Optical fiber communication
Optical fiber is the abbreviation of optical fiber. Optical fiber communication is a communication method in which light waves are used as information carriers and optical fibers are used as transmission media. From a principle point of view, the basic material elements that make up optical fiber communication are optical fibers, light sources, and light detectors. In addition to the classification of optical fibers according to manufacturing process, material composition and optical characteristics, in applications, optical fibers are often classified according to use, and can be divided into optical fibers for communication and optical fibers for sensing. Transmission medium fiber is divided into general and special two types, and functional device fiber refers to the fiber used to complete the functions of light wave amplification, shaping, frequency division, frequency doubling, modulation and optical oscillation, and it is often used with a certain functional device. The form appears.
Optical fiber communication is a communication method that uses light waves as a carrier wave and optical fiber as a transmission medium to transmit information from one place to another. It is called "wired" optical communication. Nowadays, optical fiber is far superior to the transmission of cable and microwave communication due to its transmission frequency bandwidth, high anti-interference ability and low signal attenuation. It has become the main transmission method in the world communication.
Communication system
Glossary
Optical fiber communication technology and computer technology are the two core pillars of informatization, and computers are responsible for digitizing information into the network. ; Optical fiber is responsible for information transmission. In the contemporary social and economic development, information capacity is increasing rapidly. In order to improve the transmission speed and capacity of information, optical fiber communication is widely used in the development of information technology and has become an important technology in the information field after microelectronics.
Basic optical fiber 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 through source encoding for services such as voice, image, and data; optical transmitters and modulators are responsible for converting the signals into optical signals suitable for transmission on optical fibers. 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.; while 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 fiber communication system
The optical fiber transmission system is an ideal channel for digital communication. Compared with analog communication, digital communication has many advantages, such as high sensitivity and good transmission quality. Therefore, most large-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 is 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 kind of electrical digital signal is called a digital baseband signal, which is generated by a PCM electrical terminal.
Basic structure
(1) Optical transmitter
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 from the electrical signal from the electric terminal to become a modulated light wave, and then couple the modulated light signal to an optical fiber or cable for transmission. The electric terminal is a conventional electronic communication device.
(2) Optical receiver
Optical receiver is an optical transceiver that realizes optical/electrical conversion. It consists of an optical detector and an optical amplifier. Its function is to convert the optical signal transmitted by the optical fiber or optical cable into an electrical signal by the photodetector, and then amplify the weak electrical signal to a sufficient level by the amplifying circuit, and send it to the electrical end of the receiving end for drawing.
(3) Optical fiber or optical cable
Optical fiber or optical cable constitutes the transmission path of light. Its function is to couple the dimmed signal from the sender end to the light detector at the receiver end after long-distance transmission via optical fiber or cable to complete the task of transmitting information.
(4) Repeater
The repeater is composed of a light detector, a light source and a 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 shape the distorted pulse.
(5) Passive components such as optical fiber connectors and couplers
Because the length of the optical fiber or optical cable is limited by the optical fiber drawing process and the optical cable construction conditions, and the optical fiber is drawn The length is also limited (such as 1Km). Therefore, an optical fiber line may have the problem of connecting multiple optical fibers. Therefore, the connection between the optical fibers and the connection and coupling between the optical fiber and the optical transceiver are indispensable for the use of passive components such as optical fiber connectors and couplers.
Backup system
In order to ensure the smooth flow of the system, a backup system is usually set up, which is like a disk backup. 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 smooth and correct communication can be guaranteed.
Auxiliary equipment
Auxiliary equipment is the perfection of the system, which includes monitoring 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 the various equipment that make up the optical fiber transmission system. When a failure occurs, it will automatically alert and deal with it, and automatically control the protection switching system. For long-distance communication lines with multiple relay stations and line maintenance central offices equipped with access to multiple directions and multiple systems, centralized monitoring is a necessary maintenance method.
Overview
Optical fiber is short for optical fiber. Optical fiber communication is a communication method in which light waves are used as information carriers and optical fibers are used as transmission media. From a principle point of view, the basic material elements that make up optical fiber communication are optical fibers, light sources, and light detectors. In addition to the classification of optical fibers according to manufacturing process, material composition and optical characteristics, in applications, optical fibers are often classified according to use, and can be divided into optical fibers for communication and optical fibers for sensing. Transmission medium fiber is divided into general and special two types, and functional device fiber refers to the fiber used to complete the functions of light wave amplification, shaping, frequency division, frequency doubling, modulation and optical oscillation, and it is often used with a certain functional device. The form appears.
