Underwater remote volumetric imaging with a high-resolution confocal LiDAR

Monitoring the biomass and distributions of plankton on local and global scales is critical as these organisms form the foundation of marine food webs and generate a key portion of the total oxygen in the atmosphere. Simultaneous in situ and in vivo detection is, however, technically intricate and confined to small close-range interrogation volumes with current imaging technologies. Here, we present our novel inelastic scanning confocal light detection and ranging (LiDAR) system for underwater imaging with high-spatial resolution and high-sensitivity. We report the empirically determined axial and lateral resolutions, and perform volumetric imaging with rejection of out-of-focus contributions and backscattering from intermediate layers. We additionally study the impact of turbidity of the propagation medium in the performance of our system, using controlled concentrations of Arizona test dust particles, and accomplish axially and laterally confined measurements up to approximately 9.3 attenuation lengths. Finally, we demonstrate the detection of spatially-resolved in vivo autofluorescence from sub-millimeter Acocyclops royi copepods, and the a linear correlation between phytoplankton biomass and inelastic signal for two microalgae cultures (Tetraselmis and Emiliania huxleyi). These measurements shed a light on the possible applicability of our novel instrument for non-intrusive profiling of aquatic biota in the water column.