What is an optical fiber communication system?

What is an optical fiber communication system?

Fiber optic communication systems use light as a carrier wave and extremely fine optical fibers drawn from high-purity glass as the transmission medium. Through photoelectric conversion, they transmit information using light. With the rapid development of the international internet and the communications industry, information technology has greatly boosted global productivity and the development of human society. As one of the main technological pillars of information technology, fiber optic communication will undoubtedly become one of the most important strategic industries of the 21st century.

Glossary: 

Fiber optic communication technology and computer technology are the two core pillars of informatization. Computers are responsible for digitizing information and inputting it into networks; fiber optics bear the heavy responsibility of information transmission. In contemporary social and economic development, the volume of information is increasing dramatically. To improve the speed and capacity of information transmission, fiber optic communication has been widely used in the development of informatization, becoming an important technology in the information field after microelectronics.

Basic Fiber Optic Communication System

The most basic fiber optic communication system consists of a data source, an optical transmitter, an optical channel, and an optical receiver. The data source includes all signal sources, which are signals obtained through source encoding of voice, image, and data services. The optical transmitter and modulator are responsible for converting the signals into optical signals suitable for transmission over the fiber optic cable; optical wavelength windows used successively include 0.85, 1.31, and 1.55. The optical channel includes the basic optical fiber and repeater amplifiers (EDFAs), etc. The optical receiver receives the optical signals, extracts information from them, converts them into electrical signals, and finally obtains the corresponding voice, image, and data information.

Digital Fiber Optic Communication Systems

Fiber optic transmission systems are ideal channels for digital communication. Compared to analog communication, digital communication has many advantages, including high sensitivity and good transmission quality. Therefore, most high-capacity, long-distance fiber optic communication systems adopt digital transmission methods.

In fiber optic communication systems, binary optical pulses '0' and '1' codes are transmitted in the optical fiber. These are generated by modulating the light source with binary digital signals. The digital signal is generated by sampling, quantizing, and encoding continuously changing analog signals, a process called PCM (pulse code modulation). This electrical digital signal is called a digital baseband signal and is generated by a PCM terminal unit.

Basic Components

(1) Optical Transmitter

An optical transmitter is an optical transceiver that performs electro-optical conversion. It consists of a light source, a driver, and a modulator. Its function is to modulate the light wave emitted by the light source with the electrical signal from the electrical transceiver, creating a modulated light wave, and then couple the modulated light signal to an optical fiber or cable for transmission. The electrical transceiver is a conventional electronic communication device.

(2) Optical Receiver

An optical receiver is an optical transceiver that performs electro-optical conversion. It consists of a photodetector and an optical amplifier. Its function is to convert the optical signal transmitted from the optical fiber or cable into an electrical signal via the photodetector, and then amplify this weak electrical signal to a sufficient level before sending it to the electrical transceiver at the receiving end.

(3) Optical Fiber or Cable

An optical fiber or cable constitutes the optical transmission path. Its function is to couple the modulated light signal emitted by the transmitting end to the photodetector at the receiving end after long-distance transmission via the optical fiber or cable, thus completing the information transmission task. 

(4) Repeaters

A repeater consists of a photodetector, a light source, and a decision regeneration circuit. It serves two purposes: one is to compensate for the attenuation of the optical signal during transmission in the optical fiber; the other is to perform pulse reshaping to correct waveform distortion.

(5) Passive Devices such as Fiber Optic Connectors and Couplers

Because the length of optical fibers or cables is limited by the fiber drawing process and cable construction conditions, and the drawn length of optical fibers is also limited (e.g., 1 km), a single optical fiber line may involve the connection of multiple optical fibers. Therefore, the use of passive devices such as fiber optic connectors and couplers is essential for the connection between optical fibers, and for the connection and coupling between optical fibers and optical transceivers.

Backup Systems and Auxiliary Equipment

To ensure uninterrupted system operation, backup systems are typically installed, much like disk backups. Normally, only the primary system operates. If the primary system fails, the system can immediately switch to the backup system, ensuring uninterrupted and accurate communication.

Auxiliary equipment complements the system and includes monitoring and management systems, business communication systems, automatic switching systems, alarm handling systems, and power supply systems.

The monitoring and management system automatically monitors the performance and operational status of various devices within the fiber optic transmission system. It automatically alarms and handles faults and automatically controls the protection and switching system. For long-distance communication lines with multiple relay stations and for maintenance centers with multi-directional and multi-system access, centralized monitoring is an essential maintenance method.