In a sense, the development history of modern wireless communication technology is a process of the evolution of communication frequencies from low to high.

However, with the advancement and popularization of technology, the original radio frequency communication spectrum is gradually becoming saturated, and the development of high frequency radio frequency technology such as extremely high frequency and high frequency still faces many challenges.

Therefore, another technology route with higher frequency and wider spectrum, wireless optical communication, has attracted more extensive attention in academia.

In fact, wireless optical communication is not a new technology and is not unfamiliar to us. From the ancient Chinese beacon towers to the photoelectric telegraph during World War I, its application history is far longer than that of radio frequency communication.

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However, due to the weak penetration and fast attenuation of light, this technology has not become a mainstream communication technology for a long time.

In a certain sense, the evolution of modern wireless communication technology is a history of communication frequencies evolving from low to high.

With the progress and widespread adoption of technology, the original radio frequency communication spectrum is gradually becoming saturated, and the development of high-frequency radio frequency technologies such as extremely high frequency and high frequency still faces many challenges.

As a result, another technology route with higher frequencies and a wider spectrum, wireless optical communication, has attracted broader attention in academia.

In fact, wireless optical communication is not a new technology and is not unfamiliar to us. From the ancient Chinese beacon towers to the photoelectric telegraph during World War I, its application history is far longer than that of radio frequency communication.

However, due to the limitations of light's weak penetration and rapid attenuation, this technology has not become a mainstream communication technology for a long time.Despite the continuous advancement of semiconductor light sources, photoelectric detection, and signal processing technologies in recent years, the research on high-speed wireless optical communication still mainly remains in the simulation stage, disconnected from practical applications.

Even though some studies have achieved real-time transmission, they are usually only suitable for specific scenarios and cannot communicate with other communication systems.

Based on this, Professor Wang Yongjin from Nanjing University of Posts and Telecommunications, leading a team in collaboration with Suzhou Bright Chip Optoelectronic Technology Co., Ltd., proposed and constructed a space-air-sea integrated all-optical communication network (All-Light Communication Network, ALCN) technology.

To adapt to different application scenarios, this technology integrates four specific wavelength optical communication technologies to establish its communication links.

Considering the low absorption rate of pure seawater for blue-green light, which enables long-distance communication underwater, the team members adopted blue light communication (Blue Light Communication, BLC) underwater, for controlling unmanned underwater vehicles or establishing communication between underwater equipment and buoys.In a swimming pool with a Neutral Density (ND) factor of 256 and a turbidity of 1.7, the transmission distance of the BLC link can reach 12 meters, and communication connections can be established within a 20-degree angle range.

However, the increase in turbidity and the intensification of water flow may lead to a weakening of the received signal and optical self-interference, which need to be resolved by adjusting the optical and electrical gains.

For the part above the sea surface, the researchers first used Wireless White Light Communication (WLC), a technology that can achieve communication within a range of 150 meters on land, suitable for sea surface beacons, buoys, and ships to accurately report marine conditions.

Secondly, to avoid the interference of sunlight, the research team chose Deep Ultraviolet Communication (DUVC) to establish connections with airborne equipment such as drones. In strong light environments, DUVC links can achieve day-blind communication within a maximum range of 7 meters.

It should be noted that although these LED-based communication technologies have a wider divergence angle, they also have the disadvantage of lower received optical power.Therefore, for point-to-point long-distance communication in free space, the research team chose Laser Diode-based Communication (LC) based on high optical power directed light.

Due to the small divergence angle of the laser diode, the LC link needs to rely on stabilizers and attitude sensors to maintain precise alignment during the communication process.

This network uses Ethernet switches and Wi-Fi technology to connect different optical communication links, achieving information sharing between different network nodes.

This flexible access method greatly broadens the application range of ALCN, from sensors to personal computers, to mobile devices, both wired and wireless devices can exchange data through this network.

Moreover, all wireless optical communication links use registered jack (RJ-45) network interfaces, unifying the transmission mode, which simplifies the deployment and maintenance process of the network.To meet the needs of a broader range of terminal access, the ALCN network expands the number of interfaces by daisy-chaining multiple Ethernet switches, ensuring that the network's connectivity and stability are not affected under multi-terminal deployment conditions.

In the experiment, ALCN demonstrated excellent performance. The overall average power consumption was 155 watts, with the four optical communication links accounting for 77.42% of the total power consumption.

Through continuous testing for 3.5 hours, the optical link power remained stable, showing sufficient stability and reliability.

When tested with a Pseudorandom Binary Sequence (PRBS) signal, both the BLC and DUVC links showed clear eye diagrams, indicating good signal quality and significant transmission effects.

Among the various links, the DUVC and LC links stood out. With the bias module, both types of links can achieve a transmission rate of 10Mbps, while meeting the timing constraints of the Xilinx Spartan-6 Field Programmable Gate Array (FPGA) and accommodating larger node data loads.Due to the lower 3-dB bandwidth and higher current demand of blue light and white light LEDs, the throughput of BLC and WLC links is relatively lower, with a maximum transmission rate of 2Mbps.

On the other hand, ALCN also performs well in key quantitative indicators such as packet loss ratio (PLR), latency, and jitter.

When measured using the maximum transmission unit of 1514 bytes, the overall PLR of ALCN is 2.78%, the maximum PLR is 5.80% (excluding self-check points), its transmission latency is less than 74 milliseconds, and the maximum jitter is 15 milliseconds.

It is evident that ALCN provides a promising solution for the future realization of multi-terminal, multi-service applications in the space-air-sea interconnection.

Recently, the related paper was published in Optic Express with the title "All-Light Communication Network for Space-Air-Sea Integrated Interconnection" [1].Nanjing University of Posts and Telecommunications doctoral candidate Wang Lining is the first author, with Professor Wang Yongjin serving as the corresponding author.

Next, researchers plan to address the bottleneck issues caused by LEDs through wavelength division multiplexing technology, compensating for the lower transmission rates of BLC and WLC links, thereby enhancing the overall throughput of the all-optical communication network.