Tailoring of RF coded optical pulses for underwater 3D imaging

Laser imaging of 3D targets in highly scattering environments has traditionally been done using narrow 1-2ns optical pulses in conjunction with a range-gated detector. While this approach offers range resolution determined by the width of the optical pulse and is very effective in eliminating the effects of backscatter from the turbid environment, it lacks resistance to the effects of forward scatter. The Modulated Pulse Lidar is an alternative approach developed by researchers at NAVAIR that uses longer pulses that are intensity modulated with RF coded waveforms. Radar pulse compression techniques involving the use of RF coded waveforms with large bandwidths provide range resolution that meets or exceeds that of the short pulse approach, while matched-filtering techniques are employed to suppress backscatter. More importantly, the modulated pulse approach provides resistance to forward scatter effects that the short pulse approach does not. This work focuses on the comparison of imagery generated employing both pulse schemes in the same line-scan underwater 3D imaging system over a range of water turbidities. Experimental imagery and temporal data will be presented that shows that the modulated pulse approach does in fact offer an advantage over the short pulse approach due to its resistance to forward scatter effects.