Key Technologies in Optical Communication

The appearance of optical fiber communication technology is an important revolution in the history of communication. As a broadband transmission solution, optical fiber communication has been paid special attention by people since its birth, and has maintained a strong development momentum. Especially in the period from the middle to the end of 1990s, optical communication has developed rapidly in terms of technology and related products, and established its irreplaceable core position in the field of communication.

At present, optical communication technology is developing at a speed beyond people's imagination. In the past 10 years, the optical transmission rate has increased by 100 times, and it is expected to increase by about 100 times in the next 10 years. The continuous exponential growth of IP services has brought new opportunities and challenges to the development of optical communication: on the one hand, the huge traffic and asymmetry of IP have stimulated the application and rapid development of WDM technology; On the other hand, the difference between IP services and circuit switching also challenges SDH (Synchronous Digital Series) based on circuit switching. Optical communication itself is undergoing profound changes, especially the rise and development of "optical network", which can be multiplexed, demultiplexed, routed and switched in the optical domain. It can make full use of the huge bandwidth resources of optical fiber to increase the network capacity and realize the "transparent" transmission of various services, so optical communication technology has become the focus of people's attention. This paper will briefly introduce and look forward to several important technologies in optical communication.

First, the multiplexing technology

1. time-division multiplexing (TDM)

Multiplexing technology is a good way to increase the transmission capacity of communication lines. Time-division multiplexing is used for digital communication. The digital group series starts with PDH groups, followed by SDH groups, and consists of electrical combiner/splitter and combiner/demultiplexer (MUX/DE-MUX). At present, the highest digital application rate of TDM is 10gbit/s. The digital group with the highest digital rate is directly modulated to the optical carrier on the optical fiber, which becomes the highest digital rate of optical fiber transmission. However, the optical fiber itself has great potential capacity, so it is limited by the highest rate of electricity. In fact, when the transmission rate is increased from 10Gbit/s to about 20Gbit/s, it is close to the technical limit of semiconductor technology or microelectronic technology. Even if TDM electronic devices and circuits with higher speed are developed, such as micro-vacuum optoelectronic devices and atomic electronic switches, the development and production costs will be expensive, resulting in the high price of transmission equipment and systems, which is undesirable. What's more, the influence of fiber dispersion and nonlinearity is more serious at this time, resulting in transmission difficulties. Therefore, although the laboratory rate of TDM has reached 40Gbit/s, it should be in g. Long-distance transmission on 62 optical fiber is by no means something that can be expected in the near future. On the contrary, if the WDM system based on 10Gbit/s is adopted, the high capacity of 40Gbit/s can be achieved with four wavelengths, which can not only meet the demand of medium and long-term communication capacity, but also meet the communication development in the 21st century without substantial technical difficulties.

2. Wavelength division multiplexing technology (WDM)

In the late 1980s, the world began to imagine using an optical fiber to transmit multiple optical carriers at the same time and be modulated by digital signals. If the wavelengths of these optical carriers are sufficiently spaced from each other, the digital signals of each channel can be transmitted on the same optical fiber without mutual interference, which is the wavelength division multiplexing technology used in optical fiber communication. Wavelength division multiplexing (WDM) technology is essentially the frequency division multiplexing of light, but it is often called WDM because it is convenient to express the optical carrier by wavelength. If an optical fiber uses N WDM channels with 10Gbit/s digital signals in each channel, the transmission capacity of the optical fiber will be n×l0Gbit/s, thus breaking the restriction of transmission rate caused by the electronic bottleneck. Thus, multiplexing technology is an excellent method for capacity expansion. With the maturity and application of wavelength division multiplexing (WDM) technology, the huge potential bandwidth resources of optical fiber have been fully utilized, thus making optical fiber communication the mainstream technology to support the communication transmission network. At present, the single-wavelength transmission rate of optical fiber has reached 40Gbit/s, but further improvement of the single-wavelength transmission rate will be restricted by semiconductor technology. However, WDM technology, as the main technology for capacity expansion of optical fiber transmission network, is unshakable.

