Wavelength division multiplexing refers to a technology in which signals of different wavelengths are transmitted together and separated again. At most, it is used in optical fiber communication to transmit data in multiple channels with slightly different wavelengths. Using this method can greatly improve the transmission capacity of the optical fiber link, and the use efficiency can be improved by combining active devices such as optical fiber amplifiers. In addition to applications in telecommunications, wavelength division multiplexing can also be applied to the case where a single fiber controls multiple fiber optic sensors.
WDM in telecommunication systems Theoretically, the extremely high data transmission rate in a single channel can reach the limit of the data transmission capacity that a single fiber can bear, which means that the corresponding channel bandwidth is very large. However, due to the very large bandwidth of the low-loss transmission window of silica single-mode fiber (tens of THz), the data rate at this time is far greater than the data rate that the photoelectric transmitter and receiver can accept. In addition, various dispersions in the transmission fiber have very adverse effects on the wide-bandwidth channel, which will greatly limit the transmission distance. Wavelength division multiplexing technology can solve this problem, while keeping the transmission rate of each signal at a suitable level (10 Gbit/s), a very high data transmission rate can be achieved through the combination of multiple signals. According to the standards of the International Telecommunication Union (ITU), WDM can be divided into two types: In Coarse Wavelength Division Multiplexing (CWDM, ITU standard G.694.2 [7]), the number of channels is small, such as four or eight, and the channel spacing of 20 nm is relatively large. The nominal wavelength range is from 1310nm to 1610nm. The wavelength tolerance of the transmitter is relatively large, ±3 nm, so that distributed feedback lasers without stabilization measures can be used. Transmission rates for a single channel typically range from 1 to 3.125 Gbit/s. The resulting overall data rate is therefore useful in metropolitan areas where fiber-to-the-home is not implemented. Dense Wavelength Division Multiplexing (DWDM, ITU Standard G.694.1 [6]) is a case of extending to very large data capacity and is also commonly used in Internet backbone networks. It contains a large number of channels (40, 80, 160), so the corresponding channel spacing is very small, respectively 12.5, 50, 100 GHz. The frequencies of all channels are referenced to a specific 193.10 THz (1552.5 nm). The transmitter needs to meet very narrow wavelength tolerance requirements. Usually the transmitter is a temperature-stabilized distributed feedback laser. The transmission rate of a single channel is between 1 and 10 Gbit/s, and it is expected to reach 40 Gbit/s in the future. Due to the large amplification bandwidth of erbium-doped fiber amplifiers, all channels can be amplified in the same device (except when applying the full-scale CWDM wavelength range). Problems arise, however, when gain is wavelength-dependent or when there is fiber nonlinear data-channel interaction (crosstalk, channel interference). Combining different techniques, such as the development of broadband (dual-band) fiber amplifiers, gain flattening filters, nonlinear data feedback, etc., this problem has been greatly improved. System parameters such as channel bandwidth, channel spacing, transmission power, fiber and amplifier types, modulation formats, and dispersion compensation mechanisms need to be considered to achieve the best overall performance level. Although the current fiber optic link contains only a small number of channels in a single fiber, it is also necessary to replace the transmitter and receiver that can satisfy the simultaneous operation of multiple channels, which is cheaper than replacing the entire system to obtain higher data capacity a lot of. Although this solution greatly improves the data transmission capacity, it does not need to add additional optical fibers. In addition to increasing transmission capacity, wavelength division multiplexing also makes complex communication systems more flexible. Different data channels can exist in different locations in the system, and other channels can be flexibly extracted. In this case, an add-drop multiplexer is required, and this period can be inserted into the channel or extracted from the channel according to the wavelength of the data channel. Add-drop multiplexers can flexibly reconfigure the system to provide data connections for a large number of users in different locations. In many cases, wavelength division multiplexing can be replaced by time division multiplexing (TDM). Time-division multiplexing is where different channels are distinguished by time of arrival rather than by wavelength.
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