The development of DWDM
In recent years, the fiber optic communication systems in the fast development of constantly upgrading. Since 1995, the development of WDM technology is extremely rapid, and into the fast lane, optical amplifiers (OA), the emergence of an important milestone and developed into the history of optical communication, especially based on erbium-doped fiber amplifier EDFA 1550nm window dense wavelength divison multiplexing (DWDM) system. At present, DWDM technology laboratory and commercial level have been to Tb/s average rate, the world’s leading equipment manufacturers and operating companies showed great interest in its commercialization.
WDM and DWDM applications is the same kind of technology, DWDM technology in WDM technology evolution, DWDM technology is a concrete manifestation of the WDM, WDM more universal, they are called different periods of the WDM system with WDM technology development are closely linked. In the early 1980s, people thought of and first used in the two low-loss winsow of the fiber at 1310nm and 1550nm window transmits optical wavelength signal, but with the 1550nm EDFA window of commercial, people only in 1550nm window to send multi-channel optical carrier signal. In order to distinguish it from the traditional WDM system, we call this wavelength closer spaced WDM system for dense wavelength division DWDM systems. The so called DWDM technology is the order to take full advantage the singlemode fiber low loss area of the enormous bandwidth resources according to each lightwave frequency (or wavelength) different from the low-loss window of the optical fibers is divided into several channels, the light-wave as a signal of the carrier, WDM (combiner) to a different predetermined wavelength of signal carrier combined fed into an optical fiber for transmission at the transmitting end. At the receiving end, and then decomposed by a wave of a multiplexer (demultiplexer) these different wavelengths to carry different signals separated from the optical carrier multiplexing method. Carrier signals of different wavelengths of light can be regarded as mutually independent, and thus can realize the multiplex transmission of the multiplexed optical signal in an optical fiber. Different types of light of the wavelength division multiplexer, the number of wavelengths can be multiplexed is also different, bidirectional problem easy to solve.
Although the the WDM working principle is the optical domain on the frequency division multiplexing (FDM) technology, the actual use of the WDM system is a combination of frequency division multiplexing on the optical domain, with the electric field on the time division multiplexing technology. In the optical domain, each wavelength path through the split in the frequency domain implementation, the bandwidth of each wavelength occupancy period of the optical fiber. DWDM systems is the optical signal on the frequency domain division. DWDM systems on the digital signal of each wavelength channel: SDH2.5GB / s and 10Gb / S, or Gigabit Ethernet and other digital systems, in general, not specific 1310nm/1550nm the case of a two-wavelength WDM system, the usual the WDM system means we are talking about are DWDM systems.
The advantages of DWDM
A.Large capacity transmission
The transmission capacity of DWDM systems is enormous. DWDM multiplexed optical path rate 2.5Gb / s, 10Gb / s, etc., undone the number of optical paths can be 8, 16, 32, or even more, so the transmission capacity of the system can reach 300 to 400Gb / s. Such a huge transmission capacity TDM way can not be done.
B. Saving fiber resources
For a single wavelength system, an SDH system needs a pair of optical fibers. For DWDM systems, regardless of the number of SDH systems, the entire multiplexing system only needs a pair of fibers enough.
C. Access transparent transmission smooth upgrade and expansion
As long as the increase complex optical number of lanes and equipment, you can increase the transmission capacity of the system to achieve the expansion, and the expansion will not have a negative impact on other multiplexed optical path. The upgrading and expansion of the DWDM system is smooth, but also convenient and easy, so as to maximize the protection of the construction of the intial investment. Since each multiplexing passage of the DWDM system are independent of each other, so each of the optical patch can transparently send different service signal, such as voice, data and image, etc., has brought great convenience to the user at the same time.
D. Make full use of the mature TDM technology
To improve the transmission rate of TDM way to reduce costs has great appeal, but faces many limiting factors such as manufacturing processes, electronic devices work rate restrictions. DWDM technology can take full advantage of the now mature TDM technology, even several times to increase the transmission capacity of the system several times quite easily, so as to avoid the development of more high-speed TDM technology (10Gb / s or more) are facing difficulties.
C. EDFA to achieve ultra-long haul transmission
The erbium-doped fiber amplifier (EDFA) has a high gain, wide bandwidth, low noise, etc., its light magnification range of 1530-1565nm to 1550nm wavelength range that can cover almost the entire DWDM systems. Path signal at the same time with a very wide bandwidth of EDFA can DWDM systems each multiplexed optical zoom to achieve a system of ultra long distance transmission, and also to avoid the situation of each of the optical transmission system requires a light amplification, reduce costs .
D. Fiber dispersion without excessive requirements
Terms of DWDM systems, no matter how high transfer rate of the system, the transmission capacity of how much fiber dispersion coefficient is basically a single multiplex channel rate signal on the fiber dispersion coefficient. DWDM system transmission capacity of existing laboratories, commercial level reached Tb / s, presents the following technical trends:
a. Raising the single channel bit rate and the increase in both the number of channels, in order to achieve a rate of increase of the DWDM system;
b. one of the options to achieve Tb/s system is OTDM and DWDM combined;
c. bidirectional DWDM system has appeared, and will get a certain development;
d. DWDM technology is to the metro and access network development;
e. The use of DWDM technology to build greater capacity optical transport network;
f. the band further expansion (now the C-band development of the L and S band), the channel spacing is further reduced, to further increase the transmission capacity.
Especially with the advent of no fiber optical communication, no fiber WDM systems are not restrictions and the impact of the terrain, bad weather disturbance and ability, confidentiality, and the use of open interfaces, can carry a variety of businesses.
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