Abstract
Extended range underwater optical imaging techniques can be classified into one of two broad categories; those which use synchronously scanned narrow source and receiver paths to restrict both back-scattered and forward-scattered light reaching the receiver (continuouswave laser line scan); and those which use pulsed sources and time-gating to remove back-scatter noise (pulsed laser line scan and pulse-gated laser line scan). Laser line scan systems are observed to perform at up to 5 to 6 optical attenuation lengths, but greater standoff distances are desirable for seabed imaging using the growing fleet of autonomous underwater vehicles (AUVs). Currently, a moderate physical separation between laser and receiver is necessary to reject near-field multiple back-scatter, which imposes restrictions on AUV miniaturization. Recent experiments and theoretical modeling reveal that significant imaging improvements are possible over the existing continuous-wave laser line scan systems (LLS), by using a pulsed-gated version of the LLS (PG-LLS). However, the use of such a technique has a greater advantage in reducing the overall form factor over conventional LLS imaging system, as well as providing greater depth-of-field. In this paper, we present experimental results comparing both LLS and PG-LLS systems for several source-receiver separations and standoff distances. These results compare favorably with images obtained from validated LLS image simulation tools, and indicate the potential for reducing the source-receiver separation and therefore the system size.
Highlights
Current needs for advanced undersea imaging systems include search and rescue operations, military surveillance, offshore oil pipeline inspection, and scientific observation
The source-receiver separation (S-R) is 376 mm, which is considered large for man-portable autonomous underwater vehicles (AUVs) usage
The larger-scale motivation for this work is to investigate the use of scanned pulsed laser sources with gated receivers for more compact implementations of extended range underwater laser imaging systems than are currently available
Summary
Current needs for advanced undersea imaging systems include search and rescue operations, military surveillance, offshore oil pipeline inspection, and scientific observation. Well known techniques for achieving this result include time gating, where the detection system is timed to be responsive to the arrival of the object photons after most of the nearby scattered photons have arrived These methods have the potential to determine bathymetry from the travel time of the light pulses, and as is the intent of this paper to show, pulsed-gated imager architectures can provide a more compact LLS implementation with a reduced laser-receiver separation and equivalent performance to larger CW-LLS systems. Having evaluated the accuracy of the backscatter time history model to predict the magnitude and timing of backscatter computation, the authors present laser line scan (LLS) image simulation results, to consider the relative merits of using either continuous wave (CW) or pulsed laser sources, as a function of source to receiver separation in order to analyze the expected imaging performance of future LLS payloads sized to be compatible with three common form factor autonomous underwater vehicles. Effects of water beam attenuation coefficient and scattering on image performance
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