What Is Wavelength Division Multiplexing (WDM)?

Around 1980s, Internet traffic was increasing prominent as the unceasing popularity of the Internet equipment. A call for a higher demand of communication capacity was higher and higher than ever before. Data, video and voice signals crowd transmission systems that had ample space just a few years ago. The communications industry needs room to breathe. This is just an exactly time to upgrade the capacity and enhance the speed in fiber-optic technology. By sending signals at 16 different wavelengths through each of four pairs of optical fibers, Project Oxygen ( a project of a well-known company of fiber-optic technology) researched and developed a technology which can carry 640 gigabits per second (Gbit/s) across whole oceans. The technology that makes this new bandwidth possible is called wavelength division multiplexing, or WDM. It represents the second major fiber-optic revolution in telecommunications. The definition and two common types of WDM (CWDM and DWDM) will be provided in the following text.

Introduction to Wavelength Division Multiplexing (WDM)

Wavelength Division Multiplexing (WDM）is a technology that multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths of laser light. This technique enables two-way communication become possible and enhances the capacity for the optical fiber. The technology of WDM opens vast realms of capacity in the fibers that carry phone and Internet traffic all over the world. WDM system is popular with telecommunications companies because it allows them to expand the capacity of the network without laying more fiber. By using WDM and optical amplifiers, they can accommodate several generations of technology development in their optical infrastructure without having to overhaul the backbone network. Capacity of a given link can be expanded simply by upgrading the multiplexers and demultiplexers at each end.

Types of WDM

According to different wavelength, transmission distance, channel spacing and frequent stability, Wavelength Division Multiplexing (WDM) can generally be divided into two types, Coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM).

Coarse Wavelength Division Multiplexing (CWDM)

Extended from wavelength division multiplexing(WDM), Coarse Wavelength Division Multiplexing (CWDM) is used to increase the bandwidth of an optical fiber by multiplexing several wavelength onto it. Using 18 channels with wavelengths between 1270 nm and 1610 nm, it improves the transmission speed a lot. The channel width itself is 13 nm. The remaining 7 nm is designed to secure the space to the next channel. This very wide channel spacing of CWDM allows for the use of cheaper CWDM network components, such as uncooled lasers or lower-quality multiplexers and demultiplexers. However, this broad spectral range prevents using erbium-doped fiber amplifiers, that work best around 1550 nm, and accordingly limits the maximum range of CWDM deployments to about 60 or 80 km. The maximum transmission distance is 120 km. Therefore, CWDM equipment is designed for short-range communications and is mainly used for city and access networks.

Dense Wavelength Division Multiplexing (DWDM)

Dense Wavelength Division Multiplexing (DWDM) is designed for long-haul transmission where wavelengths are packed tightly together in order to increase bandwidth over existing fiber networks. And DWDM uses much more than 18 channels with wavelength for a much wider channel spacing compared to CWDM. DWDM works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber. However, densely packed channels are still limited. First, high-precision filters are required to peel away one specific wavelength without interfering with neighboring wavelengths. Those don’t come cheap. Second, precision lasers must keep channels exactly on target. That nearly means such lasers must operate at a constant temperature. High-precision, high stability lasers are expensive, as are related cooling systems. The technology creates multiple virtual fibers, thus multiplying the capacity of the physical medium. DWDM provides ultimate scalability and reach for fiber networks. Boosted by Erbium Doped-Fiber Amplifiers (EDFAs) — a sort of performance enhancer for high-speed communications, DWDM systems can work over thousands of kilometers.Without the capacity and reach of DWDM systems, most Web 2.0 and cloud-computing solutions today would not be feasible. Establishing transport connections as short as tens of kilometers to enable nationwide and transoceanic transport networks, DWDM is working hard for keeping the data highway alive and expanding. And thus, it is more expensive than CWDM.

Conclusion

Wavelength Division Multiplexing (WDM) is a technology that multiplexes a number of optical carrier signals onto a single optical fiber. There are two common types of WDM, Coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM). Both of them are of great capacity and high speed. For short distance transmission and money-saving, CWDM is suggested. For long distance and higher speed, DWDM is available with huge capacity and good performance. You can chose a right one according to your requirement.