High spatial resolution underwater confocal LiDAR: reduction of optical aberrations in air-water interfaces using a dome port.

We study the impact of optical aberrations in underwater scanning confocal inelastic imaging arising from refraction at oblique incidences on a refractive index-mismatched air-glass-water interface. We experimentally demonstrate that optical aberrations at non-normal incidence drastically reduce the intensity of the inelastic signal and deteriorate the system resolution. At a 2.5° incidence angle, the return signal decreases to about 20% of its peak value at normal incidence. We implement passive correction using a spherical glass dome that is co-centered with the pivot point of the scanning mirror to ensure near-normal incidence on the interface irrespective of the scanning angle and depth. This configuration provides a drastic reduction in the optical aberrations within an angular range from -20° to 20°. The optical system is modeled in ray tracing software for validation. The interfacing of a scanning confocal system with a dome port unlocks near-diffraction-limited underwater imaging over wide areas without resorting to complex adaptive wavefront manipulation.