Practicality of Using Single-Photon Raman Lidar for Underwater Applications

A team of researchers has unveiled an innovative single-photon Raman lidar system that can effectively operate underwater and accurately differentiate between various substances. Additionally, the system has demonstrated its ability to detect the thickness of oil while submerged up to a distance of 12 meters. This advancement holds great potential for detecting and combating oil spills.

“Being able to differentiate substances in water and understand their distribution characteristics in the ocean is of utmost importance for marine monitoring and scientific research,” emphasized Mingjia Shangguan, the leader of the research team from Xiamen University in China. “By successfully demonstrating the remote sensing capabilities of underwater oil using our lidar system, we have opened up possibilities for effectively monitoring underwater oil pipeline leaks.”

Prior lidar systems utilizing Raman signals for underwater substance detection have been rendered impractical due to their bulky size and high energy consumption. However, in the journal Applied Optics, the researchers present their compact lidar system, which operates on just 1 μJ of pulse energy and features a 22.4 mm receiver aperture. The entire system measures 40 cm in length and has a diameter of 20 cm, enabling its underwater operation up to a depth of 1 km. The team achieved enhanced sensitivity by incorporating single-photon detection technology into their compact Raman lidar system.

“The integration of an underwater Raman lidar system onto an autonomous underwater vehicle or remotely operated vehicle offers the potential for monitoring underwater oil pipeline leaks,” stated Shangguan. “Furthermore, this technology could be deployed for exploring oceanic resources and detecting seafloor sediment types, such as coral reefs.”

Unveiling Single-Photon Sensitivity in Underwater Lidar

Traditional lidar systems designed for operation aboard ships, aircraft, or satellites can provide extensive ocean profiling, albeit with limited detection depth, particularly in rough sea conditions. In contrast, Raman lidar systems can analyze underwater environments at different depths without being influenced by sea conditions.

The functioning principle of Raman lidar involves emitting a green laser pulse into the water, which interacts with substances like oil. This interaction generates inelastic Raman signals that can be utilized for substance identification. By measuring the intensity of Raman signals at specific wavelengths, lidar can provide valuable insights into the oil content present in the water.

“Conventional Raman lidar systems rely on increasing laser power and telescope aperture to achieve remote sensing capabilities, resulting in large system sizes and high energy consumption, making it challenging to integrate them onto underwater vehicles,” explained Shangguan. “The utilization of single-photon detection technology has made this work possible by significantly enhancing detection sensitivity at the single photon level.”

To demonstrate the capabilities of their novel lidar system, the researchers conducted experiments using a quartz cell containing varying thicknesses of gasoline oil, placed 12 meters away from the system. Both the lidar system and the quartz cell were submerged at a depth of 0.6 m underwater in a large pool. The lidar system successfully detected and differentiated all the thicknesses of gasoline, ranging from 1 mm to 15 mm.

The team is currently striving to expand the number of detection channels and improve the Raman spectral resolution of their single-photon lidar system, aiming to enhance its ability to distinguish different substances in water. This would facilitate the analysis of underwater bubble types and the detection of corals and manganese nodules.

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