Whether in ground-based or non-ground-based networks, Free-Space Optical (FSO) communication and systems can achieve rapid and secure connectivity. With significant advances in space optics over the past two decades, ultra-high bandwidth signals now often travel back and forth over long distances, providing a communications canopy that extends across the globe.
Figure1: Application of a typical wireless communication concept in low earth orbit satellite constellation. Each satellite carries four optical communication terminals, which are connected with adjacent satellites in all directions to form a mesh network.
From transmitter to receiver, to all the related optical equipment in the middle, every technological leap in the past 20 years has helped wireless communication become cheaper, faster, easier to deploy and ultimately more commercially viable. According to Global Market Estimates, the value of FSO industry will increase from 4.4 billion US dollars in 2022 to 47.5 billion US dollars in 2027, with a compound annual growth rate of 34.1%.
A variety of advanced technologies are playing a role, including high-power and small linewidth lasers; Complex opto-mechanical control systems, such as fine steering mirrors, can accurately shape and guide lasers; High sensitivity detector; Adaptive and Coherent Optics for Atmospheric Aberration Compensation; A high-speed digital processing device for signal encoding and decoding; And photonic integrated chips (PICs) that help to reduce optical signal processing components.
With the development of technology, the increasing demand for data bandwidth has promoted people's interest in wireless networks. The demand for greater bandwidth around the world has prompted several companies to deploy large "constellations" that sometimes consist of thousands of satellites, most of which are connected by wireless optical systems capable of exchanging optical data signals with terrestrial terminals. Compared with traditional point-to-point microwave links, wireless optical networks provide a rapidly deployable wireless access solution with higher bandwidth, security and lower power consumption.
Figure2: Compared with traditional point-to-point microwave links, Infinite Optical Network provides a rapidly deployable wireless access solution with higher bandwidth, security and lower power consumption.
John Reid, scientific director and co-founder of Aircision in Eindhoven, the Netherlands, said: "In a satellite system, laser signals from Earth terminals must lock the satellite as it flies over the horizon, and then wait for the line of sight to drop to an acceptable level before transmitting data at maximum speed."
The first research group to report on gigabit speed laser satellite communications was the European Data Relay System (EDRS), a commercial cooperation agreement between Airbus and the European Space Agency (ESA). Part of the purpose of the network is to support the Copernicus Project, which is jointly managed by ESA and the European Commission. It is estimated that the Copernicus plan will require the on-board telecommunications infrastructure to transmit TB-level Earth observation data from space to the ground every day. Providing these data to ground stations in real time is very useful for land, sea and ice monitoring, as well as government and security services.
SpaceX's "Starlink" has 12,000 satellites, which is probably the most striking giant constellation and the largest constellation of satellites in low Earth orbit. Its satellites operate in orbit at an altitude of about 550 kilometers, relatively close to the Earth, to reduce the delay to about 20 milliseconds, and support high data rate activities such as games and streaming media at speeds of 50 to 500 Mbit/s.
Figure3: Optical communication link network, satellite links showing low Earth orbit (LEO) and geosynchronous equatorial orbit (GEO) layers, and wireless links with aircraft, balloons and ground stations.
Unlike fixed underground telecommunication fibers, these constellations can be moved wherever needed. For example, these requirements may include communication during natural or man-made disasters. Plug-in terminals from several trucks were shipped to Ukraine early last year to compensate for damage to communications links caused by Russian bombing. The ground antenna also provides an important lifeline for the Ukrainian army, through which users can connect to the nearest Starlink satellite and communicate with the nearest ground station in neighboring Poland.
High-performance but affordable optical systems that can withstand rigorous mechanical, thermal and radiative conditions will be the key to the success of such large constellations. "These systems will include optical amplifiers that meet space conditions, electronic devices with sufficient optical output power and high data rate capabilities, and space-verified pointing, acquisition and tracking algorithms to establish connections between long-distance satellites."
Figure4: Beyond interference from Earth's atmosphere, optical links between satellites send signals back and forth across the entire globe.
In quantum optics, single photon sources and detectors are used to encrypt data and transmit data safely over long distances. In fact, FSO links do not cause the same transmission loss and limited distance as quantum key distribution (QKD) technology in optical fiber networks. FSO can further utilize the traditional wavelength division multiplexing technology to send encrypted data at a higher rate.
At present, QKD is limited due to the lack of suitable optical amplifiers to overcome the limitation of optical fiber transmission, but FSO may provide a solution. Atmospheric constraints applicable to ground-to-air networks also hinder ground-based FSO communication. However, as the cost of key optical components decreases, terrestrial FSO systems have more opportunities to replace optical fiber-based networks.
The demand for faster, higher-capacity communication and data rates ensures the strong development prospects of FSO. The future trend is the increasingly common Internet of Things, connecting and communicating data between billions of devices, from cars and containers to lighting and surveillance cameras. FSO's ability to extend data exchange beyond cable infrastructure will ensure that the technology plays a key role.
Optical wireless communication technology has made great progress, but there is still a long way to go for secure high-bandwidth information transmission. At present, the Standardization Committee is formulating specifications. In the future, the demand for 6G will usher in explosive growth, and mobile systems will be launched one after another.
Wireless optical communication: satellite technology opens up strong development prospects-China exportsemi.com
