Optical fibers can transmit digital signals as well as analog signals. Currently, 90% of the world's communication services require transmission through optical fibers. With the development of fiber optic communication technology, many countries around the world have introduced fiber optic communication systems into public telecommunications networks, relay networks, and access networks.
Fiber optic broadband trunk transmission networks and access networks are developing rapidly and are the main targets of current research, development, and application. Various applications of fiber optic communications can be summarized as follows:
(1) Communication networks: Fiber optic communications are widely applied in communication networks and have become the mainstream method in modern communications. 1) Global communication networks. Since fiber optic communication systems can have very long relay distances, underwater fiber optic lines spanning oceans can be designed, such as submarine optical cables across the Atlantic and Pacific Oceans, and intercontinental optical cable trunk lines across the Eurasian continent. The first fiber optic communication system across the Atlantic (TAT-8) began operation at the end of 1988, which was realized 32 years after the opening of the first coaxial copper wire cable telephone system (TAT-1).
The TAT-8 fiber optic communication system spanned a distance of approximately 6×10³km between the east coast of the United States and Europe, and could provide a total capacity of nearly 40,000 voice channels, demonstrating the superiority of fiber optic communications in terms of capacity. Compared with coaxial cables, optical cables are much lighter in weight, making them easier to transport and lay. Moreover, if lower-loss optical fibers and optical amplifiers are used, the need for repeaters can be reduced or eliminated. Currently, there are optical cables in all oceans and most seas of the world, forming high-speed communication bridges.
Public telecommunications networks of various countries. Optical cables have the advantages of small size and large information capacity, making them the best alternative to traditional copper twisted-pair cables at the present stage. China's national first-level trunk lines, provincial second-level trunk lines, and branch lines below the county level have basically been converted to optical fiber.
Various specialized communication networks. Fiber optic systems used for communication, command, dispatch, and monitoring in electric power, railway, national defense and other departments mainly utilize the electromagnetic interference resistance of optical fibers and real-time transmission and reception of video signals.
Special communications. Optical fibers have extremely strong corrosion resistance, and the use of optical cables in flammable and explosive environments in petroleum, chemical, coal mining and other departments provides higher safety.
The use of fiber optic communication systems inside aircraft, warships, submarines, missiles, and spacecraft takes advantage of the characteristics of optical fibers being lightweight, small in size, resistant to electromagnetic interference, and having no signal radiation.
(2) Local area networks and wide area networks that constitute the Internet: Fiber optic communication systems are particularly suitable for transmitting data in digital form. Interconnections between central processing units (CPUs) and peripheral devices, between CPUs and memory, and between multiple CPUs can all be realized using optical fibers. The transmission rates of local area networks and wide area networks using optical fibers have increased to 100Mbit/s and 1Gbit/s, and can provide high-speed connections between local area networks. Optical fiber transmission can be used for local area networks and wide area networks with various different network topologies.
(3) Trunk and distribution networks of cable television networks, industrial television systems: Signals from satellite earth stations, microwave lines, antenna-received television broadcasts, and self-produced television programs can all be connected to distribution centers through optical fibers. Optical fibers can be directly connected to video distribution networks at user terminal lines. Multiple television channels can be transmitted simultaneously using multiple mutually isolated optical fibers in an optical cable or through frequency division multiplexing on a single optical fiber. Fiber optic communication networks can also be applied to data transmission in monitoring and automatic control systems of factories, banks, shopping centers, transportation, and public security departments.
(4) Integrated services optical fiber access networks: Optical fiber access networks are divided into active access networks and passive access networks, which can realize integrated access to core networks for telephone, data, video (conference television, videophone, etc.) and multimedia services, providing various community services.
(5) Optical fiber sensors: Strictly speaking, optical fiber sensors do not belong to the communications category. However, optical fiber sensors are an extremely important application field of fiber optics. Optical fiber sensors have been successfully applied in fields such as temperature measurement, pressure measurement, rotational and translational position measurement, and liquid depth measurement. For some sensors, optical fibers have dual functions: first, the sensor itself depends on certain sensitive characteristics of the optical fiber; second, information is collected and transmitted through the optical fiber to the information output terminal.
The true application of this peculiar medium of optical fiber is still merely replacing copper wires with optical fibers in existing telecommunications networks, which has somewhat improved the performance of communication networks and reduced costs, but the network topology is basically still the model from before the emergence of fiber optic communications. The potential of fiber optic communications has not been fully realized, and the application of fiber optic communication technology in current communication networks is still a classical application. With the trend of all-optical communication networks being set off worldwide, fiber optic systems are not only used for transmitting signals, but switching, multiplexing, control and routing are also all completed in the optical domain, thereby constructing true fiber optic communication networks.