Optical fiber communication is a communication method that uses light waves as a carrier wave and optical fiber as a transmission medium to transmit information from one place to another. It is called "wired" optical communication. Nowadays, optical fiber is far superior to the transmission of cable and microwave communication due to its transmission frequency bandwidth, high anti-interference ability and low signal attenuation, and has become the main transmission method in the world communication.
In 1966, the British Chinese Charles Kao published a paper and proposed to use quartz to make glass filaments (optical fibers), the loss of which can reach 20dB/km, which can realize large-capacity optical fiber communication. At that time, only a few people in the world believed, such as the British Standard Telecommunications Laboratory (STL), the American Corning Glass Company, Bell Labs and other leaders. In 2009, Gao Kun won the Nobel Prize for his invention of optical fiber. In 1970, Corning developed a quartz optical fiber with a loss as low as 20dB/km and a length of about 30m. It is said that it cost 30 million U.S. dollars. In 1976, Bell Labs established an experimental line in Atlanta, Washington, with a transmission rate of only 45Mb/s, which can only transmit hundreds of telephones, while the coaxial cable used can transmit 1800 telephones. Because there is no laser for communication at the time, light-emitting diodes (LED) are used as the light source for optical fiber communication, so the speed is very low. Around 1984, the semiconductor laser for communication was successfully developed. The speed of optical fiber communication reached 144Mb/s, which could transmit 1920 telephones. In 1992, the transmission rate of an optical fiber reached 2.5Gb/s, which is equivalent to more than 30,000 telephone lines. In 1996, lasers of various wavelengths were successfully developed, which can realize multi-wavelength and multi-channel optical fiber communication, the so-called "wavelength division multiplexing" (WDM) technology, which means that multiple optical signals of different wavelengths are transmitted in one optical fiber. . As a result, the transmission capacity of optical fiber communications is doubled. In 2000, using WDM technology, the transmission rate of an optical fiber reached 640Gb/s. Some people have big doubts about Gao Kun who invented optical fiber in 1976, but only won the Nobel Prize in 2010. In fact, it can be seen from the above optical fiber development history that despite the large capacity of optical fibers, the ultra-large capacity of optical fibers cannot be played without high-speed lasers and microelectronics. The speed of electronic devices has reached the order of gigabits/second. The emergence of high-speed lasers of various wavelengths has enabled optical fiber transmission to reach the order of terabits/second (1Tb/s=1000Gb/s). A revolution in communication technology!"
Features
①The amount of information that can be transmitted per unit time is large. The information rate of the practical level of optical fiber communication in the early 1990s was 2.488 Gbit/s, that is, a pair of single-mode optical fibers can open 35,000 telephones at the same time, and it is still developing rapidly; ②Economy. The construction cost of optical fiber communication decreases with the increase in the number of use; ③Small size, light weight, convenient construction and maintenance, etc.; ④Use of less metal, strong anti-electromagnetic interference, strong anti-radiation, good confidentiality, etc.
Basic composition
The main components of a conventional optical fiber communication system are optical fiber, light source and light detector. Optical fibers include single-mode and multi-mode fibers, and light sources include semiconductor lasers and light-emitting diodes. Medium and long-distance systems use single-mode fiber and semiconductor lasers, newly developed high-speed systems use distributed feedback (DFB) lasers, and short-distance systems can use multimode fibers and light-emitting diodes.
The conventional optical fiber communication system refers to a system in which the transmitting end modulates the intensity of the light source, and the receiving end uses a photodetector to directly detect the received optical signal (IM/DD), also known as direct intensity modulation The tethered wave optical fiber communication system, which was the master of actual use in the early 1990s. Its basic structure takes the 2.488Gbit/s system as an example, as shown in Figure 2.
The left side of Figure 2 is the electrical time division multiplexer at the transmitting end, which combines the input 155Mbit/s digital signal into a 2.488Gbit/s signal. The signal directly intensity modulates a distributed feedback laser, and then transmits the dimmed output to the single-mode fiber. On the right side of Figure 2, the light-electricity detector directly detects the dimmed light and obtains a digital signal of 2.488Gbit/S, and then decomposes the multiplexer over time to obtain a set of digital signals of 155Mbit/s.
