Composition Of WDM System
Wavelength Division Multiplexing(WDM) is a technology that transmission multi wavelength same time by one fiber .The basic principle is combined different color wavelengths of signals together (multiplexed) in transmit port，and then coupled it in one fiber transmission，At the received port , Separate the wavelengths（Demultiplex），And after further processing，Restore the original signal to send to different terminal，So this technology is called optical wavelength division multiplexing.
I. Composition of WDM system
1) Multiplex (MUX):Combined multi wavelength in one fiber
2) Optical power amplifier(BA):compensate the loss of the optical multiplexer, increase input power 。
3) Optical Line Amplifier(LA):compensate the loss of the fiber.
4) Optical Preamplifier(PA):improve the receiver sensitivity.
5) Optical dispersion compensator（DCM）：Provide dispersion compensation in long-distance transmission.
6) Demultiplex (DEMUX):A plurality of wavelengths are divided into each optical fiber to separate the channel
7) Optical interface converter (OTU)：converts conventional SDH singnals into suitable for DWDM transmission signals .
8) Monitoring channel（OSC）：Specialized monitoring system transmission channel.
II. Two techniques of wavelength division multiplexing
1) DWDM Technical brief
WDM and DWDM is in different development period of the WDM system call.In the early 1980s，People think use the two low-loss wavelength in the fiber 1310nm and 1550nm transmit each wavelength，that is 1310nm、1550nm two wavelength WDM system.with 1550nm is wildly used for business in EDFA .WDM system interval adjacent wavelengths becomes very narrow (typically less than 1.6nm)，And work in a window, sharing EDFA optical amplifier.In order to distinguish it from the traditional WDM system，People call this more closely spaced wavelengths of WDM system DWDM system. The so-called intensive, refers to the adjacent wavelength interval is concerned,WDM system in the past is tens of nanometers of the wavelength interval, the current wavelength interval of only 0.4 ~ 2nm.Dense wavelength division multiplexing technology is a specific manifestation of wavelength division multiplexing.If not specifically refers to 1310nm, 1550nm two-wave WDM system, people talk about the WDM system is DWDM system.
There are many types of equipment to realize optical wavelength division multiplexing and transmission,Each function module has a variety of ways to achieve,The specific use of what equipment should be based on site conditions and system performance to determine the focus. In general, in the DWDM system which has optical transmitter / receiver, wavelength division multiplexer, optical amplifier, optical dispersion compensator, optical monitoring channel and fiber six modules.
The nonlinear effect of fiber is the main factor that affects the performance of WDM transmission system. The nonlinear effect of the fiber is mainly related to the optical power density, channel spacing and fiber dispersion and other factors are closely related;More bigger optical power density, the smaller the channel spacing, the more severe the nonlinear optical effect；The relationship between dispersion and various non-linear effects is more complex, where the four-wave mixing is significantly increased with the dispersion close to zero. With the continuous development of WDM technology, the number of channels transmitted in fiber is more, the channel spacing is getting smaller, the transmission power is getting bigger , so the nonlinear effect of fiber on DWDM transmission system performance is more big.
The main way to overcome the nonlinear effect is to improve the performance of the fiber，Such as increasing the effective transmission area of the fiber to reduce the optical power density; In the working band to retain a certain amount of dispersion, in order to reduce the four-wave mixing effect;Reduce the dispersion slope of the fiber to expand the working wavelength range of the DWDM system, increase the wavelength interval;At the same time, we should try to reduce the polarization-mode dispersion of the fiber, and reduce the dispersion of the four-wave mixing effect on the basis of the fiber to reduce the dispersion of the work to adapt to the continuous improvement of single channel rate.
The optic source of DWDM system should required as following :
① wide wavelength range;
② As many channels as possible;
③ The spectral width of each channel wavelength should be as narrow as possible；
④ each channel wavelength and its interval should be highly stable.
Therefore, the laser light source used in the wavelength division multiplexing system is almost always distributed feedback laser (DFB-LD), and now mostly quantum well DFB laser.
With the development of science and technology progress，The light source used in the wavelength division multiplexing system is in addition to the discrete DFB-LD,Tunable lasers, surface emitting lasers，There are another two forms。One is the array of laser diodes, or the integration of the array of lasers with electronic devices，Actually optoelectronic integrated circuit (OEIC)，Compared with the discrete DFB-LD, this laser is a big step forward in technology，It is smaller, lower power consumption, high reliability, simple and convenient application.Another new light source - super continuous light source. Specifically, it should be a limiting spectrum of supercontinuum(Spectrum Sliced SupercontinuumSource)。Studies have shown that when there is a very high peak power of the short pulse into the fiber, when the nonlinear propagation will generate supercontinuum (SC) wide spectrum in the optical fiber,it will be many Spectrum Sliced Supercontinuum Source，And is suitable for wavelength division multiplexing light source, which is the so-called Spectrum ultra-continuous Sliced Supercontinuum Source
2) CWDM Technology Introduction
With the rapid development of data bussiness, will be higher and higher for bandwidth requirement of transmission network. All over the world’s pattern has formed basically for backbone optical communication network at present, market subject of optical communication transfer to metropolitan area network gradually. Operator shift their focus to the construction of metropolitan area network,need equipment manufacturers to provide a low cost solution. In view the characteristics of metropolitan area network has short distance of transmission, the variety of business and so on which are different from those of backbone network, people put forward the technology of CWDM, Coarse Wavelength Division Multiplexing.
