorption has a pulse energy of MHz  in order to be able to record at the shot noise level a vibration spectrum of several MHz [14]. Experimentally the feedback level can be controlled by playing with the efficiency of the frequency shifter (composed of two acousto-optic deflectors) and allows to work in the weak feedback regime where the LOFI vibrometer performs linearly [13][14].…”

“…The green laser used to generated the PA absorption has a pulse energy of MHz  in order to be able to record at the shot noise level a vibration spectrum of several MHz [14]. Experimentally the feedback level can be controlled by playing with the efficiency of the frequency shifter (composed of two acousto-optic deflectors) and allows to work in the weak feedback regime where the LOFI vibrometer performs linearly [13][14]. The laser output power modulation induced by the frequency shifted optical feedback is detected by using a reversed bias PIN photodiode (THORLABS/DET110) and the delivered voltage is send to a DAQ card (SPECTRUM/M3I.4111) and processed by a PC to finally obtain the displacement measurements (    …”