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1. Optical wavelength division multiplexing (WDM) technology
WDM (Wavelength Division Multiplexing, WDM) technology is simultaneously a plurality of wavelengths simultaneously optical carrier signal in an optical fiber, and each optical carrier by FDM or TDM mode, each carrying multiple analog or digital signals . The basic principle is to combine the transmission side of the optical signals of different wavelengths (multiplexing), and coupled to the same fiber optic cable for transmission on the line, turn on the receiving end of these combine separate signals at different wavelengths ( demultiplexing), and further processed to recover the original signal into a different terminal. Therefore, this technology called optical wavelength division multiplexing, optical wavelength division multiplexing technology called.
WDM technology for expansion of the network upgrade, development of broadband services, mining fiber bandwidth capacity, ultra high-speed communication etc. of great significance, especially coupled with erbium-doped fiber amplifier (EDFA) in WDM modern information networks more powerful attractive.
2. The basic configuration of the WDM system
WDM system, the basic structure is divided into two-way transmission and single-fiber bidirectional transmission in two ways. Refers to all WDM unidirectional optical path simultaneously transmitted along the fiber in a same direction, at the transmit end carrying the modulated optical signals having different wavelengths are combined by various information extended light demultiplexer, and a fiber one-way transmission, since each signal is carried by light of different wavelengths, it is not confused with each other, the receiving end through an optical multiplexer to optical signals of different wavelengths separated, complete transmission multiplexed optical signal, The opposite direction is transmitted through another fiber. Bidirectional WDM optical path refers to two different directions simultaneously transmitted in a fiber to be on the wavelength used apart from each other, the two sides to each other in order to achieve full-duplex communications. Unidirectional WDM systems currently in development and applications are more widespread, and the impact due to the bidirectional WDM in the design and application by each channel interference, light reflection effects of bidirectional path between isolation and crosstalk and other factors, the actual application of more less.
3. composed of dual-fiber unidirectional WDM system
Double-fiber unidirectional WDM system, for example, in general, WDM system mainly consists of the following five components: optical transmitter, optical relay amplifiers, optical receivers, optical supervisory channel and NMS.
1) Optical Transmitter
WDM optical transmitter is the core of the system, in addition to the central wavelength WDM system emitting lasers have special requirements, but also depending on the application of WDM systems (mainly transmission type and a transmission distance of the optical fiber) to select a certain chromaticity dispersion capacity transmitter. Signal at a specific wavelength optical signal using optical repeater from the first transmitting side terminal device converts the optical signal output from the non-specific wavelength to have a stable, re-use of the multiplexer into a plurality of optical signal path, through the optical amplifier (BA ) amplified output.
2) Optical Repeater
After a long distance (80 ~ 120km) optical transmission, optical repeaters need amplifying optical signals, the majority of optical amplifiers currently in use for the erbium-doped fiber optical amplifier (EDFA). In a WDM system must gain flattening technique, so EDFA for different wavelengths of light signals having the same amplification gain, and to ensure that the optical channel gain competition does not affect the transmission performance.
3) Optical Receiver
At the receiving end, the optical preamplifier (PA) which amplifies the transmission signal attenuation of the primary channel, using the branching filter specific wavelengths of light separated from the main signal channel optical signal, the receiver must not only meet the optical signal sensitivity, overload require power and other parameters, but also can withstand a certain optical noise signal, to have enough power bandwidth performance.
4) The optical supervisory channel
Optical supervisory channel main function is to transfer the case within the monitoring system for each channel. Node is inserted at the transmitting end of the light generated by the wavelength monitoring signal, the optical signal combiner output λs (1550nm) of the primary channel. At the receiving end, the received optical signal branching filter, respectively, output λs (1550nm) wavelength optical supervisory channel optical signal and a traffic signal. The frame synchronization bytes, overhead bytes and byte public network through the use of an optical supervisory channel to pass.
5) Network Management System
NMS through the optical supervisory channel overhead bytes transferred to other nodes or received from other nodes overhead bytes for WDM systems management, configuration management, fault management, performance management, security management, and other functions.
4. Optical wavelength division multiplexer and demultiplexer
Throughout the WDM system, the optical wavelength division multiplexer and demultiplexer WDM technology is a key component of its performance. The pros and cons of the transmission quality of the system has a decisive role. Different light wavelengths combine a signal transmission through the fiber output device called a multiplexer; on the contrary, the same multi-wavelength fiber-optic signal transmission sent decomposed into individual wavelengths output device called a demultiplexer. In principle, the device is reciprocal (two-way reversible), that as long as the demultiplexer output and inputs in turn use that multiplexer. WDM performance indicators are mainly the insertion loss and crosstalk requirements loss and frequency offset is smaller, the insertion loss is less than 1.0 ~ 2.5db, little crosstalk between channels, the isolation degree, among the different wavelength signals little effect. In the current practical application of WDM systems, there are grating optical WDM and optical dielectric membrane filter WDM.
1) Grating optical WDM
Blazed grating is on a plane can be transmitted or reflected scribe marks equal and equidistant grooves, which groove-like shape having a small ladder. When the multi-wavelength optical signal comprising the generation of a diffraction grating through the optical signals of different wavelength components will be emitted at different angles. When a fiber optical signals via the lens to a parallel beam to the blazed grating, due to the diffraction grating, a variety of different wavelengths of the optical signal parallel to the direction of the lens to return slightly different light transmission, and then focused by a lens, to a certain Law were injected into the output fiber, so that the different wavelengths of light signals at different optical fiber transmission, to achieve the object of the demultiplexed. According to the reciprocity principle, the optical wavelength division multiplexing input and output can be interchanged to achieve the purpose of reuse.
2) Dielectric film optical WDM filter
WDM systems currently work within the 1550nm wavelength zone, with 8, 16 or more wavelengths, on a pair of fibers (single fiber may also be used) constituting the optical communication system. Between each wavelength of 1.6nm, 0.8nm or narrower intervals, corresponding 200GHz, 100GHz or more narrow bandwidth.
5. The main features of WDM technology
1) Take advantage of the huge bandwidth of the fiber, the transmission capacity of a single fiber is increased several times to several times more than the single-wavelength transmission, thereby increasing the transmission capacity of the fiber, reduce costs, has great application value and economic value.
2) Because each wavelength WDM technology used independently, which can be completely different signal transmission characteristics, complete integration and separation of various signals, multimedia signal hybrid transmission.
3) As many have adopted full-duplex communication style way, so using WDM technology can save a lot of line investment.
4) Needed, WDM technology can have many applications forms, such as long-distance trunk network, broadcast distribution networks, multiple local area networks and more, so the network application is very important.
5) With the transfer rate continues to improve, many of the response speed optoelectronic devices is obviously insufficient, using WDM technology can reduce some of the high demands on the performance of the device, but also can realize large-capacity transmission.
6) The use of WDM technology routing, network switching and recovery.