The relay equipment of the conventional optical fiber communication system is shown in Figure 3.
2.2 Application Scope
Optical fiber communication is first applied between telephone offices to form a local optical fiber network, and then as a long-distance communication to form a nationwide optical fiber network, which will become a broadband communication network Skeleton. Submarine optical cable systems have also been developed for transoceanic communications or short-distance communications across islands and coasts. The famous submarine optical cable communications systems that span the Atlantic and Pacific Oceans. For example, the first trans-Atlantic system TAT-8, which was commercialized in December 1988, has 3 pairs of optical fibers in the optical cable, 2 pairs for use, and 1 pair for standby. The information rate of each pair is 280Mbit/s. The total length is 6700km, the average distance between relay stations is 67knu and the wavelength is 1.3μm, and the conventional single-mode fiber is used.
Developed countries are planning, designing and constructing fiber-optic user networks, namely fiber-to-the-home (FTTH) or fiber-to-the-roadside (FTTC). Other applications, such as optical fiber local area networks of various scales and applications in various occasions.
Technical Field
(1) Large communication capacity and long transmission distance; the potential bandwidth of an optical fiber can reach 20THz. With such a bandwidth, it only takes about one second to transmit all the textual data of human beings, ancient and modern, both at home and abroad. The 400Gbit/s system has been put into commercial use. The loss of the optical fiber is extremely low. The loss of the quartz fiber can be less than 0.2dB/km near the light wavelength of 1.55μm, which is lower than the loss of any transmission medium. Therefore, the non-relay transmission distance can reach dozens or even hundreds of kilometers.
(2) Small signal interference, good security performance;
(3) Anti-electromagnetic interference, good transmission quality, electrical communication cannot solve various electromagnetic interference problems, only optical fiber communication Not subject to all kinds of electromagnetic interference.
(4) The optical fiber is small in size, light in weight, easy to lay and transport;
(5) The source of materials is abundant, and the environmental protection is good, which is conducive to saving non-ferrous metal copper.
(6) There is no radiation and it is difficult to eavesdrop because the light waves transmitted by the optical fiber cannot escape outside the optical fiber.
(7) Optical cable has strong adaptability and long service life.
(8) The texture is brittle and the mechanical strength is poor.
(9) The cutting and splicing of optical fiber requires certain tools, equipment and technology.
(10) Shunting and coupling are not flexible.
(11) The bending radius of the fiber optic cable should not be too small (>20cm)
(12) There is a problem of power supply difficulties.
The communication method that uses light waves to transmit information in optical fibers. Because laser has significant advantages such as high directivity, high coherence, high monochromaticity, etc., the light wave in optical fiber communication is mainly laser, so it is also called laser-fiber communication.
Principle and application
The principle of optical fiber communication is: at the transmitting end, the transmitted information (such as voice) must first be converted into electrical signals, and then modulated to the laser beam emitted by the laser , The intensity of the light changes with the amplitude (frequency) of the electrical signal, and it is sent out through the optical fiber; at the receiving end, the detector converts the light signal into an electrical signal after receiving it, and restores the original information after demodulation.
As the transmission speed of information technology is updated day by day, optical fiber technology has been widely valued and applied. In the multi-computer elevator system, the application of optical fiber fully meets the requirements of a large number of correct, reliable, high-speed transmission and processing of data communication. The application of optical fiber technology in elevators greatly improves the response speed of the entire control system, and significantly improves the parallel group control performance of the elevator system. The optical fiber communication device used on the elevator is mainly composed of a light source, a photoelectric receiver and an optical fiber.
Light source
The signal output by the microcomputer control system is an electrical signal, while the optical fiber system transmits an optical signal. Therefore, in order to transmit the electrical signal generated by the microcomputer system in the optical fiber, first To convert electrical signals into optical signals. The light source is such an electro-optical conversion device.
The light source first converts the electrical signal into an optical signal, and then sends the optical signal to the optical fiber. In the fiber optic system, the light source has a very important position. Incandescent lamps, lasers and semiconductor light sources can be used as optical fiber light sources. Semiconductor light sources use semiconductor PN junctions to convert electrical energy into light energy. Commonly used semiconductor light sources include semiconductor light emitting diodes (LED) and laser diodes (LD).