Because the optical fiber manufacturing technology itself has greatly improved the transmission capacity according to the requirements of WDM system, and the optical devices such as laser tube and wavelength combiner/demultiplexer are innovative in structure and performance, the wavelength interval of multiple optical carriers on the optical fiber is reduced, thus the number of optical paths transmitted at the same time is greatly increased. In order to increase the number of optical paths transmitted on one optical fiber, dense wavelength division multiplexing (DWDM) technology was used internationally in 1995. In 1998, about 90% of long-distance communication lines used DWDM technology, which allowed one optical fiber to transmit more optical carriers at the same time, thus further increasing the transmission capacity of optical fiber. At present, the total capacity of commercial DWDM system that can transmit on one optical fiber is 400 Gbit/s. From the technical point of view, DWDM system technology is still progressing, and it is entirely possible to increase the transmission capacity of optical fiber. Therefore, it is expected that the capacity of backbone communication network in the future will soon rise from Gbit/s to Gbit/s.

　　3。 Time-division multiplexing (OTDM)

Optical time-division multiplexing refers to the technology of multiplexing multiple optical signals with high-speed optical switches for transmission. Using OTDM technology can not only obtain a high rate-bandwidth ratio, but also overcome the uneven gain of erbium-doped fiber amplifier (EDFA), nonlinear effect of four-wave mixing FWM and other factors, and solve the competition of multiplexing ports, further increasing the flexibility of all-optical networks. Although OTDM has the above advantages, its technical advantages have not been fully developed and applied, because its key technologies (high repetition rate ultrashort optical pulse source, time-division multiplexing, ultrashort optical pulse transmission technology, clock extraction technology and time division multiplexing technology) are complex and difficult to realize, and the optoelectronic devices to realize these technologies are particularly expensive. However, it can be expected that with the expansion of optical fiber transmission system, the continuous innovation of industrial manufacturing technology and the continuous improvement of optoelectronic device manufacturing level, optical time-division multiplexing will surely be achieved.

4. Optical Code Division Multiple Access (OCDMA)

As the technical foundation of the third and fourth generation mobile communication, code division multiple access (CDMA) has made great contributions to the development of communication. CDMA technology has many advantages over other technologies. For example, it has obvious advantages in improving the capacity of the system, which can solve the problems of anti-interference and anti-multipath fading in the mobile communication system. However, due to the bandwidth limitation in satellite communication and mobile communication, the advantages of CDMA technology have not been fully exerted. Optical fiber communication has abundant bandwidth resources, which can make up for this defect. The application of CDMA technology in optical fiber system can make full use of the huge bandwidth of optical fiber and give full play to its own advantages, which is the inevitable trend of CDMA technology development. As early as the mid-1980s, foreign experts studied OCDMA system. In recent years, OCDMA has become a hot technology.

Although the development of DWDM technology provides a solution to the problem of capacity expansion of optical fiber, compared with OCDMA, DWDM scheme has a major defect-it increases the cost of network structure. For most users, the existing network cost is already very expensive, while OCDMA technology provides a new way for network development. When a large number of TDM intermediate steps required in traditional SDH are eliminated, OCDMA can not only increase the utilization rate of existing optical fiber equipment, but also greatly reduce the number of optical fibers to be built in the future. Reducing the equipment in the network can not only save the cost of the equipment itself, but also reduce the cost of other construction projects, peripheral facilities and operation support systems related to the equipment, and at the same time simplify the network management through the network element layer. However, at present, OCDMA technology is not mature enough. The main obstacle to the practical application of OCDMA is incoherent optical CDMA: firstly, the number of users is limited because of the limited number of non-polar codes and the large inter-symbol interference; Secondly, the optical codec is too bulky, so it is not practical and so on.

Second, the exchange technology

1. optical packet switching technology

The concept of optical packet switching is similar to that of electrical packet switching, but it is only an extension in the optical domain, that is, the switching granularity is based on the high-speed transmission of optical packets. Although optical packets can be long or short, optical packet switching requires a very high processing capacity of nodes because the switching equipment must be capable of processing the smallest packets. The earlier all-optical switching requires that the control signals be processed in the optical domain, but the optical logic devices can't be put into practice so far, so it can only be demonstrated in the laboratory. Therefore, at present, the international practice has actually deviated from the original intention of realizing transparent packet switching in the early stage, and the optical packet switching is realized by adopting the photoelectric hybrid method, that is, the data is exchanged in the optical domain, and the control information is processed after the switching node is converted into electrical signals.

