Ranging through Shallow Semitransparent Media with Polarization Lidar

A new approach to shallow depth measurement (<2 m) using polarization lidar is presented. The transmitter consists of a 532-nm linearly polarized laser coupled with conditioning and polarization optics. The prototype lidar evaluates the differing polarization attributes of signals scattered from semitransparent media surfaces, simultaneously receiving signals polarized in the planes parallel and perpendicular to the transmitted laser signal via dual photomultiplier tubes. In the event the first surface nearly preserves the incident polarization and the second surface depolarizes the incident energy, signals scattered from the second surface are isolated from the first by a polarization analyzer in the receiver. This approach translates depth measurements into the conditions of single surface range measurements, giving the ability to resolve extremely shallow depths (e.g., 1 cm) independent of laser or detector pulse widths. This approach can circumvent dead time issues in photon-counting systems and can be applied to extremely shallow and deeper waters for depth determination. Furthermore, the approach provides an estimate of the first surface linear depolarization ratio, enabling differentiation between surfaces with variable scattering properties. Using this technique to acquire range-resolved observations through shallow semitransparent media to measure depth removes the dependency on sophisticated and, subsequently, costly lidar components by becoming independent of system bandwidth. The limiting factor in-depth resolution is driven only by the timing resolution of the time-to-digital converter. This approach allows for the use of common lasers, optics, and detector equipment, making it comparatively cheaper and less complex while achieving vast improvement in the accuracy and precision of shallow depth measurements.