Semiconductor light sources have been widely used in optical fiber transmission systems due to their small size, light weight, simple structure, convenient use, and easy compatibility with optical fibers.
Photoelectric receiver
The optical signal transmitted in the optical fiber must first be restored to the corresponding electrical signal before being received by the microcomputer system. This conversion is achieved through the optical receiver. The function of the optical receiver is to convert the optical signal transmitted by the optical fiber into an electric signal, and then the electric signal is handed over to the control system for processing. The optical receiver is based on the principle of photoelectric effect, irradiating the PN junction of the semiconductor with light, and the PN junction of the semiconductor will generate carriers after absorbing light energy, thus generating the photoelectric effect of the PN junction, thereby converting the optical signal into an electrical signal. Semiconductor receivers used in optical fiber systems mainly include semiconductor photodiodes, phototransistors, photomultipliers, and photovoltaic cells. The phototransistor can not only convert the incident light signal into an electrical signal, but also amplify the electrical signal, so that it can be well matched with the control system interface circuit, so the phototransistor is the most widely used.
Optical fiber
Optical fiber is the transmission channel of optical signal and the key material of optical fiber communication.
The optical fiber is composed of a core, a cladding, a coating and a jacket, and is a symmetrical cylinder with a multilayer dielectric structure. The main body of the core is silicon dioxide, which is doped with a small amount of other materials to increase the optical refractive index of the material. There is a cladding layer on the outside of the core, and the cladding and the core have different optical refractive indexes. The optical refractive index of the core is higher to ensure that the optical signal is mainly transmitted in the core. There is a layer of coating on the outside of the cladding, which is mainly used to increase the mechanical strength of the optical fiber so that the optical fiber is not damaged from outside. The outermost layer of the optical fiber is a jacket, which also plays a protective role.
The two main characteristics of optical fiber are loss and dispersion. Loss is the attenuation or loss of an optical signal per unit length, expressed in db/km. This parameter is related to the transmission distance of the optical signal. The greater the loss, the shorter the transmission distance. Multi-microcomputer elevator control systems generally have short transmission distances, so in order to reduce costs, plastic optical fibers are mostly used. The dispersion of the fiber is mainly related to pulse broadening. In the Mitsubishi elevator control system, optical fiber communication is mainly used for data transmission between group control and single elevator and data transmission between two parallel single elevators. The optical fiber device used by Mitsubishi Elevator is mainly composed of a light source, a light receiver and an optical fiber. The light source and the light receiver are encapsulated in the fixed plug of the optical fiber connector, and the optical fiber is connected to the movable plug.
Working process
Send: CPU serializes parallel data through a dedicated IC chip, and inserts the corresponding bit code (start, stop, check digit, etc.) according to the communication format, The output TXD sends the signal to the optical fiber connector (ie fixed plug), and then the light source in the optical fiber connector performs electrical-optical conversion. The converted optical signal sends the optical signal to the optical fiber through the optical fiber moving plug, and the optical signal is in the optical fiber. Propagating forward.
Receive: The optical signal from the optical fiber is sent to the receiver of the fixed plug through the movable plug of the optical fiber connector, and the receiver performs photo-electric restoration of the received optical signal to obtain the corresponding electrical signal , The electrical signal is sent to the RXD input terminal of the dedicated IC chip, and the serial data is changed to parallel data by the dedicated IC chip, and then transmitted to the CPU.
Application field
The application field of optical fiber communication is very wide. It is mainly used for local telephone trunk lines. The advantages of optical fiber communication can be fully utilized here, gradually replacing cables and being widely used. It is also used for long-distance trunk communication in the past, which mainly relied on cables, microwaves, and satellite communications. Now it is gradually using optical fiber communication and has formed a globally dominant bit transmission method; it is used in global communication networks and public telecommunication networks in various countries (such as China’s national first It is also used for high-quality color television transmission, industrial production site monitoring and dispatching, traffic monitoring and control command, urban cable television network, and common antenna (CATV) system. It is used in optical fiber local area network and other such as aircraft, spacecraft, ships, underground mines, electric power departments, military, corrosion and radiation, etc.
The optical fiber transmission system is mainly composed of: optical transmitter, optical receiver, optical cable transmission line, optical repeater and various passive optical devices. To achieve communication, the baseband signal must be processed by the electrical terminal and sent to the optical fiber transmission system to complete the communication process.
