ABSTRACT
This paper investigates a 1.7 mm diameter ultra-weak fiber Bragg grating (UWFBG) hydrophone towed array cable for acoustic direction finding. The mechanism of the underwater acoustic waves received by this integrated-coating sensitizing optical cable is deduced, and it is shown that the amplitude of its response varies with the direction of the sound wave. An anechoic pool experiment is carried out to test the performance of such a hydrophone array. The test array is a selection of six sensing fibers, each of which is coiled into 9 cm diameter fiber ring suspended in the water to receive acoustic signals. An average sensitivity of -141.2 dB re rad/µPa at frequencies from 2.5 kHz to 6.3 kHz was achieved, validating the detection of the azimuth of underwater acoustic waves. The ultra-thin towing cable system, with free structure, high sensitivity, and underwater target-detection capability has demonstrated great potential for future unmanned underwater vehicle (UUV) applications.
ABSTRACT
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.
ABSTRACT
We demonstrate the use of a fiber delay line (FDL) to demodulate a weak fiber Bragg grating (WFBG) array. The FDL is composed of a fiber-ring cavity, by which the delay time is matched with the interval length of the adjacent WFBGs. Two pulses reflected by the former WFBG with delay and a pulse reflected by the rear WFBG without delay overlap and interfere. The emitted sounds can be given by the interference signals of a 5-WFBG array with the FDL. Using the FDL, it is demonstrated that the simple structure can be used to demodulate the WFBG array.
ABSTRACT
The fiber laser hydrophone is compact, and it is easy to construct an array by using the multiplexing technique. However, the number of hydrophones multiplexed in the array through wavelength-division-multiplexing (WDM) technology is limited. In order to expand the array, a spatial division multiplexing (SDM) method for addressing a fiber laser hydrophone array is researched. An optical switch is used in such array, and four distributed feedback (DFB) fiber laser hydrophones are addressed by switch channels. Experimental results show that the switch channels' crosstalk crucially influences the hydrophone signals' signal-to-noise ratio (SNR). In order to get good SNR, the crosstalk of the switch channels should be lower than -20 dB. Each DFB laser in the SDM-addressed array may be replaced with a WDM-addressed array, so a DFB fiber laser hydrophone array with more than 64 elements can be constructed.