Session: 05-04-01 CCUS and Underwater Development/ Utilization

Submission Number: 156720

Deep Seabed Images Produced by the Underwater UV Laser Scanner Mounted on an Underwater Vehicle in Deep-Water Tests 

In recent years, laser scanners have attracted attention as a new seabed visualization technology that complements the advantages and disadvantages of conventional technologies such as sonars and cameras. The underwater vehicle, which equips a laser scanner, approaches the seabed and then the seabed reflection of laser irradiated toward the seabed is detected. It is expected that this will make it possible to visualize the state of the seabed that could not be detected by conventional sonars or cameras.

we are now developing an underwater UV laser scanner that uses a near-ultraviolet laser source in order to visualize unique seabed features. It aims to visualize the seabed in detail with three-dimensionally, at the same time, it is expected to visualize unprecedented seabed state by extracting reflections from the seabed that have fluorescent properties. There are deep-sea organisms that have self-luminous characteristic, and when these organisms are irradiated with a UV laser, it is expected that the bioluminescent substances in their bodies will be excited by the UV laser and fluoresce. In addition, there are deep-sea organisms and minerals that fluoresce when exposed to UV light from outside, even if they are not self-luminous characteristic. By effectively extracting the reflection of the UV laser from these organisms and/or minerals that have fluorescent properties, we can clarify seabed state that differ from those observed with visible lasers.

The underwater UV laser scanner was developed on the premise of being mounted on an underwater vehicle, and can be used at depths of up to 1,000 m. A UV pulse laser with the wavelength of 355 nm is generated by wavelength conversion and pulse modulation of a CW laser source with the wavelength of 1,064 nm. The UV pulse laser is irradiated into the seawater as a line-laser via multiple fixed mirrors and a four-sided rotating polygon mirror placed inside the system. The irradiation time per line laser is proportional to the rotation speed of the rotating polygon mirror. The rotation speed determines the number of laser measurement points that make up one line laser, which means the horizontal resolution of the seabed image produced by the underwater UV laser scanner. The UV laser irradiated into the seawater and reflected by the seabed is received by a photodetector via internal optical systems, a pinhole, and a wavelength filter. The photodetector uses a PMT (Photomultiplier Tube) with a built-in MCP (Microchannel Plate) that has high light- receiving sensitivity in the ultraviolet wavelength range, and has a multi-alkali photocathode. The round-trip propagation time of the laser is detected, at the same time, the received intensity of the signal is measured, owing to the received signal after photoelectric conversion in the MCP-PMT. In this case, multiple AGC (Auto Gain Control) functions are incorporated as a pre-stage to appropriately amplify the signal according to the amount of the propagation loss with propagation distance. Different seabed images, a distance image and an intensity image, are produced by the round-trip propagation time and the received intensity of signal. The distance image represents the three-dimensional topography of the seabed using TOF (Time Of Flight), and the intensity image represents the characteristics of the reflected seabed.

This paper describes the overview and elemental technologies of the underwater UV laser scanner. It also describes deep-sea tests conducted in the water depth of 200-900m using an underwater vehicle equipped with the underwater UV laser scanner. Moreover, seabed images produced by the underwater UV laser scanner and unique and interested features confirmed from those images, are exemplified.

Presenting Author: Shojiro Ishibashi JAMSTEC (Japan Agency for Marine-Earth and Science Technology)

Presenting Author Biography: He obtained his Ph.D. in 2013 and then joined the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). He is a senior researcher. He has participated in the development of nine underwater vehicles, and was responsible for the development of CPU control systems, navigation systems, and motion control algorithms. In 2010, he began to research the underwater laser technologies for deep-water use, and has been responsible for the development of underwater laser scanners and an underwater laser communication system until now.