Pulse versus CW Laser Line Scan Imaging Detection Methods: Simulation Results

Laser Line Scan (LLS) systems rely upon narrow angular field apertures and displacement between emitter and receiver to reduce volume backscatter from the common volume near the target. With this configuration, multiple volume scattering near the sensor becomes a significant contribution to the total backscatter return. Detection of continuous wave laser return signals is complicated at the range limit of these systems due to the temporal overlap between the target return and the backscatter return. In the compact implementation of these systems, necessary when the deployment platform is the 12" or 21" diameter AUV, this detection ambiguity can lead to a significant degradation in imaging performance. Detection methods must therefore separate the two signals and estimate the energy returning from the target alone. Indeed, the use of pulsed laser illumination under some conditions allows for temporal separation of the target and volume scattering return signals. However, this method relies upon knowledge of the target distance and establishment of a gated detection scheme. We present a comparison of simulation results for both the CW and pulsed-laser cases in a range of turbid water conditions and at differing optical attenuation lengths approaching the limiting case for these types of underwater imagers. These simulations are performed using models developed by Metron Inc. (Reston, VA), and allow both a time history and image quality comparison. The goal of this simulation work is to identify the operational and environmental conditions under which each scheme is usable for the AUV-deployed scenario.