Laser-pulse-dispersion codes for turbid undersea imaging and communications

Potential applications for networks of intelligent undersea robots are numerous and varied. Dynamic events such as storm-water runoff, harmful algal blooms, or toxic spills need to be mapped accurately and monitored by sensors as they evolve. A distributed approach to sensing can provide valuable data sets that allow marine scientists to better understand their causes and effects. Similarly, distributed seabed-imaging surveys using laser-networked robots could provide identification-quality underwater imagery across larger regions of seabed than current technology permits. In recent years, the improved capabilities of compact, autonomous underwater vehicles and relevant sensor hardware have prompted undersea laser-imaging and communications researchers to consider new concepts compatible with such platforms.

It is well known that the use of adequate separation between camera and lights is an effective method to combat the undesirable effects of particulate scatter, which proliferates in typical coastal waters.¹ Using today's powerful lights and sensitive cameras, quality imagery can be obtained up to around three attenuation lengths before a contrast limit is reached because of volumetric scatter. (One attenuation length is the dimensionless reciprocal of the attenuation coefficient.) To extend range performance, synchronous laser-line-scan (LLS) or laser-range-gated techniques can offer access up to six attenuation lengths. LLS uses narrow spatial and angular filtering to reject undesirable scattering and also provides wide total-field-of-view imagery. Several LLS systems have been packaged in towed bodies for underwater survey.²

Our recent work has focused on investigating time-resolved LLS techniques, both in simulation³ and experimentally.⁴ The goal is to further reject angular-dependent scatter, improving performance and increasing the potential for size reduction (that is necessary to configure imagers compatible with compact, untethered, undersea robots). Other research groups have explored the use of high-frequency intensity modulation combined with coherent processing to restrict the deleterious effects of scatter.⁵ Test-tank measurements examining the loss in modulation depth caused by scatter for one-way paths have also been conducted.⁶

Our current research investigates distributed-LLS concepts. These are somewhat unconventional because the imaging system's components (illuminator and sensor) are distributed among multiple robots. This has been shown in recent test-tank trials to offer a vast improvement over single-platform techniques. This approach was originally demonstrated as a diver-deployed technique in the 1970s.⁷ Our results suggest that when fully developed, the distributed-LLS concept could be used for rapid inspection, survey, and site assessment, enabling seabed-to-surface operations in hundreds of feet of water.⁸ Our ongoing test-tank experimentation at the Harbor Branch Oceanographic Institute aims to develop techniques and validate radiative-transfer models that can be used to design optimal configurations of undersea laser-sensor networks. We are focusing on distributed imaging and other closely related behaviors, including communications (both non-line-of-sight and line-of-sight) and laser localization between adjacent network agents (see Figure 1).

Figure 1. Artist impression of future-generation undersea laser-sensor network showing imaging, communications, and localization behavior.

Because of the favorable transmission of the blue/green spectral range in natural waters (as well as a desire to limit the cost and complexity of the imager hardware), we use same-wavelength lasers and intensity modulation to distinguish between illuminating sources, encode synchronization information about scanned images, or implement phase-sensitive detection for relative position estimation between agents. Analysis suggests that up to gigahertz modulation frequencies will be desirable to implement most of these required behaviors. Accurate and efficient simulation of the propagation of intensity-modulated laser light is, therefore, a central and key requirement for the development of more advanced simulations.

We are extending Metron's electro-optic detection simulator radiative-transfer model³ to allow undersea laser-sensor-network developers and operators to accurately predict network parameters in prevailing optical conditions. We are, therefore, developing a very efficient Monte Carlo model to accurately simulate the spatiotemporal propagation of narrow, focused light beams in turbid environments. This model achieves a high level of efficiency by exploiting the equivalence between the beam- and point-spread functions.⁹ The code, which has been parallelized so that it may run on multiprocessor computers, can fully and accurately resolve the effects of multiple scattering events on the time-dependent behavior of electro-optical signal propagation.

We also seek to derive approximate solutions to the radiative-transfer equation under the small-angle scattering approximation that include the effects of temporal dispersion (pulse stretching). Our analytical model consists of a regular perturbation expansion for narrow, highly collimated beams, where Dolin's small-angle approximation¹⁰ is the leading-order result. We use a multiple-scales expansion to extend its range of validity and incorporate temporal dispersion. As a result, an approximate analytical solution may be obtained without prescribing its functional form a priori. We will use exact analytical solutions to the radiative-transfer equation, where available, and Monte Carlo simulations to validate the analytical approximation. We are evaluating the range of accuracy of the models against 500ps laser-pulse propagation measurements from well-controlled and characterized particle suspensions in a 12.5m test tank.