Our current research adds another layer of enhancement over Pulsed Laser Line Scanning (PLLS) by partially rejecting the source-to-target forward scattering effects. The technique uses the same source as PLLS system, but the receiver consists of a linear array of n time-resolved bucket collectors in the cross-axis direction. The spacing between receive elements is large enough to cause measurable time-of-flight differences in the near-field. By using a near-field beamforming algorithm, which sums together the time-resolved receiver waveforms with the appropriate delays based on the geometry, the contrast ratio is improved and some of the multiple forward scattering induced blur/glow/ghosting effects are rejected as well as improving the time-of-flight based 3D image reconstruction. This paper utilizes modeling to investigate the feasibility of this method.
The compressive line sensing (CLS) active imaging system was proposed and validated through a series of test-tank experiments. As an energy-efficient alternative to the traditional line-scan serial image, the CLS system will be highly beneficial for long-duration surveillance missions using unmanned, power-constrained platforms such as unmanned aerial or underwater vehicles. In this paper, the application of an active spatial light modulator (SLM), the individually addressable laser diode array, in a CLS imaging system is investigated. In the CLS context, active SLM technology can be advantageous over passive SLMs such as the digital micro-mirror device. Initial experimental results are discussed.
Compressive sensing (CS) theory has drawn great interest and led to new imaging techniques in many different fields. Over the last few years, the authors have conducted extensive research on CS-based active electro-optical imaging in a scattering medium, such as the underwater environment. This paper proposes a compressive line sensing underwater imaging system that is more compatible with conventional underwater survey operations. This new imaging system builds on our frame-based CS underwater laser imager concept, which is more advantageous for hover capable platforms. We contrast features of CS underwater imaging with those of traditional underwater electro-optical imaging and highlight some advantages of the CS approach. Simulation and initial underwater validation test results are also presented.
Compressive sensing (CS) theory has drawn great interest and led to new imaging techniques in many different fields.
In recent years, the FAU/HBOI OVOL has conducted extensive research to study the CS based active electro-optical
imaging system in the scattering medium such as the underwater environment. The unique features of such system in
comparison with the traditional underwater electro-optical imaging system are discussed. Building upon the knowledge
from the previous work on a frame based CS underwater laser imager concept, more advantageous for hover-capable
platforms such as the Hovering Autonomous Underwater Vehicle (HAUV), a compressive line sensing underwater
imaging (CLSUI) system that is more compatible with the conventional underwater platforms where images are formed
in whiskbroom fashion, is proposed in this paper. Simulation results are discussed.
Compressive sensing (CS) theory has drawn great interest in recent years and has led to new image-acquisition techniques in many different fields. This research investigates a CS-based active underwater laser serial imaging system, which employs a spatial light modulator (SLM) at the source. A multiscale polarity-flipping measurement matrix and a model-assisted image reconstruction concept are proposed to address limitations imposed by a scattering medium. These concepts are also applicable to CS-based imaging in atmospheric environments characterized by fog, rain, or clouds. Simulation results comparing the performance of the proposed technique with that of traditional laser line scan (LLS) sensors and other structured illumination-based imager are analyzed. Experimental results from over-the-air and underwater tests are also presented. The potential for extending the proposed frame-based imaging technique to the traditional line-by-line scanning mode is discussed.
The compressive sensing (CS) theory has drawn great interest in signal processing community in recent years and led
to new image acquisition techniques in many different fields. This research attempts to develop a CS based underwater
laser serial imaging system. A Digital Mirror Device (DMD) based system configuration is proposed. The constraints
due to scattering medium are studied. A multi-scale measurement matrix design, the "model-assisted" image
reconstruction concept and a volume backscattering reduction technique are proposed to mitigate such constraints. These
concepts are also applicable to CS based imager in other scattering environment such as fog, rain or clouds. Simulation
results using a modified imaging model developed by HBOI and Metron and experimental results using a simple optical
bench setup are presented. Finally the proposed technique is compared with traditional laser line scan (LLS) design and
other structured illumination based imager.
The objective of this work was to develop and validate approaches to accurately and efficiently model channel
characteristics in a range of environmental and operational conditions for underwater laser communications systems that
use high frequency amplitude modulation (AM) or coded pulse trains. Two approaches were investigated: 1) a Monte
Carlo model to calculate impulse responses for a particular system hardware design over a large range of environmental
and operational conditions, and 2) a semi-analytic model which has the potential to be more computationally efficient
than the Monte Carlo model. The formulation of the Monte Carlo code is presented in this paper, together with test
results used to evaluate the range of accuracy of the model against 500ps laser-pulse propagation measurements from
well-controlled and characterized particle suspensions in a 12.5m test tank. A multiple scattering study using the Monte
Carlo simulation code was also performed and some results are presented. Results from the semi-analytic model will be
compared with these test tank measurements and the Monte Carlo model in a later paper.
Experimental results from two alternate approaches to underwater imaging based around the well known Laser Line
Scan (LLS) serial imaging technique are presented. Traditionally employing Continuous Wave (CW) laser excitation,
LLS is known to improve achievable distance and image contrast in scattering-dominant waters by reducing both the
backscatter and forward scatter levels reaching the optical receiver. This study involved designing and building
prototype benchtop CW-LLS and pulsed-gated LLS imagers to perform a series of experiments in the Harbor Branch
Oceanographic Institute (HBOI) full-scale laser imaging tank, under controlled scattering conditions using known
particle suspensions. Employing fixed laser-receiver separation (24.3cm) in a bi-static optical geometry, the CW-LLS
was capable of producing crisp, high contrast images at beyond 4 beam attenuation lengths at 7 meters stand-off
distance. Beyond this stand-off distance or at greater turbidity, the imaging performance began to be limited mainly by
multiple backscatter and shot noise generated in the receiver, eventually reaching a complete contrast limit at around 6
beam attenuation lengths. Using identical optical geometry as the CW-LLS, a pulsed-gated laser line scan (PG-LLS)
system was configured and tested, demonstrating a significant reduction in the backscatter reaching the receiver. When
compared with the CW-LLS at 7 meters stand-off distance, the PG-LLS did not become limited due to multiple
backscatter, instead reaching a limit (believed to be primarily due to forward-scattered light overcoming the attenuated
direct target signal) beyond 7 beam attenuation lengths. This result demonstrates the potential for a greater operational
limit as compared to previous CW-LLS configuration.
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