A Shipborne Photon-Counting Lidar for Depth-Resolved Ocean Observation

Simultaneous sensing profiles of beam attenuation coefficient and volume scattering function at 180° using a single-photon underwater elastic-Raman lidar

Lidar has emerged as a promising technique for vertically profiling optical parameters in water. The application of single-photon technology has enabled the development of compact oceanic lidar systems, facilitating their deployment underwater. This is crucial for conducting ocean observations that are free from interference at the air-sea interface. However, simultaneous inversion of the volume scattering function at 180° at 532 nm (βm) and the lidar attenuation coefficient at 532 nm (K l i d a r m) from the elastic backscattered signals remains challenging, especially in the case of near-field signals affected by the geometric overlap factor (GOF). To address this challenge, this work proposes adding a Raman channel, obtaining Raman backscattered profiles using single-photon detection. By normalizing the elastic backscattered signals with the Raman signals, the sensitivity of the normalized signal to variations in the lidar attenuation coefficient is significantly reduced. This allows for the application of a perturbation method to invert βm and subsequently obtain the K l i d a r m. Moreover, the influence of GOF and fluctuations in laser power on the inversion can be reduced. To further improve the accuracy of the inversion algorithm for stratified water bodies, an iterative algorithm is proposed. Additionally, since the optical telescope of the lidar adopts a small aperture and narrow field of view design, K l i d a r m tends to the beam attenuation coefficient at 532 nm (cm). Using Monte Carlo simulation, a relationship between cm and K l i d a r m is established, allowing cm derivation from K l i d a r m. Finally, the feasibility of the algorithm is verified through inversion error analysis. The robustness of the lidar system and the effectiveness of the algorithm are validated through a preliminary experiment conducted in a water tank. These results demonstrate that the lidar can accurately profile optical parameters of water, contributing to the study of particulate organic carbon (POC) in the ocean.

Full-day profiling of a beam attenuation coefficient using a single-photon underwater lidar with a large dynamic measurement range

A compact underwater lidar system, utilizing a single-photon detection technology, is proposed to effectively eliminate interference from the sea-air interface and enhance the accuracy of water optical property measurements. However, the high sensitivity of the single-photon detector poses challenges, including daytime operation difficulties due to strong solar radiation noise and detector saturation from near-field lidar signals. To address these issues, the laser and optical receiver of the lidar are optimized to suppress solar radiation noise, and a dual-telescope structure is introduced to improve the dynamic measurement range beyond 70 dB. In addition, a Monte Carlo simulation establishes the relationship between beam attenuation coefficients (c) and lidar attenuation coefficients (Klidar), enabling the retrieval of c profiles from Klidar. A field experiment conducted in the South China Sea, spanning from inshore to offshore waters, demonstrates the effectiveness of the lidar. The results highlight its potential applications, including the assessment of subsurface particulate organic carbon (POC).

Day and night continuous high-resolution shallow-water depth detection with single-photon underwater lidar

Single-photon lidar has emerged as a strong technology for bathymetric measurements. However, its heightened sensitivity additionally makes it susceptible to solar radiation noise, particularly in the green light wavelength where solar radiation is strong, posing challenges for its daytime operation. To address this issue, a single-photon underwater lidar system is proposed and demonstrated. This scheme has these features. 1) Underwater applications not only mitigate the impact of the air-water interface on laser transmission but also significantly attenuate solar radiation reaching the lidar due to the absorption and scattering properties of water. 2) The telescope is designed with a small aperture and narrow field of view to significantly suppress solar radiation. 3) A combination of a narrowband laser and narrowband filter technique is effectively employed to minimize residual solar radiation, thus enabling continuous bathymetric observation capabilities during both day and night. 4) After acquiring the backscattered signal from the bottom, a water depth extraction algorithm utilizing bi-Gaussian fitting is proposed. To demonstrate the robustness of the lidar and the effectiveness of the algorithm, the underwater single-photon lidar system is deployed on a ship to conduct cruise surveys of two bays in the nearshore area, as well as a full-day stationary observation experiment. The lidar measurements are highly consistent with the synchronized sonar observations. The full-day stationary observation experiment showcased its capability to deliver continuous measurements throughout the day and night. These results demonstrate the potential of the system in various applications, including high-precision underwater terrain mapping, obstacle avoidance for underwater platforms, and underwater target imaging.

