What Are the Different Types of Fiber-Optic Systems?

An optical fiber communication system is a communication system that uses light as a carrier wave and uses ultra-high-purity glass to make ultra-fine optical fiber as a transmission medium. It uses photoelectric conversion to transmit information using light. With the rapid development of the Internet business and communications industry, informatization has greatly promoted the development of world productivity and human society. Optical fiber communication, as one of the main technical pillars of information technology, will become the most important strategic industry in the 21st century.

Optical fiber is short for optical fiber. Optical fiber communication is a communication method that uses light waves as information carriers and optical fibers as transmission media. In principle, 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 processes, material composition, and optical characteristics, in applications, optical fibers are often classified according to use, and can be divided into communication optical fibers and sensing optical fibers. Transmission media optical fiber is divided into general and special two types, and functional device optical fiber refers to the optical fiber used to complete the amplification, shaping, frequency division, frequency doubling, modulation, and optical oscillation of optical waves. The form appears.

Optical fiber communication is a communication method that uses light waves as a carrier wave and uses optical fiber as a transmission medium to transfer information from one place to another. It is called "wired" optical communication. Today, optical fiber has become the main transmission mode in the world for its transmission frequency bandwidth, high interference resistance and small signal attenuation, which is far superior to cable and microwave communication. ^[1]

Optical fiber communication is the main transmission means of modern communication networks. Its development history is only one or two decades, and it has gone through three generations: short-wavelength multimode fiber, long-wavelength multimode fiber, and long-wavelength single-mode fiber. The use of optical fiber communication is a major change in the history of communication. More than 20 countries, such as the United States, Japan, Britain, and France, have announced that they will no longer build cable communication lines and are committed to the development of optical fiber communication. China's optical fiber communication has entered the practical stage.

The birth and development of optical fiber communications is an important revolution in the history of telecommunications. Satellite communications and mobile communications are listed 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 a more urgent demand for large-capacity (ultra-high-speed and ultra-long-distance) optical wave transmission systems and networks.

Optical fiber communication is the latest communication technology that uses light waves as a carrier wave to transmit information, and uses optical fiber as a transmission medium to achieve information transmission and achieve communication purposes.

The development process of communication is to continuously increase the carrier frequency to expand the communication capacity. The optical frequency as the carrier frequency has reached the upper limit of the communication carrier. Because light is an extremely high frequency electromagnetic wave, the use of light as the carrier for communication capacity is extremely large. It is thousands of times of the past communication methods and has great attraction. Optical communication is a goal that people have long pursued, and it is also an inevitable direction for the development of communication.

Compared with the previous electrical communication, the main difference between 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 material is quartz, saving metal materials, Conducive to the rational use of resources; strong insulation and anti-electromagnetic interference performance; also has the advantages of strong corrosion resistance, strong radiation resistance, good rewindability, no electric spark, small leakage, strong confidentiality, etc., can be used in special environments or military Use.

Optical fiber communication system trends

FTTH can provide users with extremely rich bandwidth, so it has been considered as an ideal access method, which plays an important role in realizing the information society, and needs large-scale promotion and construction. The fiber required for FTTH may be two to three times that of existing fiber. In the past, due to the high cost of FTTH, the lack of broadband video services and broadband content, etc., FTTH has not been mentioned on the agenda, with only a few trials. Due to the progress of optoelectronic devices, the prices of optical transceiver modules and optical fibers have been greatly reduced; coupled with the ease of broadband content, both have accelerated the practical process of FTTH.

Developed countries' views on FTTH are not exactly the same: AT & T in the United States believes that the FTTH market is small and declared in 0F62003: FTTH will not be available until 20-50 years later. American operators Verizon and Sprint are more active and will use FTTH to transform their networks within 10-12 years. Japan's NTT developed FTTH the earliest, with nearly 2 million users. China's FTTH is in a pilot phase.

FTTH Optic fiber communication system FTTH meets challenges

There are still certain advantages in the widely used ADSL technology to provide broadband services

Compared with FTTH: the price is cheap the use of the original copper network to make the construction simple for the transmission of 1Mbps-500kbps film and television programs can meet the needs. 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 of two-way services and HDTV high-definition digital television, asymmetry of uplink and downlink transmission services, ADSL is difficult to meet. Especially HDTV, after compression, its transmission rate still needs 19.2Mbps. It is being developed with H.264 technology and can be compressed to 5-6Mbps. It is generally believed that the highest transmission speed of ADSL with QOS guarantee 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 the HDTV service comes, non-FTTH is not required.

