Optical terminology
Wavelength Division Multiplexing(WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths of laser light. Each signal modulated through the data (text, voice, video, etc.) transmit in its unique zone. WDM enables telephone companies and other operators to increase the capacity of existing fiber optical infrastructure. Manufacturers have introduced a WDM system, also called DWDM (Dense Wavelength Division Multiplexing) system.
DWDM can support more than 150 light beams with different wavelengths transmit simultaneously and each light beam can reach up to 10Gb/s data transfer rate at the highest. This kind of system can provide more than 1Tb/s data transfer rate in fiber optic cable which is thinner than hair.
Optical communication is a transmission mode to carry and transmit the signal through light. People used to name with the wavelength instead of frequency in the field of optical communications. Therefore, the socalled wavelength division multiplexing (WDM, Wavelength Division Multiplexing) is essentially just frequency division multiplexing. WDM carries a plurality of wavelengths (channels) in one optical fiber, and convert it to a number of "virtual" fibers. Of course, each "virtual fiber" work independently at different wavelengths, which dramatically increases the transmission capacity of optical fiber. The economy and effectiveness of WDM systems technology, make them the primary means to expand the current optical fiber communication network. As a system concept, WDM technology usually has three kinds of multiplexing schemes, namely 1 310 nm and 1 550 nm wavelength division multiplexing, coarse wavelength division multiplexing (CWDM) and dense wave division multiplexing (DWDM).
Two wavelengths
This kind of multiplexing only use two wavelengths in the early 1970s: 1310 nm wavelength window, 1550 nm wavelength window, using the WDM technology to achieve single fiber dual windows transmission, which is the initial use of wavelength division multiplexing.
1.Coarse wavelength division multiplexing
Following the application of the backbone network and long-distance networks, WDM technology began to be used in the MAN, which mainly refers to the coarse wavelength division multiplexing technology. CWDM use 1200 ~ 1700nm wide windows, mainly used in the 1550 nm wavelength systems. Of course, the 1310nm wavelength WDM is also being developed. The adjacent channel spacing of Coarse Wavelength Division Multiplexing (large wavelength interval) is generally over 20nm, the number of which are usually 4 or 8 waves or up to 16 waves at most. When the number of channels is 16 or less, since the system uses CWDM DFB laser cooling is not required, in terms of cost, power requirements, and equipment dimensions, CWDM system has more advantages than DWDM systems, CWDM is increasingly widely accepted by the industry. CWDM need to select the expensive dense wavelength division multiplexer and "optical amplification" EDFA, and just need to use the cheap multi-channel laser transceiver as a relay, thus the cost is greatly reduced. Today, many manufacturers have been able to provide commercial CWDM systems with 2 to 8 wavelengths which are suitable for using in the cities that geographical scope is not particularly large, data services development is not very fast.
2.Dense wavelength division multiplexing
Dense wavelength division multiplexing technology (DWDM) can carry 8 to 160 wavelengths, and with the continuous development of DWDM technology, the upper limit of the sub-wave number continues to grow, and the interval is generally less than 1.6 nm, which is mainly used in long distance transmission systems. The dispersion compensation technology (multi-wavelength system to overcome nonlinear distortion - the four-wave mixing phenomenon) is required in all DWDM systems. 16 waves DWDM systems generally use conventional dispersion compensating fiber to compensate, but 40 waves of DWDM systems must use the dispersion slope compensating fiber to compensate. DWDM can be put in the same fiber to combine and transmit the different wavelengths simultaneously. To ensure efficient transmission, an optical fiber is converted to a plurality of virtual fibers. Using the DWDM technology, a single optical fiber can transmit data flow rates up to 400 Gbit/s. As manufacturers added more channels in per fiber, per second transmission speed just around the corner.
3.Technological level
On the perspective of the test level of existing transmission capacity of WDM systems, the Nortel and other companies' 1.6Tbit/s (160 (10Gbit/s) WDM system has been successful. In the subsequent exhibition, Nortel released 80Gbit / s of WDM system, the total capacity of which is 6.4Tbit/s. In addition, Lucent uses 80nm spectral width of the optical amplifier to create the world record of a number of wavelengths up to 1022. At the same time, we know that some of the indicators of the world famous company's existing WDM systems.
In China, the research and development of WDM technology are not only active, but also develop very fast. Wuhan Research Institute of Posts and Telecommunications (WRI), Peking University, Tsinghua University, the Ministry of Posts and Telecommunications have successively carried out the transmission experiments or construction of pilot project. For example, Wuhan Research Institute of Posts and Telecommunications successfully carried out 16 (2.5Gbit/s 600km unidirectional transmission system in October 1997. It showed 32 (2.5Gbit/s of WDM transmission system, and conducted transmission experiments of a capacity of 40 (10Gbit/s WDM system in October 1998 at the Beijing 98 International Telecommunications Exhibition. And the higher level WDM systems are under experiments. Huawei, Ericsson, ZTE, flames and other manufacturers all have WDM relevant layout, Huawei's WDM global market share has jumped to first place. 100G WDM products have been officially used in business, and 400G technology validation and experimental tests have been carried out in the laboratory.
