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Lead to higher yields and lower costs
May 08, 2018

CWDM is a low-cost WDM transmission technology for the access layer of metropolitan area networks. In principle, CWDM uses optical multiplexers to multiplex different wavelength optical signals to a single optical fiber for transmission. At the receiving end of the link, the optical demultiplexer is used to decompose the mixed signals in the optical fiber into different wavelengths. The signal is connected to the corresponding receiving device. The principle is shown in Figure 1. The main difference from DWDM is that CWDM has a wider wavelength spacing than the wavelength spacing of 0.2nm to 1.2nm in DWDM systems. The standard wavelength interval used in the industry is 20nm. The wavelength specified in ITU-T G.694.2 is shown in Table 1. Due to the wide wavelength spacing of the CWDM system, the technical requirements of the laser are lower. Because the wavelength interval reaches 20nm, the maximum wavelength shift of the system can reach -6.5°C to +6.5°C. The laser's emission wavelength accuracy can be relaxed to ±3nm, and within the operating temperature range (-5°C to 70°C), the temperature The wavelength drift caused by the change is still within the allowable range. The laser does not require a temperature control mechanism, so the structure of the laser is greatly simplified and the yield is improved.

In addition, a larger wavelength interval means that the structure of the optical multiplexer/demultiplexer is greatly simplified. For example, the number of filter layers of a CWDM system can be reduced to about 50 layers, while the number of layers of a 100 GHz filter in a DWDM system is about 150 layers. This leads to higher yields, lower costs, and an increase in the number of filter suppliers. It is conducive to competition. The cost of CWDM filters is more than 50% lower than that of DWDM filters, and will be further reduced with the increase of automated production technology and batch size.

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