Wavelength division multiplexing

Throughout the last decade, optical fiber has gained more and more popularity as a transmission medium. The unprecedented capacity of the fiber makes it an ideal medium for transport of digital video or other high-bandwidth signals. An increasing number of TV stations use fiber instead of coaxial cable even for in-house applications. A fiber cable typically consists of a number of individual fibers. Four, 12, 24, 48 or more fibers in a cable are common. Today the majority of installed fiber is so-called single-mode fiber, even for in-house distances. Multimode fibers are mostly used for short-haul data applications.

If you have reached a stage where you have used all your installed fibers, the time has come to consider putting more signals on the same fiber instead of deploying a new fiber cable. There are different ways to transport more data on a single fiber. One can use time division multiplexing (TDM), where many signals of the same type are multiplexed together electrically before they are put on a single wavelength. An alternate solution is to transmit each optical signal on a different wavelength, known as wavelength division multiplexing (WDM).

Figure 1. Comparison of bandwidth needed for coarse WDM and dense WDM. The CWDM channels are spaced 20nm apart due to the drift of the laser wavelengths (max 13nm peak to peak) and will fit a maximum of eight channels in the range from 1470nm up to 1610nm. The DWDM channels are spaced about 0.8nm apart, and due to temperature stabilization can be put close together. Only 1/3 of the DWDM channels possible are shown in the figure.

This is analogous to transmitting different radio channels on different frequencies through air. Recalling the school experiments with white light and prisms is also useful in understanding WDM. The visible white light can be split (demultiplexed) into its components by a prism in the same way as the invisible WDM wavelengths on the fiber can be demultiplexed at the receiving end by an optical filter.

It is quite common to talk about different colors of light instead of wavelengths when describing WDM systems. A number of different wavelengths will, in this case, be denoted as a set of colors. A WDM channel is a signal running on a unique wavelength. Each WDM channel is completely independent of the other channels, both with regards to bit rates, as well as protocols, so running a mixture of SDI, HD-SDI, SDH/SONET, Gigabit Ethernet and Fast Ethernet on the same fiber is easy to do with WDM.

Multichannel WDM exists in two flavors; one is called dense WDM (DWDM) and the other is called coarse WDM (CWDM). When it comes to transporting lots of digital video over a single fiber, DWDM as a technology is very effective. On the other hand, if you have a short fiber span and need a few channels more, CWDM, with its lower cost per channel, can be a good alternative to laying new fiber cable.

Let us take a closer look at the two implementations of the WDM technology. DWDM uses temperature-stabilized lasers in order to fix the center wavelength and narrowband filters, giving many densely spaced channels. Typical channel spacing for broadcast-class DWDM equipment is 100GHz, corresponding to a channel spacing of approximately 0.8nm, thereby avoiding the need for wavelength lockers. The wavelengths used are specified in ITU-T recommendation G.694.1 and the technology is well proven.

Figure 2. Example of single-mode fiber attenuation for the wavelength-range 1280-1620nm. The CWDM channels proposed by the ITU are indicated at the bottom of the figure. As indicated, four channels around the water peak of standard single-mode fiber cannot be used due to the high attenuation of the fiber. Use of all channels needs a special low-water-peak fiber.

CWDM, on the other hand, uses non-stabilized lasers in combination with broadband filters, which gives a coarse spacing of 20nm between channels. CWDM transmitter cards have lower power consumption than DWDM transmitter cards since there is no need for temperature control of the laser diodes. The CWDM wavelengths are standardized in ITU-T Rec. G.694.2. The difference in bandwidth usage between CWDM and DWDM is shown in Figure 1.

Network Electronics’ flashlink system offers efficient solutions for both CWDM and DWDM applications. The flashlink CWDM architecture is based on the well-established 4+1 architecture used in the flashlink DWDM system. Four channels per frame and one upgrade port allow a four-channel system to be upgraded to more channels in the future. The architecture also offers added design flexibility.

The new ITU specification opens for 18 CWDM channels on a special type of fiber, using wavelengths spanning from 1270-1610nm (see Figure 2). The difference in fiber attenuation over the wavelength range, including the water peak at 1383nm of the standard single-mode fiber, limits the practical distance for CWDM systems with more than eight channels.

Figure 3. Example of a combined CWDM and DWDM system, incorporating the 4+1 system architecture. A single CWDM channel is replaced with a set of DWDM channels, thereby increasing the number of channels on the fiber in a cost-efficient way. This can be done with several CWDM channels in the 1530-1610nm range. Here, as well as for all optical systems, the optical transmission budget must be calculated and verified before the installation of equipment.

Most current systems use the wavelengths from 1470nm up to 1610nm. Fiber reducing the water peak attenuation is available for new installations, but more than 99 percent of the already installed fiber is standard single-mode fiber. CWDM is, therefore, best suited for in-house applications and shorter distances with a low channel count. If the future bandwidth need is expected to exceed eight channels per fiber, DWDM would be a better solution. It offers several tens of available channels in the range from 1530-1610nm (see Figure 1). The uniformity of the fiber attenuation over the DWDM wavelengths is better than the CWDM range, as seen in Figure 2, so for medium- and long-haul applications DWDM would be the best solution, even for low channel counts.

The flashlink system has solutions for both CWDM and DWDM, and offers the flexibility to upgrade a CWDM system with a number of DWDM channels, as shown in Figure 3, for the best of both worlds. Multichannel WDM is a technology that will be part of optical networks for many years to come.

Ronny Sletteng is senior specialist of optical systems for Network Electronics.

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