2. Optical Burst Switching Technology (OBS)

The concept of optical burst switching appeared in the early 1980s. However, at that time, both the telephone network and the data network were technically mature, and there was no need to deal with voice or data in bursts. Therefore, the concept of optical burst switching was not as important and developed as circuit switching and packet switching at that time. Actually, in every circuit switching, the switching granularity contains many voice bursts, but it is obviously a waste of resources to make a call application for each burst. In the early data network, a burst represents a large piece of data. In order to occupy less network resources and improve the success rate of transmission, the burst data is split into multiple packets and then transmitted, without taking bursts as units. However, with the continuous development of technology, the growth rate of transmission rate greatly exceeds the growth rate of processing rate. If the old grouping method is still used for processing, the network processing equipment will be overloaded for a long time, which is not conducive to the improvement and optimization of network performance. Therefore, it is necessary to further improve and simplify the processing of network nodes. Optical burst switching improves the processing granularity, which is a good solution. By sending control information in advance, optical burst switching is performed at each node. After electrical conversion, processing, and reservation of resources, the node transmits burst data, which can always be kept in the optical domain, and the trouble of processing packet headers one by one in packet switching is avoided. There are two types of optical burst switching nodes: core nodes and edge nodes. The edge is responsible for reassembling data, such as encapsulating user packet data in the access network into burst data, or vice versa; The task of the core node is to complete the forwarding and exchange of burst data. Different from optical packet switching, only the wavelength of the transmission control packet in the optical fiber needs to be illuminated. Electrical conversion, the wavelength of transmitting burst data does not need light? Electrical transformation. In addition, the entrance fiber delay line (FDL) in optical packet switching is used to buffer burst data, which can be omitted.

At present, the communication network is developing towards the optical Internet, and there are two obvious trends: one is IP as the core, and the data service will be in the future 5? Become the leading business within 8 years: Second, the lower layer of IP layer is photochemical, and optical transmission and optical switching become the main development direction. At present, except that WDM has become the first choice for all kinds of network upgrading and expansion, there are still many disputes about optical switching: one opinion basically denies optical switching, holding that it is expensive and technically infeasible, and insisting on the network development mode of IP high-end router plus WDM transmission; the other opinion admits optical switching, but is influenced by IP packet, and insists that the future optical switching is only optical packet switching. In the near future, using high-performance high-end routers and mature WDM transmission to transmit signals on several wavelengths in POS (Packet Over SDH), ATM or GE(Gigabit Ethernet) mode to upgrade the Internet (not the real optical Internet) is indeed a simple and feasible solution. However, if the number of wavelengths is increasing and the signal transmission rate is increasing, each packet of each wavelength has to be processed, which will greatly increase the burden of routers, and the network QoS (Quality of Service) will not be guaranteed. Fortunately, MPLS (Multi-Protocol Label Switching) appeared at this time, and now high-end routers can successfully solve these two problems, but routers still handle each wavelength in a hop-by-hop manner, so the degree of solution is limited after all. Therefore, it should be an inevitable development direction to adopt optical switching technology in optical Internet.

Third, optical Internet technology

Optical Internet, also known as IP over WDM, in short, the IP network running directly on the optical layer is the optical Internet. With the rapid growth of IP data services in exponential form and the continuous maturity and perfection of WDM technology, how to use the ultra-large fiber bandwidth capacity brought by WDM to transmit data services has become a global research hotspot.

The load-bearing of IP services on WDM optical network must be based on the most mature and advanced network transmission technology at present, and make use of various resources of the existing network, including equipment, networking mode, network protocol and signal format, etc. Therefore, there are many different implementation methods, such as IP over ATM over WDM, IP over SDH over WDM, IP over WDM, etc. However, with the emergence of various new technologies and the continuous updating of equipment and networking methods, many redundant functions among all levels of the network will be cancelled, and the multi-layer protocol stack will collapse and simplify. But instead of simply discarding some layers, different functions of each layer, such as ATM switching, SONET/SDH multiplexing/demultiplexing and IP layer addressing, are reasonably decomposed and combined. The important functions of the intermediate layer are infiltrated into the IP layer and WDM optical layer respectively, and finally developed into IP over WDM.

The direct IP over WDM mode saves the ATM and SDH layers in the middle, and is built on a pure optical transmission backbone network, which has rich bandwidth management and facilities protection and recovery capabilities. It makes full use of G-bit or T-bit routing switching technology and WDM optical interconnection technology, and directly transmits IP packets on the optical network through certain adaptive encapsulation, thus greatly reducing the functional overlap between network layers, reducing the complexity of network management and the cost of network operation, improving transmission efficiency, and facilitating different networks. Therefore, the architecture of optical Internet attracts the attention of all circles of communication, and it will become the mainstream technology of IP network and optical network interconnection in the future. In addition, it should be noted that although it is called IP over WDM, in fact, IP is not directly carried on WDM network. There must be an adaptation layer with simplified functions between them. IP data used for entering WDM optical network is properly encapsulated and provided with corresponding.