It is suitable for optical fiber analog communication system, but also suitable for optical fiber digital communication system and data communication system. In an optical fiber analog communication system, electrical signal processing refers to processing such as amplification and pre-modulation of baseband signals, while electrical signal reverse processing is the inverse process of originating processing, that is, processing such as demodulation and amplification. In optical fiber digital communication systems, electrical signal processing refers to amplifying, sampling, and quantizing baseband signals, that is, pulse code modulation (PCM) and line code encoding processing, etc., and electrical signal reverse processing is also the inverse process of origination. For data optical fiber communication, electrical signal processing mainly includes amplifying the signal, which is different from the digital communication system in that it does not require code conversion.
Development
Optical fiber communication is the main transmission method of modern communication network. Its development history is only ten or twenty years. It has experienced three generations: short-wavelength multimode optical fiber, long-wavelength multimode Optical fiber and long-wavelength single-mode optical fiber. The use of optical fiber communications is a major change in the history of communications. More than 20 countries including the United States, Japan, Britain, and France have announced that they will no longer build cable communications lines, and are committed to the development of optical fiber communications. China's optical fiber communication has entered the practical stage.
The birth and development of optical fiber communication is an important revolution in the history of telecommunications. Satellite communication and mobile communication are juxtaposed as technologies in the 1990s. After entering the 21st century, due to the rapid development of Internet services and the growth of audio, video, data, and multimedia applications, there is an even more urgent need for large-capacity (ultra-high-speed and ultra-long-distance) lightwave transmission systems and networks.
Optical fiber communication is the latest communication technology that uses light waves as carrier waves to transmit information, and optical fiber as the transmission medium to achieve information transmission and achieve the purpose of communication.
The development process of communication is the process of continuously increasing the carrier frequency to expand the communication capacity. As the carrier frequency, the optical frequency has reached the upper limit of the communication carrier. Because light is a very high-frequency electromagnetic wave, it uses light. As a carrier, the communication capacity is huge, which is thousands of times that of the past communication method, and has great appeal. Optical communication is a goal that people have long pursued, and it is also the inevitable direction of communication development.
Compared with the previous electrical communication, the main difference of optical fiber communication is that it has many advantages: its transmission frequency bandwidth, large communication capacity; low transmission loss, long relay distance; thin wire diameter, light weight, raw materials It is quartz, which saves metal materials and is conducive to the rational use of resources; it has strong insulation and anti-electromagnetic interference performance; it also has the advantages of strong corrosion resistance, strong radiation resistance, good windability, no sparks, small leakage, strong confidentiality, etc. , Can be used in special environment or military.
Trend
FTTH can provide users with extremely rich bandwidth, so it has always been regarded as an ideal access method. It plays an important role in realizing the information society. It also needs large-scale promotion and Construction. The fiber required for FTTH may be 2 to 3 times that of the existing laid fiber. In the past, due to the high cost of FTTH, lack of broadband video services and broadband content, FTTH has not been on the agenda, and only a few trials. Due to the advancement of optoelectronic devices, the price of optical transceiver modules and optical fibers has been greatly reduced; coupled with the ease of broadband content, all have accelerated the practical process of FTTH.
The views of developed countries on FTTH are not exactly the same: AT&T in the United States believes that the FTTH market is small, and 0F62003 declared that FTTH will not have a market until 20-50 years later. US operators Verizon and Sprint are more active and will adopt FTTH to transform their networks within 10-12 years. Japan's NTT is the first to develop FTTH and has nearly 2 million users. China's FTTH is in the pilot phase.
FTTH encounters challenges
The widely used ADSL technology still has certain advantages to provide broadband services
Compared with FTTH: ①Low price ②Use the original The copper wire network makes the construction of the project simple ③It can meet the demand for the transmission of 1Mbps-500kbps film and television programs. Mass promotion of FTTH is restricted.
For broadband services to be developed in the near future, such as: online education, online office, conference TV, online games, remote diagnosis and treatment and other two-way services and HDTV high-definition digital television, uplink and downlink transmission asymmetric business, ADSL is difficult to satisfy. Especially HDTV, after compression, its transmission rate still needs 19.2Mbps. It is being developed with H.264 technology and can be compressed to 5 to 6 Mbps. It is generally believed that the highest transmission rate of ADSL that guarantees QOS is 2Mbps, and it is still difficult to transmit HDTV. It can be considered that HDTV is the main driving force of FTTH. That is, when HDTV services arrive, FTTH is a must.
