Optical fiber communication system consists of what equipment is introduced the development of optical fiber communication technology

Fiber optic communication system, what does fiber optic communication system mean

　　fiber optic communication system

　　Optical fiber optical fiber communication system is a communication system that uses light as the carrier, uses ultra-fine optical fiber drawn from extremely high-purity glass as the transmission medium, and uses light to transmit information through photoelectric conversion. With the rapid development of international Internet business and communication industry, informatization has brought great impetus to the development of world productivity and human society. As one of the main technical pillars of informatization, optical fiber communication will surely become the most important strategic industry in the 21st century. Optical fiber communication technology and computer technology are the two core pillars of informatization. The computer is responsible for digitizing information and inputting it into the network; optical fiber is responsible for information transmission. In the development of contemporary society and economy, the information capacity is increasing day by day. In order to improve the transmission speed and capacity of information, optical fiber communication is widely used in the development of informatization, and has become an important technology in the information field after microelectronics technology.

　　Basic Fiber Optic Communication System

　　The most basic optical fiber communication system consists of data source, optical transmitter, optical channel and optical receiver. The data source includes all signal sources, which are signals obtained by encoding voice, image, and data services through the source; the optical transmitter and modulator are responsible for converting the signal into an optical signal suitable for transmission on the optical fiber. The light wave windows used successively are 0.85, 1.31 and 1.55. The optical channel includes the most basic optical fiber, as well as the relay amplifier EDFA, etc.; the optical receiver receives the optical signal, extracts information from it, and then converts it into an electrical signal, and finally obtains the corresponding voice, image, data and other information.

　　Digital Optical Communication System

　　Optical fiber transmission systems are ideal channels for digital communications. Compared with analog communication, digital communication has many advantages, such as high sensitivity and good transmission quality. Therefore, most of the high-capacity and long-distance optical fiber communication systems adopt digital transmission methods.

　　In the optical fiber communication system, the binary light pulse "0" code and "1" code are transmitted in the optical fiber, which are generated by the on-off modulation of the light source by the binary digital signal. The digital signal is generated by sampling, quantizing and encoding the continuously changing analog signal, which is called PCM (pulse code modulation), that is, pulse code modulation. This electrical digital signal is called a digital baseband signal, which is generated by the PCM electrical terminal.

　　Basic composition of optical fiber communication system

　　(1) Optical transmitter

　　The optical transmitter is an optical transceiver that realizes electrical/optical conversion. It consists of light source, driver and modulator. Its function is to modulate the light wave emitted by the light source by the electrical signal from the electric terminal to become a modulated light wave, and then couple the modulated light signal to the optical fiber or cable for transmission. The electric terminal machine is the conventional electronic communication equipment.

　　(2) Optical receiver

　　The optical receiver is an optical transceiver that realizes optical/electrical conversion. It consists of a photodetector and an optical amplifier. Its function is to convert the optical signal transmitted by the optical fiber or optical cable into an electrical signal through the photodetector, and then amplify the weak electrical signal to a sufficient level through the amplifier circuit, and send it to the electric terminal at the receiving end to drain.

　　(3) Optical fiber or optical cable

　　An optical fiber or an optical cable constitutes a transmission path of light. Its function is to couple the light-dimmed signal sent by the sending end to the optical detector at the receiving end after long-distance transmission by optical fiber or optical cable, so as to complete the task of transmitting information.

　　(4) Repeater

　　The repeater is composed of photodetector, light source and decision regeneration circuit. It has two functions: one is to compensate the attenuation of the optical signal when it is transmitted in the optical fiber; the other is to close the pulse of waveform distortion.

　　(5) Passive components such as optical fiber connectors and couplers

　　Since the length of the optical fiber or optical cable is limited by the optical fiber drawing process and the construction conditions of the optical fiber cable, and the drawing length of the optical fiber is also limited (such as 1Km). Therefore, there may be a problem of connecting multiple optical fibers in one optical fiber line. Therefore, the connection between optical fibers, the connection and coupling between optical fibers and optical transceivers are indispensable for the use of passive devices such as optical fiber connectors and couplers.

　　Backup Systems and Auxiliary Equipment

　　In order to ensure the smooth flow of the system, a backup system is usually set up, just like the backup of the disk. Under normal circumstances, only the main system works. Once the main system fails, it can be switched to the backup system immediately, so that the communication can be guaranteed to be smooth and correct.

　　Auxiliary equipment is the perfection of the system, which includes monitoring and management system, official communication system, automatic switching system, alarm processing system, power supply system, etc.

　　Among them, the monitoring and management system can automatically monitor the performance and working status of various equipment that make up the optical fiber transmission system, and automatically alarm and deal with it when a fault occurs, and implement automatic control of the protection switching system. For long-distance communication lines with multiple relay stations and line maintenance central offices with multi-direction and multi-system access, centralized monitoring is a necessary maintenance method.

