Digital passband processing of wideband-modulated optical signals for enhanced underwater imaging

Abstract

Radar modulation, demodulation, and signal processing techniques have been merged with laser imaging to enhance visibility in murky underwater environments. The modulation provides a way to reject multiple scattered light that would otherwise reduce image contrast and resolution. Recent work has focused on the use of wideband modulation schemes and digital passband processing to resolve range details of an underwater scene. Use of the CLEAN algorithm has also been investigated to extract object features that are obscured by scattered light. Results from controlled laboratory experiments show an improvement in the range resolution and accuracy of underwater imagery relative to data collected with a conventional short pulse system.

Range-Doppler processing of the underwater radar-encoded laser system

“…The processing chain for the radar-encoded laser system is presented; more details can be found in the references [9][10] . The radar-encoded laser system transmits a chirp modulated pulse.…”

“…The use of high-frequency modulation allows this technique to exploit experimentally-measured cutoff frequencies of both backscatter and forward scatter [5][6][7] . The chirp waveform is a commonly used modulation scheme 5,[8][9][10] . By applying pulse compression to the chirp waveform, one can achieve resolution comparable to a short pulse lidar system while utilizing the modulation to reduce the effects of scattering.…”

Initial laboratory and field testing of the modulated underwater laser imaging system

“…In this approach, the radar-encoded laser system transmits a pulse on the order of tens of nanoseconds, with a radar-like intensity-modulation applied during the pulse. One common modulation scheme is the chirp waveform [7][8][9] . By applying pulse compression to the chirp waveform, resolution comparable to a conventional short pulse lidar is achieved while simultaneously the modulation reduces the effects of scattering.…”

Underwater Object Segmentation Based on Optical Features

“…Two key aspects to consider for underwater artificial light are optical characteristics such as its strength and spectrum, and its collimation, which is the region covered by the light. The former is accounted for by many underwater imaging and image processing methods, such as those extending underwater visibility [ 9 , 10 ]. However, no existing methods determine the importance of light collimation for underwater computer vision tasks.…”