In this article, we study the non-linear coupling between the stationary (i.e. the beating modulation signal) and transient (i.e. the laser quantum noise) dynamics of a laser subjected to frequency shifted optical feedback. We show how the noise power spectrum and more specifically the relaxation oscillation frequency of the laser are modified under different optical feedback condition. Specifically we study the influence of (i) the amount of light returning to the laser cavity and (ii) the initial detuning between the frequency shift and intrinsic relaxation frequency. The present work shows how the relaxation frequency is related to the strength of the beating signal and the shape of the noise power spectrum gives an image of the Transfer Modulation Function (i.e. of the amplification gain) of the nonlinear-laser dynamics.The theoretical predictions, confirmed by numerical resolutions, are in good agreements with the experimental data.
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.
In this article, we study the nonlinear dynamics of a laser subjected to frequency shifted optical reinjection coming back from a vibrating target. More specifically, we study the nonlinear dynamical coupling between the carrier and the vibration signal. The present work shows how the nonlinear amplification of the vibration spectrum is related to the strength of the carrier and how it must be compensated to obtain accurate (i.e., without bias) vibration measurements. The theoretical predictions, confirmed by numerical simulations, are in good agreement with the experimental data. The main motivation of this study is the understanding of the nonlinear response of a laser optical feedback imaging sensor for quantitative phase measurements of small vibrations in the case of strong optical feedback.
We present how a Laser Optical Feedback Imaging (LOFI) setup can be used for the optical detection of ultrasound in Photo-Acoustic Tomography (PAT). A PAT image is reconstructed by an inversion algorithm using surface displacement measurements made at several locations with our LOFI setup and following the optical irradiation with a pulsed Nd:YAG laser of a sample with absorbing inclusions. The width of the reconstructed inclusions and the SNR of the reconstructed images are firstly studied on the numerical model of a sample with 3 absorbing inclusions (i.e. with 3 acoustic punctual sources). Finally an experimental PAT image of a phantom composed of two polyamide tubes with an internal diameter of 800 µm filled with red ink and submerged at-3.5 mm depth in a tank filled with water is reconstructed. Experimentally, the water surface displacement measurements has been made with our LOFI vibrometer which provides an amplitude sensitivity of 1nm (for a single-shot measurement) in a detection bandwidth of roughly 1 MHz adapted to the detection of the polyamide tubes. Under our experimental conditions, the surface energy densities, of the LOFI focalized beam for the detection and of the pulsed Nd:YAG laser used for the irradiation, are compatible with the MPE (Maximum Permissive Exposure) for future biomedical measurements. The SNR and the resolution of the reconstructed PAT images are in good agreement with the theoretical predictions.
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