　　The real development of modern optical communication is only in the past 30 to 40 years, and the birth of lasers and optical fibers played a leading role. First, Maiman invented the ruby ​​laser in 1960. The strong coherent light generated by the laser provided a reliable light source for modern optical communications. This single-wavelength laser has the same properties as ordinary radio waves, which can be modulated to carry information. Early optical communications using lasers also traveled through the atmosphere. But it was soon discovered that many factors such as fog, rain, clouds, and even a group of birds flying by occasionally would interfere with the propagation of light waves, so they could only be used for short-distance communication. Obviously, a cable like radio frequency or microwave communication was needed. Or a light wave communication transmission line like a waveguide to overcome these effects and realize long-distance stable transmission of information.

　　In 1965, E. Miller reported a lens optical waveguide composed of a series of lenses in a metal hollow tube. It can avoid the shortcomings of atmospheric transmission, but its structure is too complicated and the precision requirements are too high to be practical. On the other hand, research on optical fibers is well underway. As early as 1951, medical glass fiber was invented, but the loss of this early optical fiber was too large (greater than 1000dB/km), and it could not be used as a transmission medium for optical communication. In 1966, CK Kao and GA Hockman published a paper on the development of optical fiber communication. Famous papers of historical significance. After analyzing the main reasons for the high loss of optical fiber transmission, they pointed out that if the impurities in the glass can be completely removed, the loss can be reduced to 20dB/km-equivalent to the level of coaxial cables, then the optical fiber can be used for optical transmission. communication. Encouraged by this expectation, Corning finally produced an optical fiber with a loss of 20dB/km in 1970, thus paving the way for the development of optical fiber communication. The study on the spectral characteristics of the fiber found that it has three low-loss transmission windows, namely the short wavelength window of 850nm and the long wavelength window of 1300nm and 1500nm. Then, with the emergence of new manufacturing methods and the continuous improvement of technological levels, the loss of optical fibers has been continuously reduced. By 1979, the loss of single-mode fiber at 1550nm wavelength had dropped to 0.2dB/km, close to the theoretical loss limit of silica fiber.

　　Moreover, the frequency of light waves is high, and the bandwidth resources of optical fibers are also very considerable, which is unmatched by any other transmission medium. It can be said that optical fiber is an ideal transmission medium that communication workers dream of, and has almost perfect qualities:

　　virtually unlimited bandwidth;

　　Almost zero loss:

　　Almost zero signal distortion

　　Almost zero power consumption

　　almost zero material consumption

　　almost zero footprint

　　Almost zero price.

　　Therefore, optical fiber is the foundation of the information superhighway, creating a new era of today's information revolution.

　　While the loss of optical fibers is continuously reduced, the research on light sources is also progressing very rapidly. In 1962, the GaAs semiconductor laser diode (LD) came out, which meant that modern optical communications had a small-volume, high-speed light source. The emission wavelength of GaAs-LD is 870nm, which is shifted to the short-wavelength low-loss window of optical fiber after doping aluminum. Later, GaAs-LD realized room temperature work for a long time. 1300nm and 1550nm LD light sources were manufactured by using the quaternary alloy InGaAsP. Because LD is expensive, high-brightness LEDs suitable for optical fiber communication have also been developed. In this way, with the successful development of various wavelengths, high efficiency, long life, and high speed semiconductor light sources that meet the requirements of optical fiber transmission, the practicality and great development of optical fiber communication have come naturally. The power output from the LD into the single-mode fiber is about 1mW. In optical fiber communication, dBm is often used as the power unit, which is the relative power expressed in dB based on 1mW.

　　In addition, in the research of optical receivers, high-efficiency and high-speed semiconductor photoelectric conversion devices (such as APD and PIN) in various wavelength ranges have also come out one after another. In 1973, SDPersonick published a paper on the analysis of PCM digital optical receivers, which solved the design problems of optical receivers in modern optical fiber communication systems. The sensitivity of the digital receiver is very high, such as the signal of 2.5Gb/s can reach -30 dBm (1 microwatt) in the early stage. That is not for the seemingly small 1mW transmission power, when the fiber loss is 0.2dB/km, the transmission distance in terms of loss alone can reach more than 100km.

　　In addition, in order to meet the needs of system applications, various optical passive devices (such as optical fiber active connection data, optical attenuators, optical wavelength division multiplexers, isolators and splitters, etc.) and special equipment (such as optical fiber grafting machines, Time domain reflectometer, optical power meter, etc.) are also supporting commercial use one after another.

　　Around 1974, many countries carried out various indoor optical fiber communication transmission experiments. After 1976, various practical optical fiber communication systems appeared. In 1980, the 45Mb/s optical fiber communication system FT-3 of American Telegraph and Telephone Company was commercialized. Since the 1980s, it has entered a period of rapid development of optical fiber communication, and has experienced the development process from short wavelength to long wavelength, from multimode fiber to single mode fiber, and from low speed to high speed. So far, the development of commercial optical fiber communication systems has gone through four generations, namely the first generation system (1980-) of 850nm wavelength multimode fiber, the second generation system (1983-) of 1300nm wavelength single-mode fiber, 1550nm single-mode fiber single-frequency The third generation system (199l-) of the laser and the fourth generation system (1995-) using the optical amplifier. The total length of optical cables laid all over the world is tens of millions of kilometers, and hundreds of thousands of kilometers have been laid in my country, forming a land and submarine optical fiber network that spreads all over the country and the world. Systems from 2.5-10Gb/s have been put into practical use and have been widely used, and 40Gb/s ultra-high-speed optical fiber communication technology is developing very rapidly. The picture shows the development of the capacity of the communication system. It can be seen that the exponential growth can only be realized after the optical fiber communication is adopted.