FTTH Optical fiber communication system FTTH solution

There are usually two types of P2P point-to-point and PON passive optical networks.

The F2P solution has advantages one by one: each user transmits independently, without affecting each other, and the system is flexible; it can use inexpensive low-speed optoelectronic modules; the transmission distance is long. Disadvantages: In order to reduce the optical fibers and pipes that users go directly to the office, an active node that aggregates users needs to be placed in the user area.

PON solution-advantages: simple passive network maintenance; in principle, can save optoelectronic components and optical fibers. Disadvantages: expensive high-speed optoelectronic modules need to be used; electronic modules that distinguish users from different distances need to be used to avoid conflicts between uplink signals of users; transmission distance is shortened by the PON ratio; downlink bandwidth of each user occupies each other. When it is not guaranteed, it is not only to expand the network, but also to replace the PON and user modules. (According to market price, PEP is more economical than PON)

There are many types of PON, generally as follows: (1) APON: ATM-PON, which is suitable for ATM switching networks. (2) BPON: Broadband PON. (3) OPON: OFP-PON using universal frame processing. (4) EPON: PON using Ethernet technology, GPON is a PON of Gigabit Ethernet. (5) WDM-PON: It adopts wavelength division multiplexing to distinguish users' PON. Since users are related to the wavelength, maintenance is inconvenient, and it is rarely used in FTTH.

Wireless access technology is developing rapidly. It can be used as WLAN's IEEE802.11g protocol, with a transmission bandwidth of up to 54Mbps and a coverage range of more than 100 meters. It is commercially available. If wireless access WLAN is used for user data transmission, including uplink and downlink data and uplink data of VOD on demand TV, for general users, the uplink is not large, and IEEE802.11g can be satisfied. The FTTH using optical fiber is mainly used to solve the downlink transmission of HDTV broadband video. Of course, it can also include some downlink data when needed. This forms a "fiber-to-home + wireless access" (FTTH + wireless access) home network. Such a home network is particularly simple if a PON is used, because the PON has no uplink signals, and no electronic module for ranging is needed, which greatly reduces the cost and facilitates maintenance. If the user group of the PON to which it belongs is covered by the wireless metropolitan area network WiMAX (1EEE802.16) and is available, it is not necessary to build a dedicated WLAN. The use of wireless in the access network is the trend, but the wireless access network still needs to be densely supported by the optical fiber network adjacent to the user, which is almost the same as FTTH. FTTH + wireless access is the future development trend.

Development of optical switching in optical fiber communication systems

In fact, it can be expressed as: communication input + exchange.

Optical fiber only solves the transmission problem, and it also needs to solve the problem of optical switching. In the past, communication networks were composed of metal cables, and electronic signals were transmitted, and electronic switches were used for exchange. Except for a short section of the user's end, the communication network is an optical fiber, which transmits optical signals. A reasonable method should use optical switching. However, because the optical switch device is immature, only the "optical-electrical-optical" method can be used to solve the optical network exchange, that is, the optical signal is converted into an electrical signal, which is electronically exchanged before being converted into an optical signal. Obviously this is an unreasonable solution, which is not efficient and economical. Large-capacity optical switches are being developed to implement optical switching networks, especially the so-called ASON-Automatic Switching Optical Network.

Figure 4 Fiber

The information usually transmitted in the optical network is usually xGbps, and the electronic switch is not capable. Electronic exchange is generally implemented in lower order groups. And optical switching can achieve high-speed XGbDs switching. Of course, it's not to say that everything should be exchanged with light, especially low-speed, small-particle signals. Mature electronic exchange should be used, and there is no need to use immature ones.

Large-capacity optical switching. At present, in data networks, signals appear in the form of "packets", so-called "packet switching" is used. 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.

Small-channel and large-capacity optical switching has been practical. For example, it is used for protection, downlink and small-path scheduling. Generally use mechanical light switch, thermo-optic switch to achieve. Due to the limitation 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 switching" in optical switching. Optical fiber communications rarely use optical time division switching.

