Aiming at the requirements of low power consumption, miniaturization, and massive data processing, a modular distributed optical fibre acoustic sensing (DAS) demodulation system based on ultra-weak fibre Bragg grating (UW-FBG) is proposed. A highly integrated circuit and embedded system is used for the time-domain positioning and data acquisition of the gratings. Field programmable gate array (FPGA) demodulates the phase changes of each sensing section in real time. The demodulation outputs are uploaded through a Gigabit Ethernet connection. Through innovative software and hardware design, the demodulation system power consumption is less than 20 W, and the device volume is 250 mm× 180 mm× 45 mm, and the output data size is about 2.6% of the conventional methods. All the outputs are then tested on a piezoelectric transducer (PZT) platform. Results show that the background noise within 5 Hz–1280 Hz range is 2.16 pε/√(Hz), and the strain sensitivity reaches 484 pε/√(Hz)@200 Hz. The correlation coefficient of voltage and phase is 0.9991, which shows good linearity. Combined with a common commercial laptop to demodulate the micro-vibration signals, this system could provide a cost-effective scheme for acquiring acoustic-level micro-vibration signals and on-line field monitoring.

The demodulation algorithm based on 3 × 3 coupler in a fiber-optic hydrophone array has gained extensive attention in recent years. The traditional method uses a circulator to construct the normal path-match interferometry; however, the problem of increasing the asymmetry of the three-way signal to be demodulated is easily overlooked. To provide a solution, we report a pretreatment method for hydrophone array based on 3 × 3 coupler demodulation. We use cubic spline interpolation to perform nonlinear fitting to the reflected pulse train and calculate the peak-to-peak values of the single pulse to determine the light intensity compensation coefficient of the interference signal, so as to demodulate the corrected three-way interference signal. For experimental verification, ultra-weak fiber Bragg gratings (uwFBGs) with reflectivity of −50 dB are applied to construct a hydrophone array with 800 sensors, and a vibratory liquid column method is set up to generate a low-frequency hydroacoustic signal. Compared to the traditional demodulation algorithm based on a 3 × 3 coupler, the pretreatment method can improve the consistency of interference signals. The Lissajous figures show that cubic spline interpolation can improve the accuracy of monopulse peak seeking results by about 1 dB, and intensity compensation can further lead to a much lower noise density level for the interference pulse amplitude—specifically, more than 7 dB at 5~50 Hz—and the signal-to-noise ratio is improved by approximately 10 dB at 10 Hz. The distinct advantages of the proposed pretreatment method make it an excellent candidate for a hydrophone array system based on path-match interferometry.

No abstract

For the axial stretching fiber laser hydrophone, a fiber laser is often exposed to flexural interference. A theoretical model for the flexural vibration of a fiber laser was therefore constructed in this paper to explore the low frequency anti-interference mechanism for the frequency response of an axially tensioned fiber laser hydrophone (FLH). A specific packaging structure was used for finite element comparison and simulation. Packaged FLHs were tested for frequency response. The simulation and test results reveal that the flexible attachment at both ends of a fiber laser leads to lower amplitude of flexural vibration compared with rigid attachment, which therefore promotes a flat acoustic response curve of an FLH. The analysis given in this paper can be taken as a basis for improving the packaging technique.

No abstract