4.The developing prospects
WDM is a multiplexing technique in the optical domain and it forms an optical layer network "alloptical network", which will be the highest stage of optical communications. The establishment of a WDM and OXC (optical cross-connect) -based optical network layer, the achievement of all-optical network users to connect end to end, the using of a pure "all-optical network" to eliminate bottlenecks photoelectric conversion will be the future trend. WDM technology is still based on the way of point to point , but as the first step of all-optical network communication, the point to point WDM technology is also the most important step, the application and practice of which are based on the development of all-optical network.
5.Application
DWDM can combine and transmit the different wavelengths in the same fiber simultaneously. In order to ensure effectiveness, an optical fiber is transmitted into multiple virtual fibers. So, if you intend to reuse eight fiber optic carrier (OC), ie an 8- way optical fiber transmission signals, the transmission capacity will reach from 2.5 Gb/s up to 20 Gb/s. Becasue of the DWDM technology, a single optical fiber can transmit the maximum 40Gb/s. As manufacturers added more channels in per fiber, the transmission speed terabit per second is just around the corner.
6.Technology
Wavelength Division Multiplexing (WDM) is a technology which uses a multiplexer (also known as multiplexer, Multiplexer) at the transmitter to join two or more different wavelengths of light carrier signal (carry a variety of information) at the confluence of the sender , and coupled to the light the same fiber lines for transmission of technology; At the receiving end, the demultiplexer (also known as a demultiplexer or demultiplexer said, Demultiplexer) separate various wavelengths of light carrier, and then the optical receiver for further processed to recover the original signal. This simultaneous transmission of two or a number of different wavelengths of light signals in the same fiber technology is called wavelength division multiplexing (as the picture below).
WDM is essentially a frequency-division multiplexed FDM technology in optical domain. Each wavelength channel is divided by a frequency domain implementation. Each wavelength channel occupies the bandwidth of the optical fiber section. The wavelength of WDM system is different, that is a specific standard wavelength. In order to distinguish SDH system ordinary wavelength, sometimes called color optical interface, and the optical interface of ordinary light system as "white light mouth" or "white mouth.
Different communication system design makes the interval between each wavelength also different. According to different channel spacing, WDM can be subdivided into CWDM (Coarse Wavelength Division Multiplexing) and DWDM (Dense Wavelength Division Multiplexing). The channel spacing of CWDM is 20nm, and DWDM channel spacing from 0.2nm to 1.2nm. So compared with DWDM, CWDM is called coarse wavelength division multiplexing technique.
7.Feature
(1) Large-capacity transmission.
Since the multiplexed optical channel rate of WDM system can reach 2.5Gbit / s, and 10Gbit / s, etc., while the number of channels can be 4,8,16,32 path, and even more, the transmission capacity of the system can reach 300 -400Gbit / s, or even greater.
(2) Fiber resources conservation.
For a single wavelength system, an SDH system will need a pair of fibers; and for WDM system, no matter how many SDH sub-systems, entire multiplexing system requires only a pair of optical fibers. For example, for 16 2.5Gbit / s systems, single-wavelength systems require 32 optical fibers, while the WDM system requires only two fibers.
(3) Each channel transparent transmission, smooth upgrade, expansion.
Just adding multiplex channels number and devices can increase the transmission capacity of the system to achieve the expansion. Because each multiplex channel of WDM systems is independent from each other, so each channel can separately transparently transmit different service signals, such as voice, data, and images, etc., not interfering with each other, which brings the user great convenience.
(4) The use of EDFA to achieve ultra-long haul transmission.
EDFA has the advantages of a high gain, wide bandwidth, low noise, etc., and its optical zoom range can reach 1530 (1565nm, but its gain curve is relatively flat section 1540 (1560nm), which can cover almost the operating wavelength range of 1550nm WDM system. So with a very wide bandwidth of EDFA we can just multiplex optical path signal while amplifying WDM system, to achieve ultra-long haul transmission, and avoid the situation that each of the optical transmission system needs an optical amplifier. The long transmission distance of WDM can reach up to several hundred kilometers while saving a lot of repeaters, and reducing costs.
(5) Improving of the reliability of system.
Since most of the WDM system is photovoltaic device which is with high reliability, therefore, the reliability of the system can be guaranteed.
(6) Can be composed of all-optical networks.
All-optical network is the future direction of the optical fiber transmission network. In the all-optical network, from top to bottom, the various services are all in the optical path through scheduling the optical signal, thereby eliminating bottlenecks of E / O converter in electronic devices. WDM systems can be mixed with OADM, OXC, to form a high degree of flexibility, reliability, survivability of all-optical networks to meet the development of bandwidth transport networks.
8.Advantage
A key advantage of DWDM is the irrelevance of its protocol and transmission speed. Because DWDM-based network can use IP protocol, ATM, SONET / SDH, Ethernet protocols to transfer data and the processing data is between 100 Mb / s and 2.5 Gb / s, the DWDM-based network can transmit different types of data traffic in a laser channel transmission at different speeds. From QoS (Quality of Service) viewpoint, DWDM-based networks now quickly respond to changing bandwidth requirements and protocols changes of customers in a low cost way. Science and technology is increasingly updating. The using of 1600G, 800G and 400G are very common at the national trunk, provincial and municipal trunk. Take 1600G as an example, theoretically, an optical fiber can go 160 10G services in the fully equipped cable, greatly improving the utilization of the optical fiber. Of course, the demand of cable quality is also high. The theoretical values and actual values are also biased. In the practical application, to avoid failure rate we rarely use a hundred channels on the same fiber business.
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