FTTH solutions
There are usually two categories of P2P point-to-point and PON passive optical networks.
The advantages of the F2P scheme: each user transmits independently, does not affect each other, and the system is flexible; inexpensive low-speed optoelectronic modules can be used; and the transmission distance is long. Disadvantages: In order to reduce the number of optical fibers and pipes that users go directly to the office, an active node that summarizes users needs to be placed in the user area.
PON solution-advantages: passive network maintenance is simple; in principle, it can save optoelectronic devices and optical fibers. Disadvantages: need to use expensive high-speed optoelectronic modules; need to use electronic modules that distinguish users from different distances to avoid conflicts between users’ uplink signals; transmission distance is shortened by the PON ratio; the downlink bandwidth of each user occupies each other, if the user’s bandwidth is When there is no guarantee, not only is the network expansion required, but also the PON and the user module need to be replaced to solve the problem. (According to the market price, PEP is more economical than PON)
There are many kinds of PON, generally as follows: (1) APON: namely ATM-PON, suitable for ATM switching network. (2) BPON: Broadband PON. (3) OPON: OFP-PON using general frame processing. (4) EPON: PON using Ethernet technology, GPON is a Gigabit Ethernet PON. (5) WDM-PON: Use wavelength division multiplexing to distinguish users' PONs. Because users are related to wavelengths, it is inconvenient to maintain and is rarely used in FTTH.
Wireless access technology is developing rapidly. It can be used as the IEEE802.11g protocol for WLAN, with a transmission bandwidth of up to 54Mbps and a coverage range of more than 100 meters, which is already commercially available. If wireless access to WLAN is used for user data transmission, including: uplink and downlink data and VOD uplink data on demand TV, the uplink is not large for general users, and IEEE802.11g is sufficient. The FTTH using optical fiber is mainly to solve the downlink transmission of HDTV broadband video, of course, it can also contain some downlink data when needed. This forms a "fiber to the home + wireless access" (FTTH + wireless access) home network. If this kind of home network adopts PON, it is particularly simple, because this PON does not have the upstream signal, does not need the electronic module of the distance measurement, the cost is greatly reduced, the maintenance is simple. If the user group belonging to the PON is covered by the wireless metropolitan area network WiMAX (1EEE802.16) and can be used, then there is no need to build a dedicated WLAN. The use of wireless access network is a trend, but the wireless access network still needs to be supported by an optical fiber network densely distributed near users, which is almost the same as FTTH. FTTH+ wireless access is the future development trend.
The development of optical switching
In fact, it can be expressed as: communication input + exchange.
Optical fiber only solves the transmission problem, but also needs to solve the optical switching problem. In the past, communication networks were composed of metal cables, which transmitted electronic signals, and used electronic switches for exchanges. The communication network, except for a short section at the end of the user, is all optical fiber, which transmits optical signals. A reasonable method should use optical switching. However, due to the immaturity of optical switch devices, the only way to solve the optical network exchange is "optical-electrical-optical", that is, the optical signal is converted into an electrical signal, and then the optical signal is changed back to the optical signal after the electronic exchange. Obviously an unreasonable method, it is not efficient and uneconomical. Large-capacity optical switches are being developed to realize optical switching networks, especially the so-called ASON-automatic switching optical networks.
The information usually transmitted in the optical network is generally at a speed of xGbps, and electronic switches cannot handle it. Generally, electronic exchange should be realized in the lower order group. The optical switch can realize the exchange of high-speed XGbDs. Of course, it is not to say that everything needs to be exchanged with light, especially for low-speed, small-particle signal exchange, mature electronic exchange should be used, and there is no need to use immature
high-capacity optical exchange. At present, in the data network, signals appear in the form of "packets", using the so-called "packet switching". The particles of the bag are relatively small and can be exchanged electronically. However, after a large number of packets in the same direction are aggregated, when the number is large, a large-capacity optical switch should be used.
Optical switching with fewer channels and large capacity has been practical. For example, it is used for protection, drop off, and small-volume channel scheduling. Generally, mechanical optical switches and thermo-optical switches are used to achieve this. Due to the limitations of the volume, power consumption and integration of these optical switches, the number of channels is generally 8-16.
