Development of a free-drifting submersible digital holographic imaging system

Submersible film-based holographic systems have demonstrated the unique ability of holography to provide high resolution, three-dimensional, in-situ images of marine organisms. Inherently, use of film limits the frame rate and total number of holograms. This paper describes a submersible, free-drifting, digital holographic cinematography system that has a real-time fiber optic communication link. This system collects dual-view digital holograms at a rate of 15 frames per second, enabling the user to observe the behavior of marine plankton and distinguish motile organisms from abiotic particles. In order to follow the particles in time, the sample should have as little motion relative to the cameras as possible. To achieve this goal, the submersible is neutrally buoyant, and has high drag generating elements at the height of the sample volume. In addition, the components surrounding the sample are streamlined and designed to minimize the local flow disturbance. The data from the two digital cameras and other sensors are transmitted at 120 MB/s through a 1 km long, 250 μm diameter, fiber optic cable to an acquisition system located on a research vessel. The optical fiber is spooled out from the submersible by a powered mechanism, as the submersible drifts away from the vessel Releasing the fiber out at a rate greater than that of the drifting speed minimizes the transmission of forces through the cable, effectively decoupling the submersible from the cable and vessel dynamics. A variable buoyancy system provides vertical position control while still allowing the system to drift vertically with the surrounding fluid, i.e., follow internal waves. The dual-view holo-camera records two in-line holograms from orthogonal directions, each with a volume of 40.5 cm³. Without lenses, the resolution in the 3.4 cm³ volume where the beams cross each other, is about 7.4 μm in all three directions. Outside of the overlapping region, the resolution in the beam axial direction is lower, but the lateral resolution remains 7.4 ³. An optional 2× lens doubles the resolution, but reduces the sample volume. The first field deployment for this system took place in June 2005, in the Ria de Pontevedra, Spain. It was used for examining thin layer- s of harmful algal blooms.