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Ultra-long WDM transmission


In recent years, WDM technology mainly in two directions toward the ultra-long-range and high-capacity transmission development, in view of market demand ultra-long distance transmission and network trends, ultra-long distance transmission solutions.


Ultra-long haul transmission technologies


Ultra-long haul transmission is restricted dispersion effects, nonlinear effects and other physical disorders. In order to complete long distance transmission, must take appropriate technology in both the terminal and the line for processing.


Long-distance electricity transmission domain processing


When the dynamic routing requirements and long-distance optical transmission requirements simultaneously, relying solely on the terminal modulation techniques to improve dispersion tolerance sometimes still can not solve the problem. At this point, you can consider using electric field treatment, support systems to improve the dispersion tolerance.


EDC is the optical domain DCM dispersion compensation module functions transferred to the electric field to deal with, through the use of finite impulse response filter (FIR) technology, the optical signal in the fiber long-distance transmission via the photoelectric conversion into electrical signals made after A new method of equalization process. Currently, electric field processing methods known dispersion compensation has a forward equalization (FFE), backward equalization (DFE), a maximum likelihood sequence estimation (MLSE), FFE and DFE may also be cascaded use.


WDM system for transmitting light is not visible, but they are transmitted in an optical fiber, each beam occupies some bandwidth, each interference-free transmission, the demultiplexer at the receiving end (equivalent to the optical band-pass filter ) various colors of light signals separately. Since the optical frequency domain signal difference is relatively large, people prefer to define the difference between a wavelength frequencies, which is called wavelength division multiplexing. WDM is essentially a frequency division multiplexed optical domain FDM technology. Each wavelength channel is divided by the frequency domain, each of which occupies one fiber channel wavelength bandwidth. WDM systems using wavelength is different, that is, a specific standard wavelength, in order to distinguish it from an ordinary wavelength SDH system, sometimes referred to as color optical interfaces, and said optical interface system of the ordinary light is "white light mouth" or "white mouth . "


When people talk about WDM system, sometimes talks to DWDM. WDM and DWDM is a term used in different period of development of WDM systems. In the early 1980s, people thought and the first use of a low-loss optical fiber in the two windows 1310nm and 1550nm window for each transfer window a channel optical wavelength signal is 1310nm, 1550nm wavelength division two WDM system. With adjacent wavelengths 1550nm window EDFA commercialization, WDM system becomes very narrow spacing (generally less than 1.6nm), and the work within a window, shared EDFA optical amplifier.


In order to distinguish it from the traditional WDM system, people call this wavelength interval closer WDM system for DWDM system. The so-called intensive means adjacent wavelength interval, the wavelength interval over a WDM system is several tens of nm, wavelength interval is now only 0.4-2nm. DWDM technology is actually a manifestation of specific wavelength division multiplexing. If you do not especially 1310nm, 1550nm WDM WDM system of two outside, people talk about the WDM system is DWDM systems. WDM system in addition to greatly improve the transmission capacity, but also can reduce system cost, and its main features are:


(1) cost savings. EDFA transparency can zoom multiple wavelength, thereby greatly reducing the number of SDH regenerator and reduce system cost. In the national transmission backbone, the longer the distance, the more cost savings. Particularly suited to land a huge country.


(2) improve the reliability of the system. Since most of the photovoltaic device WDM system, and a high reliability of the photovoltaic device, so that the system reliability can be guaranteed.


(3) can increase the carrying signal transmission performance. Since WDM system greatly reduces the processing electronics, in particular SDH regenerative repeaters use, thus reducing the jitter accumulation, additional WDM system good optical design ensures SDH client signals error-free operation.


(4) can take advantage of the huge bandwidth of the fiber, so that a single optical fiber to increase transmission capacity than single-wavelength transmission times to several times.


(5) data format WDM channel is transparent, i.e., have nothing to do with the electrical signal rate modulation scheme. A WDM system can carry a variety of formats "business" signal, ATM, IP, or in the future there may be a signal, WDM transmission system is transparent to complete, for the "business" level signals, each wavelength WDM like " virtual "optical fiber the same.


WDM technology does not guarantee unlimited all-optical signal relay transmission distance, now, WDM signal 2.5G or 10G, after more than 400-600 km transmission, but also the need for electrical regenerative relay, depending on the device to ensure the transmission of renewable electricity for regeneration after re-transmission, but it is inevitable that the entire system is complicated and expensive.


In the long-distance transmission system, regenerative repeater is synonymous with increasing input costs. The so-called renewable electricity refers to the distance between the two stations can transmit electrical regeneration longest distance. For ordinary WDM systems, generally every after 80km there is an optical amplifier EDFA amplification of the signal light, to maintain a long distance electrical regeneration, must allow as much as possible the number of segments optical transport segment. This greatly reduces the number of photoelectric conversion, thereby reducing system cost.


For WDM transmission system, the current major factors causing the transmission distance is limited: OSNR OSNR, dispersion and nonlinearity. Dispersion problem can be completed through the dispersion compensating fiber. Restricted OSNR OSNR is through the introduction of RAMAN amplifier, super FEC technology and solve.


Ultra-long haul transmission is an important step forward in the light transmission, the inevitable result is RAMAN amplifiers, super FEC, dispersion compensation technology applications such as the development of new, ultra-long distance transmission will greatly reduce transport costs, improve transmission quality and reliability of the system sex. Land area especially for China, a huge country, the technology has broad market prospects and applications.




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