Inelastic Confocal LiDAR for localized underwater measurements

We present an underwater confocal continuous-wave (CW) LiDAR sensor capable of remote fluorescence measurements in a spatially confined probe volume. The system is based on a blue 450nm CW laser diode focused at a distance by a pair of lenses in a telescope configuration. The inelastic signal is collected coaxially and deflected by a dichroic mirror through a lens to be imaged onto an avalanche photodiode. A pinhole placed in the probe’s conjugate plane eliminates the out-of-focus light to ensure axial confinement. The Rayleigh length of the laser beam, in combination with the pinhole, set the near-diffraction limited measurement volume. 3D imaging is accomplished through a combination of a dual-axis galvanometer scanning mirror and a linear z-translation stage for one of the telescope lenses. The sensor’s spatial resolution is assessed by measuring the Point Spread Function at representative distances in controlled underwater conditions using sub-resolution microbeads as probes. The longitudinal and transverse resolutions are, respectively, around 5mm and 50μm at 0,6m distance, and 14mm and 65μm at 1,2m. A theoretical model describing the spatial confinement through the overlap integral between the transmitted beam and the virtually back propagated pinhole to the target space is also proposed and compared with the experimental results. Optical aberrations induced by the air-glass-water refractive index mismatch at non-normal incidence angles are discussed. Due to the inherent properties of our confocal inelastic LiDAR, superior three-dimensional resolutions can be achieved at distances of up to 5m when compared to ns-pulsed time-of-flight LiDARs. This novel tool can potentially be integrated in a submersible vehicle to enable in situ detection and imaging of sub-mm to mm-sized organisms (as plankton) in the water column.