Optical space division switching: Generally, optical switches can be used to switch optical signals from one fiber to another. Optical switches for space separation include mechanical, semiconductor and thermo-optical switches. Using integrated technology, MEMS micro-motor optical switches have been developed with a size as small as mm. 1296x1296MEM optical switch (Lucent) has been developed and is experimental.

Optical wavelength switching: A specific wavelength is assigned to each switching object. Therefore, a specific wavelength can be transmitted to communicate with a specific object. The key to realize optical wavelength exchange is to develop a practical variable wavelength light source, optical filter, and integrated low-power and reliable optical switch array. 640x640 semiconductor optical switch + AWG has been developed to combine the cross-connected test system (corning) of space division and wavelength. The use of optical space division and optical wavelength division can constitute a very flexible optical switching network. Japan NTT conducted a field test using wavelength routing switching in Chitose City, with a radius of 5 kilometers, a total of 43 terminal nodes (5 nodes on trial), and a rate of 2.5Gbps.

The optical network that is switched automatically, called ASON, is the direction for further development.

Development of integrated optoelectronic devices

Like electronic devices, optoelectronic devices must also be integrated. Although not all optoelectronic devices need to be integrated, a considerable part is required and can be integrated. The developing PLC-planar optical waveguide circuit, like a printed circuit board, can assemble optoelectronic devices on it, or it can be directly integrated into an optoelectronic device. To achieve FTTH or ASON, new, small, inexpensive and integrated optoelectronic devices are required.

Optical fiber communication system market

As everyone knows, the IT industry bubble in 2000 made the optical fiber communication industry's production scale develop explosively and over-production. Whether it is optical transmission equipment, the prices of optoelectronic devices and optical fibers have plummeted. Especially for optical fiber, the price is 1200 yuan per kilometer during the foam period, and the price is about 1 kilometer at Y100, which is cheaper than copper wire. When will the market for fiber optic communications recover?

According to the statistics and forecast of RHK's investment in the communications industry in North America, as shown in Figure 2. It was the lowest point in 2002, which is equivalent to going back 4 years. It has picked up, but it cannot be recovered. Based on this speculation, it will only recover in 2007-2008. The market for optical fiber communications has also improved with the IT market. These improvements are, to a great extent, driven by FTTH and broadband digital TV.

After all, FTTH is the demand of the information society, 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, with the needs of the market, the profits of devices and equipment will gradually rise, and 2007-2008 may be good. However, despite the successful anti-dumping in the optical fiber industry, the price is still too low 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 television, and it is rising slowly. It is understood that some large companies have sealed several fiber optic factories, and according to market conditions, production can be started at any time. As a result, supply is always greater than demand. Supply is in short supply to increase prices, which is a general market law. Therefore, if the optical fiber industry wants to make a profit, 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 by the challenges of ADSL and the development of digital TV HDTV. The social environment and conditions for the large-scale construction of FTTH in China have not yet been met, and it may take some time. However, the Beijing Olympic Games needs the promotion of HDTV and the reduction of equipment prices, which will promote the development of FTTH. It is expected that FTTH will be promoted in China in 2007-2008. However, the so-called central business district CBD of some large cities has relatively strong economic power and has already adopted fiber-to-residence PTTP for construction. In general, China's FTTH is in the pilot stage. The role of the pilot, on the one hand, is to explore technology and construction experience, on the other hand, it also plays the role of competition to seize users. Therefore, telecom operators and local owners are actively piloting FTTH in order to develop broadband services. Therefore, broadcast operators are faced with huge challenges. Broadcasters should speed up the process of developing digital television, and enrich the content of programs and adopt a competitive business model. If broadcasters want to develop VOD on-demand TV, they also need to transform the cable television network in both directions. If they use optical fiber networks, they can more fully adapt to future technological development and market demand.

Fiber Optic Communication System Broadband China Strategy

The Ministry of Industry and Information Technology put forward in the "Twelfth Five-Year Plan" for broadband network infrastructure issued in May 2012. By 2015, the country will basically realize "urban fiber-to-the-house access, rural broadband access to rural areas and villages." The access bandwidth of urban households reaches 20 megabits per second, and the access bandwidth of rural households reaches 4 megabits per second. The fiber-to-the-home coverage of 200 million households has reached more than 40 million users, and the fiber-to-the-home rate of new residential buildings in cities has reached over 60%.

"ADSL is the main access method and technology in China's broadband market, while other countries with high broadband speeds are mainly based on optical fiber access." Said Zhao Zisen, an academician of the Chinese Academy of Engineering, achieving fiber-to-the-home is one of the most important broadband strategies. ring.