Electronic exchange generally has "space division" and "time division" methods. There are "space division", "time division" and "wavelength exchange" in optical switching. Optical time division switching is rarely used in optical fiber communications.
Optical space division switching: Generally, optical switches can be used to transfer optical signals from one fiber to another. Optical switches for air separation include mechanical, semiconductor and thermo-optical switches. Using integrated technology, the MEM micro-motor optical switch was developed, and its volume is as small as mm. The 1296x1296MEM optical switch (Lucent) has been developed, which is experimental.
Optical wavelength exchange: assign a specific wavelength to each exchange object. Thus, by sending a certain wavelength, it is possible to communicate with a certain object. The key to realizing optical wavelength switching is the need to develop practical variable-wavelength light sources, optical filters, and integrated low-power reliable optical switch arrays. A cross-connection test system (corning) combining 640x640 semiconductor optical switch + AWG space division and wavelength has been developed. Using optical space division and optical wavelength division can form a very flexible optical switching network. Japan's NTT conducted a field test using wavelength routing and switching in Chitose City, with a radius of 5 kilometers, a total of 43 terminal nodes, (trying 5 nodes), and a rate of 2.5Gbps.
The optical network of automatic switching, called ASON, is the direction of further development.
Development of integrated optoelectronic devices
Just like electronic devices, optoelectronic devices must also be integrated. Although not all optoelectronic devices need to be integrated, a considerable part is needed and can be integrated. The PLC-planar optical waveguide circuit under development is like a printed circuit board on which optoelectronic devices can be assembled or directly integrated into an optoelectronic device. Whether to realize FTTH or ASON, new, small, inexpensive and integrated optoelectronic devices are needed.
The market for optical fiber communications
As we all know, the IT industry bubble in 2000 caused an explosive development of the optical fiber communications industry and overproduction of products. The prices of optical transmission equipment, optoelectronic devices and optical fibers have plummeted. Especially for optical fiber, the price per kilometer was ￥1200 during the bubble period, and the price was about Y100 for 1 kilometer, which was cheaper than copper wire. When will the optical fiber communications market recover?
According to RHK's statistics and forecasts on North American communications industry investment, 2002 was the lowest point, which is equivalent to a four-year regression. There has been a rebound, but it cannot be recovered yet. Based on this speculation, it will be restored only in 2007-2008. The optical fiber communication market also improves with the IT market. These improvements have been driven to a large extent by FTTH and broadband digital TV.
FTTH is the demand of the information society after all, and the optical fiber communication market must have a beautiful scene. FTTH in developed countries has begun construction and there is already a considerable market. Generally speaking, the profits of devices and equipment will gradually pick up as the market needs, and 2007-2008 may be good. However, in the optical fiber industry, despite the success of anti-dumping, the price is still sluggish, and the profit is very small. In fact, in the world, the production scale of optical fiber is too large, and the development speed of FTTH is affected by the social environment, including the economic conditions of the citizens and the development of digital TV, and the rise is slow. It is understood that some large companies have sealed up several optical fiber factories and can start production at any time according to market conditions. As a result, supply always exceeds demand. It is a normal market law to increase prices when supply exceeds demand. Therefore, if the optical fiber industry wants to make substantial profits, it may be a matter after 2009. China's economically underdeveloped regions and small towns still need to build optical fiber lines, but the amount of optical fiber is still in the range of oversupply.
For the Chinese market, FTTH will be delayed due to the challenges of ADSL and the development of digital TV HDTV. The social environment and conditions for the massive construction of FTTH in China are not yet available, and it may take some time to wait. However, the Beijing Olympics requires the promotion of HDTV and the decline of equipment prices, which will promote the development of FTTH. It is expected that FTTH will start to be promoted in China in 2007-2008. However, there are also so-called central business district CBDs in some large cities, which have relatively strong economic power, and have already adopted fiber-to-residence PTTP for construction. In general, China's FTTH is in the pilot phase. The role of the pilot is to explore technology and construction experience on the one hand, and on the other hand, it also plays a role in competition to seize users. Therefore, telecommunications operators and local owners are actively piloting FTTH in order to develop broadband services. Therefore, broadcast operators are facing huge challenges. Broadcasters should speed up the process of developing digital TV, enrich the program content and adopt a competitive business model. If broadcasters want to develop VOD on-demand TV, they also need to transform the cable TV network in both directions. If the fiber optic network is adopted, it can more fully adapt to future technological development and market demand.
