Optical detection, ranging, and ima

Optical detection, ranging, and imaging of targets in turbid water is complicated by absorption and scattering. It has been shown that using a pulsed laser source with a range-gated receiver or an intensity modulated source with a coherent RF receiver can improve target contrast in turbid water. A blended approach using a modulated-pulse waveform has been previously suggested as a way to further improve target contrast. However only recently has a rugged and reliable laser source been developed that is capable of synthesizing such a waveform so that the effect of the underwater environment on the propagation of a modulated pulse can be studied. In this paper, we outline the motivation for the modulated-pulse (MP) concept, and experimentally evaluate different MP waveforms: single-tone MP and pseudorandom coded MP sequences.In this work, we have revisited the modulatedpulse concept for optical imaging and detection underwater. Both single-tone and pseudorandom modulated pulses were examined. For underwater applications, the modulated-pulse waveform blends the benefits from both the short-pulse range-gated and intensity modulated techniques. We have shown that in clear water, the PN-MP stands as an improvement over the ST-MP due to their excellent correlation properties of pseudorandom codes. The PN-MP waveforms also appear to exhibit the ability to distin guish target returns from multiply forward-scattered returns. In high backscatter environments, the highpass filtering technique previously described for the ST-MP can be used for the PN-MP as well, even though some spectral content of the pseudorandom waveform is removed in the process. Future studies will examine custom code design given a priori knowledge of the impulse response of the underwater channel or a priori knowledge of the desire to use a high-pass filtering technique, both of which can be used to mitigate backscatter. When coupled with other techniques like photoreceiver gating, additional clutter suppression may be realized. It is our hope that this study provides an entry point for further study into waveform design for optical imaging, ranging, and detection for underwater applications.