This article presents the design, development, and testing of an ultracompact underwater pulsed digital holographic camera (named weeHoloCam) that has been successfully deployed in the North Sea. With a footprint of 9 cm diameter × 60 cm long and weighing just 3.5 kg, to our knowledge, this is the lightest and most compact system of its kind in the world. It can be easily adapted for mounting on remotely operated underwater vehicles (ROVs) and autonomous underwater vehicles (AUVs). weeHoloCam can record 12 cm ³ of the water column per hologram at 20 Hz and has a capacity to continuously record up to 200 000 holograms, each 5 MB in size. Along with the holographic camera, a field-programmable gate array (FPGA)-accelerated hologram analysis suite has been developed. The 838 megapixel/s reconstruction speed demonstrated is the highest reported speed for a single FPGA implementation to date. This is an extremely important development as rapid processing of recorded holograms is vital for the viability of subsea holographic cameras. weeHoloCam was successfully trialed in the North Sea, recording close to a hundred thousand holograms and extracting particle images within those holograms at a rate of 190 per min. weeHoloCam has the potential to play an important role in several oceanic studies: spatial and temporal monitoring of plankton species; study of plankton dynamics; study of vertical transport of floc; and monitoring microplastic pollution in the ocean, to name but a few.

A continuous-wave, diode-pumped, room-temperature ${\text{Tm:YAlO}}_{\text{3}}$ laser was built, characterized, and compared with a theoretical model. Spectroscopic parameters of the laser crystal, such as the pump band (3H4 manifold) lifetime, the upper laser level (3F4 manifold) lifetime, and the absorption coefficient were measured. The experimental data were used to calculate the quantum efficiency of Tm self-quenching of the 3H4 manifold.

Measuring the distribution, characteristics and dynamics of marine microscale plankton and other particulate matter is essential to understand the vertical flux of elements in the marine environment. Digital holographic microscopy is a powerful approach for measuring these and studying their 3-D trajectories in a relatively large observation volume. This article demonstrates a compact, in-line digital holographic microscope that allows large-volume and high-resolution recording of marine particles through combining a continuous wave laser and a short exposure CMOS camera with efficient global shutters. A resolution of better than 10 μm is demonstrated in air and the minimum distinguishable size of targets recorded in water is approximately 20 μm. The maximum volumetric throughput of the setup is 1904 mL/s. The microscope can take motion blur free holograms of particles moving at up to 490 mm/s in theory, and has been tested in the ∼200-mm/s flowing water. The orientation of the measured volume improves the ability of digital holography in profiling sinking rates and active vertical migration. The system was tested onboard a research vessel to record a range of live plankton and other particles. The motion of some samples, including the sinking motion and swimming motion, was analyzed using custom developed image processing software. The experimental results show that the combination of high resolution and a large volume over which motion of sparse-distribution particles can be tracked, can improve the ability to differentiate between different types of marine particle and identify behaviors of live plankton.

Studying the amount, variety, and distribution of microscale particles and plankton at the global scale at different times and seasons is very important to understand ocean environments, and by reference, the global environment. It is important to image them in their natural habitats. Many optical techniques have been developed to meet this requirement. Most techniques suffer from one or another drawbacks, such as limited depth-of-field or low volume. Digital holography is an advanced optical imaging technique to image micro-objects, and it provides high-resolution recording, large depth-of-field and recording volume, and 3D viewing and tracking. This paper describes basic principles of in-line digital holography and provides common image processing methods. Four submersible digital holographic cameras, eHoloCam, RamaCam, weeHoloCam, and LISST-Holo, are introduced, as well as their image processing software. At the end, some limitations and challenges in the underwater holographic imaging systems are mentioned, and some possible solutions are discussed.