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Utilizing hydrophone arrays for detecting underwater acoustic communication (UWAC) signals leverages spatial information to enhance detection efficiency and expand the perceptual range. This study redefines the task of UWAC signal detection as an object detection problem within the frequency–azimuth (FRAZ) spectrum. Employing Faster R-CNN as a signal detector, the proposed method facilitates the joint prediction of UWAC signals, including estimates of the number of sources, modulation type, frequency band, and direction of arrival (DOA). The proposed method extracts precise frequency and DOA features of the signals without requiring prior knowledge of the number of signals or frequency bands. Instead, it extracts these features jointly during training and applies them to perform joint predictions during testing. Numerical studies demonstrate that the proposed method consistently outperforms existing techniques across all signal-to-noise ratios (SNRs), particularly excelling in low SNRs. It achieves a detection F1 score of 0.96 at an SNR of −15 dB. We further verified its performance under varying modulation types, numbers of sources, grating lobe interference, strong signal interference, and array structure parameters. Furthermore, the practicality and robustness of our approach were evaluated in lake-based UWAC experiments, and the model trained solely on simulated signals performed competitively in the trials. [ABSTRACT FROM AUTHOR]

Source depth estimation is an important yet very difficult task for passive sonars, especially for horizontal linear arrays (HLAs). This paper proposes an efficient two-step depth estimation scheme using narrowband and broadband constructive and deconstructive striation patterns due to interference between the direct (D) and sea surface reflected (SR) arrivals at an HLA on the bottom of deep water. First, the horizontal source-array ranges are derived from triangulation results of solid angle estimates by subarray beamforming. The applicable areas of the method in deep water are investigated through Mento Carlo simulations, assuming different subarray partitioning ways of a given HLA aperture. Second, cost functions are built to match the measured beam intensity striations with modeled ones. To mitigate the spatial smoothing effect of the beam intensity striations during beamforming, a criterion of the largest subarray aperture is established, and a computationally efficient way is presented to model the replicas by the D-SR time delay templates at a single element of the array calculated by ray theory. The performance degradation due to limited source range spans, the distortion of the beam intensity striations, and range estimation errors has been analyzed. Two experimental datasets verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Relaxation oscillations in [Er.sup.3+]-doped and [Yb.sup.3+]/[Er.sup.3+] co-doped fiber grating lasers were investigated. Intensity noise models were established for both cases in which the fluctuations of pump power and cavity loss were analyzed. Simulation indicates that the relaxation oscillation is induced by the pump power fluctuation, and fluctuation of the cavity loss can broaden the relaxation oscillation peak and enhance the amplitude. The relaxation oscillation of a laser with [Er.sup.3+]-doped fiber exhibits a lower frequency and larger amplitude than that of an [Yb.sup.3+]/[Er.sup.3+] co-doped fiber grating laser. Distributed Bragg reflector fiber grating lasers with [Er.sup.3+]-doped fiber and [Yb.sup.3+]/[Er.sup.3+] co-doped fiber were fabricated, and the relaxation oscillations were measured. For the former, the relaxation oscillation frequency is at the magnitude of several hundreds of kilohertz and lineally shifts toward the higher frequency with the pump power. For the latter, no obvious relaxation oscillation was measured. OCIS codes: 060.2310, 060.2370, 060.2340.

Continuously sensing of water temperature is of critical importance to water environment, fluid dynamic and water resources researches. Coastal acoustic tomography (CAT) is a remote sensing technique in spatial temperature distribution estimating and has been widely used in many coastal regions. Continuous observations of water temperature in small-scale waters with distance less than 1 km are seldom conducted, as it is difficult to get high-resolution observation results. Here, a horizontal temperature field within 300 m distance was mapped in a sound transmission experiment with three 50 kHz CAT systems. The maximum inversion error temperature for the horizontal temperature field was 0.09°C. The root-mean-square error between the CAT-observed and range-averaged temperature that were calculated from one-way travel time data along three transmission paths were 0.03°C, 0.028°C and 0.043°C, respectively. The obtained results confirmed that CAT can be used in recovering the horizontal distributions of water temperature fields in lake. [ABSTRACT FROM AUTHOR]

To verify the performance of longitudinal waves induced by laser phased arrays (LPA) for detection and quantitative evaluation in internal defects, the finite element method (FEM) is utilized to establish the models of LPA scanning processing. The interaction of longitudinal wave and internal defect is analyzed. Besides, the two components of the reflected longitudinal waves (the longitudinal wave component r LL and the shear wave component r LS r LS ) are focused on the imaging of defects with the synthetic aperture focusing technique (SAFT) and total focusing method (TFM) algorithms. It shows that the imaging of internal sub-millimeter defect is obtained using the LPA. The defect size and location are simultaneously calculated, with the relative error being 6.7% and 2.9%, respectively. The proposed longitudinal wave-based LPA is a promising method for the imaging and evaluation of internal micro defects. [ABSTRACT FROM AUTHOR]