Underwater wireless optical communication: A high-speed and flexible alternative for communication
The ocean covers nearly 71% of the earth, and it stores abundant biological and mineral resources. The vast space and abundant resources of the ocean emphasize its scientific and strategic values, and mankind has never stopped exploring the ocean. The development of the autonomous underwater vehicle (AUV) promotes the exploration of the ocean. AUVs need to communicate with underwatersensor networks, satellites, scientific research ships and other facilities during their underwater operation to have massive information exchanges on observation results and operation orders. Traditional underwater communication with cables limits the flexibility of AUVs and makes the installation and maintenance difficult. Wireless acoustic communication has been widely used in underwater applications, but it has become difficult for acoustic communication to meet the increasing demand on capacity for communication in some specific situations. Meanwhile, radio frequency wireless communication in underwater applications still faces challenges in terms of transmission distance and the stability of communication.
Underwater wireless optical communication (UWOC) with high bandwidth and strong anti-electromagnetic interference ability has a promising prospect in underwater applications. However, the absorption and scattering of light signals by water as well as the complex underwater environment limit the transmission distance and the quality of UWOC. With the improvements of light sources and detectors, and the development of signal processing technology, the performance of UWOC is further enhanced. Currently, the transmission distance can reach several hundreds of meters, and the short-distance communication rate can reach a few dozens of Gbps. In addition, UWOC has already been successfully implemented in underwater exploration equipment. In the future, UWOC can become an important part of the integrated communication network of sea, land and air.
The research group of Professor Xu Jing from the optical communication laboratory of ocean College, Zhejiang University, published a review entitled "Link structure of underwater wireless optical communication and progress on performance optimization" in Vol. 47 Issue 9, 2020 of Opto Electronic Engineering. This review first overviewed some representative research results of UWOC, and summarized the key technologies in these researches. Then, the basic structure of an UWOC link was introduced, and the existing technologies as well as the research progresses were analyzed from three aspects, namely the transmitting end, the receiving end and the channel characteristics. This article compared the characteristics of light emitting diode (LED) with that of laser diode (LD) as light sources, and then the characteristics of several detectors such as photodiode and avalanche diode. The modulation, coding and equalization technologies applied in the transmitting and receiving end were also briefly introduced. The Monte Carlo numerical simulation method in channel research was introduced in details, and the importance of turbulence research was emphasized. Next, several important research directions of UWOC performance optimization were discussed. Firstly, using OAM multiplexing, WDM, OFDM and other multiplexing technologies can effectively improve the transmission rate. Secondly, high sensitivity single photon detector can be used to expand the coverage of UWOC link. Thirdly, the link alignment issues can be mitigated by using active alignment system and the transceiving configurations which favors alignment. At the end, it was pointed out that the practical implementation of UWOC is an important step in the development of UWOC.
Fig. 1 UWOC link basic configuration
Fig. 2 The received optical power distribution of a laser beam after passing through a (a) 4 m, (b) 8 m, (c) 12 m costal water channel; (d) 4 m, (e) 8 m, (f) 12 m harbor water channel
About The Group
The optical communication laboratory group, affiliated with the Ocean College, Zhejiang University, lead by Prof. Xu Jing, has been committed to researches on underwater wireless optical communication (UWOC) and deep-sea observation technology, and it has made an achievement in improving the coverage and data rate of UWOC by using single photon detectors, novel light sources and signal processing technology. The group members include Prof. Xu Jing, Associate Researcher Zhang Zejun and postgraduates. Prof. Xu Jing has published more than 90 papers in peer-reviewed SCI journals and international conference proceedings in the field of optical communication. He has 1 USA patent and 7 Chinese patents, and is leading several scientific research projects funded by NSFC.
Zhang Yufan, Li Xin, Lv Weichao, et al. Link structure of underwater wireless optical communication and progress on performance optimization[J]. Opto-Electronic Engineering, 2020, 47(9): 190734.