Sensing profiles of the volume scattering function at 180° using a single-photon oceanic fluorescence lidar

A novel oceanic fluorescence lidar technique has been proposed and demonstrated for remotely sensing the volume scattering function at 180° (βf), which can be used to further retrieve the profiles of the absorption coefficient of phytoplankton (aph) at 532 nm and chlorophyll concentration (Chl). This scheme has these features. 1) The single-photon detection technology is employed to enhance the detection sensitivity to the single-photon level, enabling the oceanic lidar to obtain fluorescence backscatter profiles. 2) In terms of algorithms, the Raman backscattered signals of the water are utilized to normalize the backscattered signals of chlorophyll fluorescence, effectively minimizing the depth-dependent variation of the differential lidar attenuation coefficient (Δ K l i d a r f r). To reduce the contamination of fluorescence signals in the Raman backscatter signals, a Raman filter with a bandwidth of 6 nm was chosen. Subsequently, a perturbation method is utilized to invert the βf of the fluorescence lidar. Finally, aph and Chl profiles can be inverted based on empirical models. 3) The value of Δ K l i d a r f r used in inversion is obtained through a semi-analytic Monte Carlo simulation. According to theoretical analysis, the maximum relative error of βf for Chl ranging from 0.01 mg/m3 to 10 mg/m3 is less than 13 %. To validate this approach, a field experiment was conducted aboard the R/V Tan Kah Kee in the South China Sea from September 4th to September 5th, 2022, resulting in continuous subsurface profiles of βf, aph, and Chl. These measurements confirm the robustness and reliability of the oceanic single-photon fluorescence lidar system and the inversion algorithm.

Sensing the profile of particulate beam attenuation coefficient through a single-photon oceanic Raman lidar

A lidar technique has been proposed and demonstrated for remotely sensing particulate beam attenuation coefficient (cp) profiles using the Raman backscattered signal from water. In Raman lidar, the backscatter coefficient at 180° can be considered constant, allowing for the determination of the lidar attenuation coefficient (Klidar) from the Raman backscattered signal. This scheme has these features. 1) The bandwidth of the filter that used to extract the Raman component from the backscattered signal of the lidar was optimized to ensure sufficient lidar signal strength while minimizing the influence of chlorophyll fluorescence on inversion. 2) A receiving telescope with narrow field of view (FOV) and small aperture was utilized to suppress multi-scattering components in the backscattered signal. 3) A relationship between the beam attenuation coefficient (c) and Klidar was established after simulations via a semi-analytic Monto Carlo. 4) The value of cp was obtained by subtracting the attenuation coefficient of pure seawater (cw) from c. According to the theoretical analysis, the maximum relative error of cp is less than 15% for chlorophyll concentrations up to 10 mg/m3. Due to the water Raman backscattered signal being several orders of magnitude lower than the elastic backscattered signal, a single-photon detector is required to significantly improve the detection sensitivity to the single-photon level. To validate this approach, a field experiment was conducted aboard the R/V Tan Kah Kee in the South China Sea from September 4th to September 5th, 2022, and continuous subsurface profiles of cp were obtained. These measurements confirm the robustness and reliability of the oceanic single-photon Raman lidar system and the inversion method.

Bathymetry of a macro-pulsed chaotic laser based on a 520 nm laser diode subject to free space optical feedback

We generate a macro-pulsed chaotic laser based on pulse-modulated laser diode subject to free space optical feedback, and demonstrate the performance of suppressing backscattering interference and jamming in turbid water. The macro-pulsed chaotic laser with a wavelength of 520 nm as a transmitter is used with a correlation-based lidar receiver to perform an underwater ranging. At the same power consumption, macro-pulsed lasers have higher peak power than in the continuous-wave form, enabling the former to detect longer ranging. The experimental results show that a chaotic macro-pulsed laser has excellent performance of suppressing the backscattering of water column and anti-noise interference compared with traditional pulse laser, especially by multiple accumulations about 10∼30 times, and target position can still be determined when SNR is -20 dB.