CWDM is a low cost WDM transmission technology that face to metropolitan area network access layer, it has continue the technology advantage of high bandwidth of dense wavelength division multiplexing, At the same time has the advantage that multi business interface, low cost, low power consumption and small volume, so it has very broad application prospects in metropolitan optical networks.
Compared with the DWDM system, an important difference between them is that the channel spacing is different in the spectrum. Due to the use of the CWDM channel chase band (20 + 6-7nm),so it can use cheaper components, such as uncooled lasers and thin film filters. So, in the same application, CWDM has a greater cost advantage than DWDM.
The overall structure shown in Figure 1 of N-channel wavelength reuse CWDM system:
① light wavelength conversion unit (OTU);
② wavelength division multiplexer: Demultiplexer / multiplexer (ODU / OMU).
CWDM advantages have the following:
① large capacity
② Transparent transmission of data
③ long distance transmission
④ compatible with existing fiber
⑤ flexible networking
⑥ economy and reliability
⑦ smooth expansion capacity
As show in picture 2，though ITU G.695 customized CWDM O、E、S、C、L five wavelength total 18 wavelength，However, due to the current metropolitan area laying too much for G652, G655 fiber in the E-band there is a "water peak"，Resulting in light transmission in this band when the window is too much attenuation, the business can not normally operate。The new "full-wave" fiber eliminates the "water peak" near the 1400-nm E-band,，The attenuation of 18 wavelengths is relatively smooth, allowing CWDM to transmission more business。So now CWDM 1470nm ~ 1610nm a total of eight wavelength is wildly used in transmission business.
CWDM standard wavelength:
With the escalating bandwidth and network development, appear more multi-service transmission in one lines and multi-rate coexistence, complex network topology requires the more fast delivery services , And take into account the multi-service access and convergence capabilities, high integration, low cost, green and other requirements.
Figure 3 show Sharetop 4U integrated platform front view, is a new generation of large-capacity, multi-service access to the optical network platform,The device uses advanced transmission technology and a high degree of integration technology, for all-optical network, used in national, provincial, provincial trunk, local metropolitan area network and a variety of private network construction.
Sharetop 4U integrated platform Features:
① excellent performance, long distance transmission, large capacity architecture, highly integrated design, smooth upgrade expansion, comprehensive and perfect protection, rich business access, intelligent control plane, economic energy.
② equipment capacity is large, the current use of single-mode fiber transmission, CWDM per channel maximum transfer rate of up to 10G, the total capacity of up to 320G.
③ equipment networking flexibility, wavelength division multiplexing equipment can be composed of point to point, point to multipoint, chain, ring, single-fiber one-way, single-fiber two-way network topology.
④ Support multi-rate services Flexible and transparent access, provide business interface, support Ethernet, PDH, SDH, CATV and private network and other services, while providing a variety of rate options: 155M, 1.25G, 2.5G, 10G various rate support Backward compatible.
With the new sub-frame design, slot design is compact, single sub-frame support 16 general business slot, the functional unit board can be flexible access, not slot restrictions. A wide range of functional cell boards, single board integration more interfaces, the same system capacity, the device compactness, take up less space, improve system integration, and effectively save room space. With green, low power, low radiation, non-toxic, renewable materials.
As shown in Figure 4, the 4U integrated platform of sharetop technology uses 1 + 1 redundant backup power supply design, through the way of LOAD-SHARING for the platform to provide maximum reliability guarantee. Support a variety of power supply disk selection, adapt complex room environment. Platform built-in four high-performance cooling fan, with the platform as a whole ventilation design, to achieve perfect control of temperature, to ensure reliability of the whole operation.
The main board introduced as shown in Figure 5-8 of sharetop’s 4U integrated platform.
Sharetop CWDM Metro pointsto multi point ethernet solution show in figure 9：
Point-to-multipoint applications can be implemented using CWDM's terminal multiplexing device and add-drop multiplexing devices. It is mainly used to solve the service transmission between the core nodes and multiple branch nodes. The respective services do not interfere with each other.
Use CWDM rental wavelength to solve the big customer application program，As shown in Figure 10. In the figure, each service node is distributed on the extension line of the optical fiber line, and the fiber resources of each node to the central node are saved by CWDM, and different wavelengths and exclusive bandwidths are used among the nodes, and each wavelength can carry the service Including 100M, GE, 10GE IP business, 155M / 622M / 2.5G POS business and other 1.25G following rate of business. Wavelength leasing use of this can be widely used, the advantage is: to save fiber resources, you can quickly carry out new business; different wavelengths of information isolation, good security performance; business upgrades easy, single wavelength can carry a rich business type, You can upgrade your bandwidth from 100M to 1000M or 2.5G without any changes to your device.