Objective The optical characteristic parameters of marine water are one of the key points of marine research. Most of the optical characteristics vary with depth, and oceanic lidar is one of the effective technical means to detect water profile information. Based on the requirement of water optical profile detection, a two-wavelength oceanic lidar system with blue and green light is developed. The light source uses blue and green wavelength with small attenuation coefficient in water to obtain a larger detection depth. The system has dual wavelength and polarization detection channels, which can be used to obtain the backscattered echo signal and polarization signal simultaneously, and can realize the continuous detection of nearshore and ocean water. This paper first introduces the design scheme of the lidar system, including transmitting, receiving, acquisition and control subsystem and auxiliary facilities, and then describes the data preprocessing methods, including quality control, peak alignment, background noise removal and deconvolution. The reliability of the lidar system is verified by the detection experiment in the offshore. The results show that in clean ocean waters, the attenuation coefficient of 486 nm lidar is less than 532 nm, which means that 486 nm has better detection performance in open water.

Methods The lidar system consists of transmitting, receiving, acquisition and control subsystems and auxiliary facilities (Fig.1-2). The transmitting subsystem is used to transmit 486 nm and 532 nm linearly polarized light to detect the target. The receiving subsystem is responsible for receiving and detecting the echo signal of the water target and realizing the photoelectric conversion. The acquisition and control subsystem is responsible for high-speed data acquisition and storage, system integrated control and state monitoring. Auxiliary facilities are used to ensure that the lidar works in a stable working environment. Then the data preprocessing methods are introduced, including quality control (Fig.5), peak position alignment (Fig.6), background noise removal (Fig.7) and deconvolution (Fig.8).

Results and Discussions In order to verify the reliability of the system, the detection experiment is carried out in the offshore, and the backscattered signal of the water (Fig.10) is obtained, and the attenuation coefficient profile information (Fig.11) is obtained by combining the optical inversion algorithm. The results show that in the test oceanic waters, the laser radar system can obtain the optical characteristic parameter profile of the effective detection depth of about 30 m in the dynamic range of 2 and 3 orders of magnitude at the wavelength of 486 nm and 532 nm respectively (Fig.9). At the same depth, even though green light is more energetic, about 1.5 times that of blue light, compared with 532 nm, the attenuation coefficient of 486 nm lidar is smaller, indicating that 486 nm wavelength has better detection performance (Fig.12). From the general distribution trend of optical characteristic parameters, the attenuation coefficient of lidar obtained by the system increases with the increase of depth.

Conclusions The blue-green dual-wavelength oceanic lidar system is developed to obtain the backscattered echo signal of blue-green band and the profile information of attenuation coefficient of lidar synchronously in clean oceanic waters, and realize the continuous detection of oceanic water. The results show that the attenuation coefficient of 486 nm lidar is less than 532 nm, which means that 486 nm has better detection performance in open water. The data results verify the reliability of the system and the performance of the blue-green wavelength detection channel, and provide a strong support for the application potential of oceanic lidar in the field of oceanic detection.