　　In order to give full play to the bandwidth potential of optical fiber, overcome the influence of optical fiber loss and dispersion, extend the relay distance, expand the transmission capacity and reduce the cost, it has always been the development goal of optical fiber communication. Various new optical fiber communication technologies are constantly emerging, and the code rate distance product of the system has been continuously improved. It increases by an order of magnitude almost every 4 years. These new technologies include:

　　(1) The integration and modularization of active and passive optical devices and system terminals to improve speed and performance. Simplify structure and reduce costs are the most important technical basis for system development;

　　(2) Wavelength Division Multiplexing (WDM, Wavelength Division Multiplexing) technology realizes ultra-high-speed and ultra-large-capacity transmission on a single optical fiber;

　　(3) Optical amplifier technology, especially the application of erbium-doped fiber amplifier (EDFA, Erbium-Doped Fiber Amplifier) ​​and optical amplifier in long-distance trunk system and user distribution system;

　　(4) Soliton communication technology;

　　(5) High-speed optical fiber network technology, all-optical network technology, etc.

　　The purpose of developing these new technologies is to better meet the growing demand for information. Among them, the perfect combination of WDM technology and optical amplifier technology has greatly improved the performance and communication capacity of optical fiber communication systems, and has become a shining pearl of modern optical communication technology and a bridge leading to all-optical communication networks.

　　The basic structure of the digital optical fiber communication system is shown in 6.1.5, which includes PCM terminal, input interface, optical transmitter (Tx), optical fiber line, optical repeater, output interface and optical receiver (Rx), etc.

　　A typical point-to-point optical fiber communication system mainly includes several parts such as sending and receiving information electrical terminals, optical sending and receiving terminals, and transmission optical fibers. From the optical transmitter to the optical receiver is the transmission channel of optical information, called optical channel, and its task is to transmit information reliably and effectively from the beginning to the end. The functions of each part are as follows:

　　(1) The information signals to be transmitted by the PCM electric terminal include voice, image and computer data, etc. The electric terminal is the terminal equipment such as the carrier machine, image equipment and computer in conventional electric communication. For digital communication, the signal needs to undergo A/D and D/A conversion in the electric terminal to convert it into a digital signal.

　　(2) The optical transmitter includes a light source (LD or LED) and its drive circuit. The electrical signal from the electrical terminal is encoded and modulated to the light source to generate an optical signal carrying information and complete the electrical-optical (E/O) conversion .

　　(3) The transmission optical fiber or optical cable transmits the optical signal emitted by the light source to the remote receiving end, which can be a multi-mode optical fiber or a single-mode optical fiber.

　　(4) The optical receiver completes the light-to-electricity (O/E) conversion. The received optical signal is detected by the optical detector and converted into an electrical signal, then amplified and demodulated, judged and regenerated, and sent to the electrical terminal to recover the original signal.

　　In the long-distance optical fiber communication system, repeaters need to be installed at intervals to convert optical signals that have become very weak and distorted after long-distance transmission into electrical signals. The optical signal is sent to the optical fiber for transmission, which is the traditional optical-electrical-optical repeater (Figure 1.2.2(a)). Now, however, optical amplifiers, especially EDFAs, have matured, with high gain, high output power, low noise, large bandwidth, and code rate penetration. They can completely replace optical-electrical-optical repeaters and are driving the development of optical fiber communication technology. Revolution - a new generation of all-optical communication technology (Figure 1.2.2(b)). Figure 1.2.2(c) is a schematic diagram of a WDM system. Several—hundreds, thousands of wavelengths are transmitted together in a single fiber, and EDPA relay amplification is used to increase the transmission capacity several times—hundreds, thousands times, representing the development direction and research hotspots of a new generation of high-speed and large-capacity optical fiber communication technology.

　　A combination of several point-to-point communication systems constitutes a communication network (Figure 1.2.3) to provide communication between users in different places. Communication network can be divided into public communication network and private communication network. The public communication network provides communication services to users in the whole society, such as telephone network and public data network. A dedicated communication network is a communication network serving specific users or units, such as communication networks, computer networks, state networks, etc. of railways, electric power, military and other departments. These networks have traditionally used cables or microwaves, but today the amount of communication information has increased dramatically, and they are no longer competent, and the use of optical fiber communication technology has become the general trend.

　　The basic contents of optical fiber communication technology are:

　　(1) Optical fiber transmission theory and technology, optical fiber devices;

　　(2) Signal transmission principles, modulation and demodulation methods, signal coding and channel multiplexing, etc.;

　　(3) light source and optical transmitter;

　　(4) Optical detectors and optical receivers;

　　(5) Design, structure and application of optical fiber communication system;

　　(6) Optical fiber communication technology, such as optical amplifier technology, WDM technology, all-optical network technology