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.

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.

In this paper, we demonstrate a three-dimensional imaging system based on the laser frequency comb. We develop a compact, all-fiber mode-locked laser at 1 μm, whose repetition frequency can be tightly synchronized to the external frequency reference. The mode-locked state is achieved via the saturable absorber mirror in a linear cavity, and the laser output power can be amplified from 4 mW to 150 mW after a Yb-doped fiber amplifier. Three-dimensional imaging is realized via the spectral interferometry with the aid of an equal-arm Michelson interferometer. Compared with the reference values, the measurement results show the difference can be below 4 μm. Our system could provide a pathway to the real industry applications in future.

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.

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.

Water temperature and salinity are key parameters in many fields such as industry, forestry and agriculture. In this paper, we, theoretically and experimentally, demonstrate a method which is capable of water temperature and salinity measurement based on a laser frequency comb at 518 nm. We have developed a simple Michelson interferometer system. By scanning a mirror on a precision displacement platform, a pair of cross-correlation patterns can be obtained. The real-time optical distance information from these cross-correlation patterns can be used to calculate the optical distance difference changes. Temperature and salinity can be measured via these changes, aided by the empirical formulas. Compared with the reference values, our results show the differences of below 0.12 °C for temperature measurements, and 0.06‰ for salinity measurements. The obtained results indicate that our method can offer a powerful scheme for future temperature and salinity measurement.