Underwater plenoptic cameras optimized for water refraction.

By inserting a microlens array (MLA) between the main lens and imaging sensor, plenoptic cameras can capture 3D information of objects via single-shot imaging. However, for an underwater plenoptic camera, a waterproof spherical shell is needed to isolate the inner camera from the water, thus the performance of the overall imaging system will change due to the refractive effects of the waterproof and water medium. Accordingly, imaging properties like image clarity and field of view (FOV) will change. To address this issue, this paper proposes an optimized underwater plenoptic camera that compensates for the changes in image clarity and FOV. Based on the geometry simplification and the ray propagation analysis, the equivalent imaging process of each portion of an underwater plenoptic camera is modeled. To mitigate the impact of the FOV of the spherical shell and the water medium on image clarity, as well as to ensure successful assembly, an optimization model for physical parameters is derived after calibrating the minimum distance between the spherical shell and the main lens. The simulation results before and after underwater optimization are compared, which confirm the correctness of the proposed method. Additionally, a practical underwater focused plenoptic camera is designed, further demonstrating the effectiveness of the proposed model in real underwater scenarios.

Experimental demonstration of a transmitter with a 50° divergence angle, 50 Mbps rate, and 476 mW optical power for underwater wireless optical communication based on an engineered diffuser.

In practical application of underwater wireless optical communication (UWOC), the transmitter should have a larger divergence angle to make it easier to establish a communication link, besides high modulated rate and high optical power. Laser diodes (LD) are suitable to design such transmitter, thanks to their simpler structure and much faster switching speed. However, it is difficult to implement for widespread use in ocean engineering because of its quite small divergence angle. For this, we present a simple way to enlarge the divergence angle for an LD transmitter based on an engineered diffuser in this paper. First, we design a blue LD transmitter that has 476 mW output power, 50 Mbps rate, and 50° divergence angle. Then, using such transmitter, we establish a UWOC system in a large experimental tank with 13.3 m communication distance and about 0.26m-1 attenuation coefficient of water. The results show that if the deviation of the transmitting direction is up to ±25∘, the communication system is workable. Emission light from the transmitter could cover a 42.5% solid angle of the hemisphere space. The combination performances of speed, angular coverage, and optical power are suitable for ocean engineering. Also, it implies that a light field could be designed by using a suitable engineered diffuser for UWOC. The method presented in this paper is simple and pragmatic, which is useful to reduce the difficulty in establishing communication links and is easy to popularize.

The Color Improvement of Underwater Images Based on Light Source and Detector.

As one of the most direct approaches to perceive the world, optical images can provide plenty of useful information for underwater applications. However, underwater images often present color deviation due to the light attenuation in the water, which reduces the efficiency and accuracy in underwater applications. To improve the color reproduction of underwater images, we proposed a method with adjusting the spectral component of the light source and the spectral response of the detector. Then, we built the experimental setup to study the color deviation of underwater images with different lamps and different cameras. The experimental results showed that, a) in terms of light source, the color deviation of an underwater image with warm light LED (Light Emitting Diode) (with the value of Δa*2+Δb*2 being 26.58) was the smallest compared with other lamps, b) in terms of detectors, the color deviation of images with the 3×CMOS RGB camera (a novel underwater camera with three CMOS sensors developed for suppressing the color deviation in our team) (with the value of Δa*2+Δb*2 being 25.25) was the smallest compared with other cameras. The experimental result (i.e., the result of color improvement between different lamps or between different cameras) verified our assumption that the underwater image color could be improved by adjusting the spectral component of the light source and the spectral response of the detector. Differing from the color improvement method with image processing, this color-improvement method was based on hardware, which had advantages, including more image information being retained and less-time being consumed.

Effect of high carbon incorporation in Co substrates on the epitaxy of hexagonal boron nitride/graphene heterostructures.

We carried out a systematic study of hexagonal boron nitride/graphene (h-BN/G) heterostructure growth by introducing high incorporation of a carbon (C) source on a heated cobalt (Co) foil substrate followed by boron and nitrogen sources in a molecular beam epitaxy system. With the increase of C incorporation in Co, three distinct regions of h-BN/G heterostructures were observed from region (1) where the C saturation was not attained at the growth temperature (900 °C) and G was grown only by precipitation during the cooling process to form a 'G network' underneath the h-BN film; to region (2) where the Co substrate was just saturated by C atoms at the growth temperature and a part of G growth occurs isothermally to form G islands and another part by precipitation, resulting in a non-uniform h-BN/G film; and to region (3) where a continuous layered G structure was formed at the growth temperature and precipitated C atoms added additional G layers to the system, leading to a uniform h-BN/G film. It is also found that in all three h-BN/G heterostructure growth regions, a 3 h h-BN growth at 900 °C led to h-BN film with a thickness of 1-2 nm, regardless of the underneath G layers' thickness or morphology. Growth time and growth temperature effects have been also studied.