Optical fiber communication, as the main mode of wired communication, has constituted the largest backbone communication network on the earth, and the total length of China's optical cable has been as long as 45.66 million kilometers, accounting for about 42% of the total length of the world's optical cables, it can be said that in the field of wired communication, China's optical fiber network deployment is leading the world. In the field of wireless communication, there are radio waves, microwave communications, millimeter wave communications, underwater acoustic communication and other ways, free space laser communication, as another communication mode in the field of wireless communication, due to the laser beam directivity is good, the security performance is strong, the available spectrum is wide, with the advantages of large communication capacity, etc., has been widely concerned and studied. With the development of marine resources and deep-sea exploration technology, underwater laser communication has gradually become a research hotspot in recent years
In space exploration, wireless laser communication technology can be used in inter-satellite and satellite-to-ground light transmission to realize the detection of deep space universe and the transmission of telemetry and remote sensing data. In the wireless optical communication equipment and application scenarios based on the ground platform, information transmission can be realized through wireless laser communication on the roof of different buildings, and in some emergency communication scenarios such as insufficient optical fiber coverage or high difficulty in laying optical cables, and even disaster relief, wireless laser communication can quickly guarantee the communication needs in real time. In the marine and underwater environment, which occupies more than two-thirds of the earth's surface, the traditional communication method is to use wired cables or optical cables to achieve communication, or to use radio frequency and sound waves to achieve underwater wireless communication, such as the use of sonar to detect nuclear submarines. In the ocean, the deep sea area accounts for about 50% of the earth's surface, just like the exploration of the universe, human exploration of the deep sea has never stopped, China's independent research and development design of Jiaolong No. 1 constantly refreshed the depth of diving, for human beings to dive into the deep sea to provide excellent equipment. However, with the exploration and utilization of the ocean, more and more sensors and detectors are deployed in the ocean, and it is imperative to build an interconnected high-speed communication network underwater.
Traditional underwater acoustic communication uses sound waves as the transmission medium, and the development is relatively mature, but due to the characteristics of slow sound wave transmission speed and narrow communication bandwidth, it is increasingly unable to adapt to the real-time and high-speed and high-bandwidth requirements of underwater audio, sensor network processing data transmission in the process of human exploration and utilization of the ocean. At the same time, because the light and inexpensive optical fiber cannot be used as the medium of binding optical signal transmission as on land, the traditional underwater acoustic communication equipment will have the disadvantages of large size, high power consumption and high cost with the increase of underwater sensing network nodes and the increase of complexity. In addition, the use of underwater acoustic communication for networking will also increase the threat of network security, and sound waves are easier to detect and intercept than lasers.
People often use frequencies to divide different light waves, for example, sunlight belongs to natural light, also known as white light, visible light, belongs to the frequency range of light waves that can be seen by the human eye. The use of light waves to achieve communication, one is to load the transmitted information onto the light waves, which belongs to the research of modulation technology; Second, the light wave should be given enough transmitting power so that the light wave can propagate forward in the transmission channel; The third is to receive the light waves, and then demodulate the required information. So what frequency of light waves can meet the above three basic requirements? For example, in long-distance communication scenarios such as fixed network communication, submarine cable, and power communication, optical fiber is the most commonly used transmission medium, and the light wave transmitted in the optical fiber mainly has three wavelengths of light, namely 850nm, 1310nm and 1550nm, which is due to the fact that when different wavelengths of light waves are transmitted in the optical fiber, there will be absorption loss of optical fiber to light wave power, and the absorption loss at these three wavelengths is relatively small. Therefore, these three wavelengths become the "communication window" of optical fiber communication. Similarly, when the wireless optical signal is transmitted in water, the medium becomes fresh water or seawater, and when light waves of different wavelengths are transmitted in water, there is also a "communication window", that is, blue-green light in the 450-550nm band.
Further comparison between light wave and sound wave, it is not difficult to find that the transmission rate of light wave underwater is higher than that of sound wave, which will help reduce the information transmission delay, and is conducive to real-time information interaction when establishing a wireless sensor network underwater, which is of great significance for scenarios with high real-time requirements such as underwater safe operation monitoring, underwater lifesaving and deep-sea diving detection. The frequency of the laser is much higher than that of the sound wave, which makes the underwater wireless laser communication have a wide modulation bandwidth, which can realize the modulation and transmission of information at a high bit rate, which is conducive to the transmission of high-definition and high-definition submarine images under seawater, and can meet the requirements of large-capacity communication of underwater wireless sensor network. The directivity of the laser beam is good, and the beam scattering angle is small, which is conducive to long-distance transmission, which makes the laser not easy to transmit in all directions like sound waves when transmitting underwater, and is easy to cause interference between different transmitters and receivers, and signal interference will not be conducive to the establishment of underwater wireless sensor network and effective information transmission; And due to the good directivity of the laser, once intercepted, it is easier to find the eavesdropper, thereby eliminating the hidden dangers of network security, and the security is higher than that of traditional underwater acoustic communication.
Compared with the development of fixed-line optical communication, underwater wireless optical communication is still relatively slow, and the technology is still far from being practical. The establishment of an optical communication network under water can refer to the optical fiber communication network that has been established and operated on land, which has made China's fixed network complete the optical fiber transformation and is moving towards an all-optical network, further improving the transmission rate, reducing the transmission delay, and increasing the transmission capacity. Underwater wireless optical communication also has to face and solve the problems of communication rate, communication capacity and communication security, and still have to solve the two eternal themes of communication transmission and exchange, but the development of underwater wireless optical communication is destined to solve more problems than traditional fixed network optical communication, for example, in optical fiber communication, optical fiber as a transmission channel, the nature is relatively stable, it is easy to establish a mathematical model of the channel, and the loss of optical fiber can be reduced as much as possible by improving the production process. The dispersion caused by the optical fiber can be compensated by the Dispersion Compensation Fiber (DCF); However, the complexity of the underwater environment, the organic and inorganic substances contained in the water, and the organisms in the water make it more difficult to establish the mathematical model of the underwater optical communication channel than the mathematical model of the channel in optical fiber communication.
The research on underwater optical communication at home and abroad has been in full swing, and people's determination and interest in ocean exploration are unprecedented, which has promoted the development of underwater optical communication. 5G has arrived, will 6G be far away? Wireless laser communication for the development of next-generation communication will also exert force in the 6G era, providing important support for the construction of an integrated communication network of sky, ground and sea!
The content is brief and error-prone, and valuable suggestions are welcome! ）
Selected references:
1.m. chaplin. water absorption spectrum[eb/ol]. 2016.
2.Research on Underwater Two-way Wireless Optical Communication for Deep-Sea Applications, Han Biao, Xi'an Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, 2018
3.Study on the propagation characteristics of optical pulses in the mixed channel of atmosphere and seawater, Wei Anhai, 2014
4.Research on Several Key Technologies of Underwater Coherent Laser Wireless Communication System, Yiming Li, University of Electronic Science and Technology of China, 2019
5.Research on Underwater Laser Communication Technology, Meng Linhu, Xi'an University of Technology, 2019
6.Visible light communication for 6G, Chi Nan, 2020
7.Zhang Yufan, Li Xin, Lv Weichao, Chen Jiawang, Zheng Minhui, Xu Jing. Progress in link composition and performance optimization of underwater wireless optical communication[J].Opto-Electronic Engineering,2020,47(09):3-13.
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