Broadband China Strategy
In the "Twelfth Five-Year Plan for Broadband Network Infrastructure" issued by the Ministry of Industry and Information Technology in May 2012, Realize "Urban fiber to the building into the home, rural broadband into the village into the village". The access bandwidth of urban households reaches 20 Mbit/s, and the access bandwidth of rural households reaches 4 Mbits/sec. The fiber-to-the-home coverage reaches 200 million households with more than 40 million users, and the fiber-to-the-home rate of newly built residential buildings in cities reaches more than 60%.
"The access mode and technology of my country's broadband market is mainly ADSL, while other countries with high broadband speed are basically fiber access." said Zhao Zisen, academician of the Chinese Academy of Engineering, realizing fiber to the home It is the most important part of the broadband strategy.
The academician of the Chinese Academy of Sciences Gan Fuxi said that optical fiber communication has the advantages of large information capacity, long transmission distance, and small signal interference. In the world's communication systems, more than 90% of the information is transmitted via optical fibers. In the next 5 to 10 years, my country's large-scale implementation of fiber to the home requires more than 100 million kilometers of fiber each year, which will bring good opportunities for the development of the domestic fiber optic communications industry.
According to the latest statistics of the International Telecommunication Union, 112 countries and economies have launched broadband strategies in the world. The implementation of the broadband strategy will surely bring about the great development of optical fiber access and make the optical fiber broadband industry one of the industries with the fastest growth and the largest development space in the entire information and communication industry.
展望
光纤通信发展总趋势为：不断提高信息率和增长中继距离。系统的优值用“信息率”与“距离”的乘积表示，该值每年约增加一倍；发展光纤网，特别是光纤用户网-光纤到户；采用新技术，特别是掺稀土金属的光纤放大器，光电集成和光集成。
①90年代初商用光纤通信系统的最高水平为2.488Gbit/s系统。实验室里实验系统信息率为8、10、16Gbit/s，相应的无中继距离为76、80、65km，信息率已高达20Gbit/s。单机的速率过高，大规模集成电路的电时分复用和解复器的速率将提高，要求激光器必须能在极高速率下稳定工作。如采用1.55μm波长，用常规单模光纤，将出现色散过大，码间干扰过大等都是技术上的困难。经济上也不合算。可采用光波分复用（OWDM）来提高信息率，实验室里复用数量用高达100个622Mbit/s的系统作复用，波长间隔为0.lnm，传输距离为50km，用非相干接收。还可采用副载波调制（SCM）来增加系统容量，将在光缆电视系统中应用。
掺稀土金属铒的单模光纤放大器的成功，大大增加了系统的灵敏度和传输距离。近期发表的常规系统的环路试验，在此环路里有4支掺铒光纤放大器，传输速率为2.4Gbit/s和5Gbit/s，计算结果表明传输距离达21000km和9000km。波长为1.55μm，采用色散位移光纤。这个试验系统将在新的横跨太平洋和大西洋的光缆系统里实用。
用光波分复用提高速率，用光放大增长传输距离的系统，为第五代光纤通信系统。
新系系统中，相干光纤通信系统，已达现场实验水平，将得到应用。光孤子通信系统可以获得极高的速率，实验结果已达32Gbit/s，20世纪末或21世纪初可能达到实用化。在该系统中加上光纤放大器有可能实现极高速率和极长距离的光纤通信。
②光纤用户网-光纤到户，采用同步光纤网（SONET）或同步数字体系（SDH）和建立光纤用户网是实现宽带业务的两大步骤。
光纤用户网有不同结构，其中之一如图5所示，中心局与远区局的连接，即本地网，可以用环状网路以提高网路的灵活性和效率。远区局到用户的网可以单星形或双星形网路。
③掺铒光纤放大器具有增益高、带宽宽、噪音低、易与传输光纤连接、易于制造等优点，可作前置放大、线路放大和末级放大。可提高系统灵敏度，增长传输距离。把它用在用户网里，可扩大网的范围，也可增加用户数量，对光纤通信的发展将起重大作用。掺铒光纤放大器只工作在1.55μm，还需探索掺另一种稀土金属的光纤，得到在1.3μm工作的放大器。
另外，为提高系统的可靠性和经济性，需要光电集成和光集成，对此已有不少实验成果。
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