Demodulation Technique Based on Laser Interference for Weak Photo-Acoustic Signals on Water Surface

Abstract

To detect underwater sound-generating targets, a water surface acoustic wave laser interference and signal demodulation technique is proposed in this paper. The underlying principle of this technique involves casting a laser beam onto the water surface disturbed by an underwater acoustic source and creating interference between lights reflected by the surface and reference lights. A data acquisition and processing system was employed to obtain water surface acoustic wave information from the interference signals by means of demodulation, thus allowing detection of the underwater target. For the purpose of this study, an interference detection platform was set up in an optical dark chamber. High-frequency water surface fluctuations were introduced in the reference optical path as the phase generated carriers to create laser interference signals in two different paths, which received demodulation based on an improved arc tangent demodulation algorithm and characteristic ratio algorithm, respectively, in view of their different frequencies. Water surface wave information was then derived from such low-frequency and high-frequency signals. According to test results, in the frequency range of 200 Hz–10 kHz, the frequency detection accuracy was better than 1 Hz. The amplitude measurements exhibited high repeatability, with a standard deviation lower than 2.5 nm. The theory proposed in this paper is therefore experimentally verified with good results.

Detection of Water Surface Acoustic Waves Using Sinusoidal Phase Modulation Interferometer and Prenormalized PGC-Arctan Algorithm

“…By taking the quadrature interference signal component I(t) as the horizontal coordinate axis and the in-phase interference signal component Q(t) as the vertical coordinate axis, a Lissajous figure can be constructed. It is assumed that the signal-visibility-related coefficient B 0 and carrier phase delay V c are constant in Equation (5), and other parameters are determined by referring to the characteristics of measured signals. The waveforms of functions J 1 (C) and J 2 (C) in the range of independent variable C from 0 to 4 rad are shown in Figure 2a, and the effects of different phase modulation depths on the Lissajous figure are shown in Figure 2b-d.…”

“…Currently, the optical detection methods of such weak water surface waves are mainly divided into optical intensity modulation method [3], light scattering method [4], laser Doppler interference method [5][6][7][8], and optical diffraction method [9][10][11]. Some of these detection methods have been applied to the detection of physical properties of media, such as in the study of surface tension, viscosity of medium, and molecular motion; some have been used in the detection of underwater acoustic information.…”

Special Issue on Precision Dimensional Measurements

“…Yang et al developed non-contact and real-time measurement of high strain rate Kolsky bar with temporal speckle interferometry [39]. Zhang et al present a demodulation technique for weak photo-acoustic signals of laser interferometer on water surface [40]. The last paper was written by Qiu et al on the experimental analyses on multiscale structural and mechanical properties of ε-Si/GeSi/C-Si materials [41].…”