Advanced sensors and signal processing algorithms are significant for the use of remotely-operated vehicles and autonomous underwater vehicles. The distance/length measurement is the basis of many sensing functions, including positioning, tracking, surface reconstruction, and pose determination. Optical-based ranging sensors have been proven as a promising tool and obtain up to micrometer-level accuracy when combined with dual-comb interference. Applying this approach to underwater scenarios is feasible, but one must handle the issue that the ranging signal is significantly affected by environmental disturbances and system noises. However, it has been rarely reported that processing algorithms are tailored to the dual-comb signal to improve the quality of measuring signals. This article presents an enhanced underwater dual-comb ranging (DCR) solution via an improved variational mode decomposition (VMD). Specifically, we design a fitness function considering desired dual-comb interferogram characteristics. Accordingly, we optimize vital parameters and decompose the interested ranging signal to ensure final interferogram quality. Experiments verify that our method is superior to others and can improve the signal-to-noise ratio and restore the Gaussian-like shape of interferograms simultaneously. To the best of the authors knowledge, it is the first time DCR is boosted via VMD, and answers the question about interferogram shaping. The proposed solution may find important applications in ranging and imaging tasks underwater, as well as extend their working range and robustness against non-ideal environments.

Underwater distance/length sensing serves a fundamental function and plays a crucial role in a wide range of applications, including topography and geomorphology in remote sensing. Advanced light detection and ranging (LiDAR) systems and algorithms are significant for underwater tasks, such as objection positing, target searching, and rescuing. In this article, we propose a dual-comb interferometer-based LiDAR system to achieve accurate underwater absolute distance measurement. Specifically, the system applies a well-designed pulse coding strategy that significantly expands the nonambiguity range (NAR) for underwater distance measurements, allowing for precise long-distance measurements in a single attempt. Besides, we introduce a correction process to handle the water group refractive index problem. Compared with reference values, experimental results show that the range fluctuation is within $\pm 15~\mu $ m at a 12 m measurement range, and the Allan deviation is $0.62~\mu $ m over an average time of 100 s. In practice, this LiDAR solution with a micrometer-level precision is promising for various applications in ocean engineering.

Underwater distance measurement and length sensing, as fundamental functions of geoscience, are integrated into underwater robots and vessels in various forms of sensor payloads and play a crucial role in a wide range of remote sensing applications. Advanced laser-based detection systems and algorithms are essential for underwater tasks, such as detecting damage in underwater pipelines and conducting detailed structural health assessments. In this paper, we experimentally demonstrated an arbitrary underwater ranging system by the chirped pulse interferometry with a mode-locked laser. Water's inherent characteristic introduces dispersion, enabling unique distance discrimination. Adjusting the repetition frequency allows us to acquire the interference spectrum. A long fiber delay line, stabilized by the servo-controlled system, eliminates the ranging dead zone. The widest fringe of the interference spectrum, corresponding to an instantaneous repetition frequency, indicates the arbitrary measured distance. Leveraging a charge-coupled device detection module, we achieved an update rate of 13.5 Hz for the interference spectrum. Experimental results show a precise distance difference within 59.8 μm at the 5 m range underwater without the dead zone.

Measuring sound velocity plays an important role in ocean exploration and perception technology. In this paper, we propose a method to directly measure underwater sound velocity using dual-comb lasers and matched filtering. Among them, the acousto-optic effect between the frequency comb and ultrasonic waves is applied to calibrate the time of flight. The dual-comb interferometry technique is utilized to determine the underwater sound velocity. Furthermore, matched filtering is applied to enhance the signal-to-noise ratio of the signal. Experimental results demonstrate that the method achieves a kHz-scale sound velocity measuring rate, and enjoys an uncertainty of better than 0.037 m/s for a single measurement. Moreover, compared with commercial sound velocity profilers, the method exhibits a standard deviation of 0.0154 m/s during long-term measurements.

Full-polarization light detection and ranging (LiDAR) is a powerful remote sensing technology that provides comprehensive information about object distance, material properties, surface texture, and orientation. However, existing full-polarization LiDAR systems typically rely on time-division or amplitude-division polarization schemes, requiring multiple measurements and computational processing to reconstruct the full-Stokes vector. This limits real-time performance and increases system complexity. Here, we present a compact full-polarization LiDAR, termed VRP-LiDAR, which leverages the spatial polarization modulation capability of a vortex retarder (VR) in conjunction with a polarization camera and a photodetector. This approach enables real-time, single-shot acquisition of both distance information and full-Stokes polarization data without requiring multiple exposures or moving parts. Experimental validation confirms the effectiveness of VRP-LiDAR, demonstrating its potential for compact and robust polarization-based target detection in remote sensing applications.