Academician of the Chinese Academy of Sciences Gan Fuyi said that optical fiber communication has the advantages of large information capacity, long transmission distance, and small signal interference. In communication systems around the world, more than 90% of the information is transmitted through optical fibers. In the next 5 to 10 years, the fiber required for the implementation of fiber-to-the-home in China on a large scale is expected to be more than 100 million kilometers per year, which will bring a good opportunity for the development of the domestic fiber optic communication industry.

According to the latest statistics from the International Telecommunication Union, there are 112 countries and economies in the world that have launched broadband strategies. The implementation of the broadband strategy will inevitably lead to the development of optical fiber access, and make the optical fiber broadband industry one of the fastest growing and largest developing industries in the entire information and communication industry.

Optical fiber communication system outlook

The overall development trend of optical fiber communication is: continuously improve the information rate and increase the relay distance. The good value of the system is expressed by the product of "information rate" and "distance", which is about doubled every year; the development of fiber optic networks, especially fiber subscriber networks-fiber to the home; the use of new technologies, especially rare earth metal-doped fibers Amplifier, optoelectronic integration and optical integration.

The highest level of commercial optical fiber communication systems in the early 1990s was 2.488Gbit / s. The information rate of the experimental system in the laboratory is 8, 10, and 16 Gbit / s, and the corresponding non-relay distances are 76, 80, and 65 km. The information rate has reached 20 Gbit / s. The speed of the single machine is too high, and the speed of the electrical time division multiplexing and demultiplexer of the large-scale integrated circuit will be increased. It is required that the laser must work stably at an extremely high speed. If 1.55m wavelength is used and conventional single-mode fiber is used, excessive dispersion and excessive inter-symbol interference are technical difficulties. Economically uneconomical. Optical wavelength division multiplexing (OWDM) can be used to increase the information rate. The number of multiplexes in the laboratory is multiplexed with up to 100 systems of 622 Mbit / s, with a wavelength interval of 0.1 nm, a transmission distance of 50 km, and non-coherent reception. Sub-carrier modulation (SCM) can also be used to increase system capacity, which will be used in optical cable television systems.

The success of rare-earth metal erbium-doped single-mode fiber amplifiers has greatly increased the sensitivity and transmission distance of the system. The loop test of the conventional system recently published, there are 4 erbium-doped fiber amplifiers in this loop, the transmission rate is 2.4Gbit / s and 5Gbit / s, and the calculation results show that the transmission distance reaches 21000km and 9000km. The wavelength is 1.55 m and a dispersion-shifted fiber is used. The test system will be used in new cable systems across the Pacific and Atlantic Ocean.

The system that uses optical wavelength division multiplexing to increase the rate and uses optical amplification to increase the transmission distance is the fifth generation optical fiber communication system.

In the new system, the coherent optical fiber communication system has reached the field experimental level and will be applied. Optical soliton communication systems can achieve extremely high speeds, and the experimental results have reached 32 Gbit / s, which may be practical in the late 20th or early 21st century. Adding fiber amplifiers to this system makes it possible to achieve extremely high speed and long distance fiber communication.

Optical fiber user network-fiber-to-the-home, adopting synchronous optical network (SONET) or synchronous digital system (SDH) and establishing an optical fiber user network are two major steps to realize broadband services.

Optical fiber user networks have different structures. One of them is shown in Figure 5. The connection between the central office and the remote office, that is, the local network, can use a ring network to improve the flexibility and efficiency of the network. The remote office-to-subscriber network can be a single star or double star network.

Figure 6 Example of the structure of an optical fiber subscriber network

Erbium-doped fiber amplifiers have the advantages of high gain, wide bandwidth, low noise, easy connection with transmission fiber, and easy manufacturing. They can be used for pre-amplification, line amplification, and final-stage amplification. Can improve system sensitivity and increase transmission distance. Using it in a user network can expand the range of the network and increase the number of users, which will play a significant role in the development of optical fiber communications. Erbium-doped fiber amplifiers only work at 1.55m, and another rare earth metal-doped fiber needs to be explored to obtain amplifiers that work at 1.3m.

In addition, in order to improve the reliability and economics of the system, optoelectronic integration and optical integration are needed, and many experimental results have been achieved ^[3] .