To monitor the impacts of climate change on coral reefs, several underwater electro-optic serial imaging sensors have been proposed with the intent of improving image quality by reducing the effects of scattering and attenuation. Laser Line Scan (LLS) underwater imaging is a serial imaging technique. The procedure involves the optical scanning of a narrow instantaneous field of view (IFOV) receiver in a synchronous fashion with a highly collimated laser source over a wide swath of seabed. Widely regarded as the optimal technology for extended range underwater optical imaging, LLS achieves up to 6 attenuation lengths in turbid sea water. These imagers, which typically utilize moderate-power green continuous wave (CW) lasers, require an adequate laser-receiver separation to reduce image degradation due to near-field backscatter. Currently available systems are large and require too much power to make them suitable for modern unmanned underwater platforms such as the man-portable unmanned underwater vehicle (UUV). For compact implementations of CW-LLS, the detection of target signals becomes obstructed by temporal overlap from volume scatter in turbid water, as well as further loss of target dynamic range due to ambient light during shallow water operations. To increase their operational range and provide high quality imagery, detection methods must be capable of separating the target and volume scattering signals to estimate the energy returning from the target alone and reduce the effect of high ambient light levels.
Recent work has therefore focused on investigating time-resolved pulsed LLS techniques, both in simulation and experimentally, using high repetition rate pulsed laser sources and gated photo detector hardware. The work includes detailed examination of environmental and system noise contributions for these single pulse per pixel imaging architectures, noise mitigation and performance enhancement via pulse deconvolution processing techniques, and the use of more advanced pulsed laser waveforms with coherent processing to reduce undesirable contributions from scattered light. Pulsed-gated serial imaging architectures also have the potential to generate seabed range maps and provide additional depth cues regarding object size and shape from the travel time of the light pulses. From a system packaging perspective, these systems are amenable to a more compact implementation by reducing laser-receiver separation. Under certain conditions it has been demonstrated that separation between the target and scattering volume return signals is possible, thereby increasing the imaging contrast and stand-off distance.
Current research also investigates distributed serial laser imaging concepts, which are somewhat unconventional because the imaging system components (scanned illuminator and staring photon bucket-style of receiver) are distributed among multiple platforms. Originally demonstrated as a diver-deployed technique in the 1970s, this approach has been shown in recent test tank trials to offer a vast improvement of range and image quality over single-platform techniques. The approach also has been test tank demonstrated using a frequency division multiple access communication technique to implement a multistatic configuration, the method even demonstrated that image acquisition is possible through the air-water interface. The Ocean Visibility and Optics Laboratory at Harbor Branch Oceanographic Institute has recently tested a prototype of the multiple field-of-view, distributed serial laser imager in a range of very turbid estuarine conditions off the east coast of Florida. The system demonstrated viable alternative to single platform laser imaging in challenging optical conditions (c > 3m-1), where state-of-the-art pulsed-gated LLS systems are no longer operable.