With the development of the computers, mobile phones and some other things, there is an increasing eagerness for more traffic volume of telecommunication. So WDM, with more bandwidth and faster data transmission rate, comes into being.

WDM is a technology to send multiple different wavelength lasers on a single optical fiber. As shown in the picture below, there are different signals coming from different channels. when through the multiplexer, they can be transmitted on a single fiber without obstructed by each other at a high speed. And then, when they are through the demultiplexer, they will be allocated into different channels. The multiplexer and the demultiplexer are the most important parts in WDM systems, just like transmitter and receiver. When signals are transmitted to the network medium on the fiber links, they will be amplified. And after through the network medium, the signals will still be amplified on the fiber links untill they are received by the receiver.

Currently, there are two types of it in the market. CWDM, short for coarse wavelength division multiplexing, is a low-cost WDM transmission technology. Another type is DWDM, namely, dense wavelength division multiplexing. The primary difference between them is the channel spacing. The channel spacing of CWDM is wider than DWDM, so that the number of its channels on the same link could be reduced relatively. As a result, its optical interface components does not need to be precise as same as DWDM.

Now the technology of WDM is widely used in optical networks. Why can it be so widely used? The reasons is closely linked to its features. First, it has Super capacity transmission technology. The transmission capacity can be up to 300-400 Gbit/s or even larger. Second, it can save fiber resources. No matter how many SDH subsystems there are, the whole reuse system only needs a pair of optical fiber. Third, it can work with EDFA that strengthens and restores the attenuated signals in the long-distance transmission, so that it can reduce the cost. Forth, it can improve the reliability of the system. Because most WDM systems are the photoelectric devices which have high reliability. This is a guarantee for the system reliability.

As the optical communication technology progressed further, WDM will be developed in some aspects. In terms of DWDM, Something still needs to be improved such as cost, so that more customers can adopt it.