Overview of WDM Technical Principles
WDM technology is a relatively advanced optical fiber communication technology called optical wavelength division multiplexing technology. That is, light of different speeds is mixed together for transmission in a single optical fiber. The digital signals carried by these optical signals of different wavelengths can be of the same speed, the same format, or different data formats of different speeds. At the receiving end, these combined signals of different wavelengths are separated, and further processed, and the original signals are recovered and sent to different terminals. Therefore, this technology is called light wave division multiplexing, or light wave division multiplexing technology for short.
Here, an optical fiber can be regarded as a “multi-lane” public road. The traditional TDM system only uses one lane of this road. Increasing the bit rate is equivalent to accelerating the driving speed on this lane to increase the unit time. Of transportation. The use of DWDM technology is similar to the use of unused lanes on public roads to obtain huge undeveloped transmission capabilities in optical fibers. WDM technology is of great significance for network expansion and upgrading, development of broadband services, tapping of optical fiber bandwidth capabilities, and realization of ultra-high-speed communications.
Basic structure of WDM system
The basic structure of the WDM system is mainly divided into two modes: dual fiber unidirectional transmission and single fiber bidirectional transmission. Unidirectional WDM means that all optical paths are transmitted in the same direction on an optical fiber at the same time. At the transmitting end, the dimmed signals with different wavelengths carrying various information are combined together by an optical extension device, and are combined in one optical fiber. In optical fiber unidirectional transmission, because each signal is carried by light of different wavelengths, there is no confusion between each other. The optical multiplexer at the receiving end separates the optical signals of different wavelengths to complete the transmission of multiple optical signals. The opposite direction is transmitted through another optical fiber. Two-way WDM means that the optical path transmits in two different directions simultaneously on a fiber, and the wavelengths used are separated from each other to achieve full-duplex communication between the two parties.
The WDM system generally consists of four parts: optical transmitter, optical relay amplifier, optical receiver, and optical monitoring channel.
- Optical transmitter: As the core equipment of the WDM system, at the transmitting end, the optical signal output from the terminal equipment is first converted into a signal with a stable specific wavelength by an optical transponder, and then multiplexed The multi-channel optical signal is synthesized by the device, and the output is amplified by the optical power amplifier.
- Optical relay amplifier: After long-distance (80~120km) optical fiber transmission, the optical signal needs to be optically relayed and amplified. In the WDM system, gain flatness technology must be used to make the EDFA have the same amplification gain for optical signals of different wavelengths, and to ensure that the gain competition of the optical channel does not affect the transmission performance.
- Optical receiver: At the receiving end, the optical preamplifier amplifies the transmitted and attenuated main channel signal, and uses a splitter to separate the optical channel of a specific wavelength from the main channel optical signal. The receiver must not only meet the requirements for the optical signal Sensitivity, overload power and other parameters must be able to withstand certain optical noise signals.
- Optical monitoring channel: Optical monitoring channel is set up for the monitoring of WDM optical transmission system. ITU-T recommends using 1510nm wavelength and a capacity of 2Mbit/s. Relying on high receiving sensitivity (better than -48dBm) at low rates, it can still work normally. But it must go down the light path before EDFA, and go up the light path after EDFA.
In the entire WDM system, the optical wavelength division multiplexer and demultiplexer are the key components in the WDM technology, and their performance is decisive for the transmission quality of the system. A device that combines signals of different light source wavelengths and outputs them through a transmission fiber is called a multiplexer; conversely, a device that separates the multi-wavelength signals sent from the same transmission fiber into individual wavelengths and outputs them separately is called a demultiplexer. In principle, the device is bidirectional and reversible, that is, as long as the output and input of the demultiplexer are used in reverse, it is a multiplexer. The performance indicators of the optical wavelength division multiplexer mainly include access loss and crosstalk. The loss and frequency deviation are required to be small, and the access loss should be less than 1.0~2.5db. The crosstalk between channels is small, the isolation is large, and the influence between signals of different wavelengths is small. To
Advantages of WDM system:
- Ultra-large capacity and ultra-long distance transmission:
The transmission bandwidth of the ordinary optical fiber used at present is very wide, but its utilization rate is still very low. The use of DWDM technology can increase the transmission capacity of an optical fiber several times, tens of times or even hundreds of times over the single wavelength transmission capacity. Now the highest capacity optical fiber transmission system is 3.2Tbit/s.
- Transparent transmission of data:
Because the DWDM system multiplexes and demultiplexes according to the difference of the optical wavelength, it has nothing to do with the signal rate and electrical modulation mode, that is, it is “transparent” to the data. What the WDM system accomplishes is transparent transmission. For “business” layer signals, each optical wavelength channel in the WDM system is like a “virtual” optical fiber.
- High flexibility, economy and reliability of networking:
The new type of communication network formed by WDM technology is simpler than the network structure formed by the traditional electrical time division multiplexing technology, and the network has a clear hierarchy. The scheduling of various services can be realized only by adjusting the wavelength of the corresponding optical signal. The flexibility, economy and reliability of the network brought about by this are obvious.
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