Wavelength Division Multiplexing (WDM) Expands Fiber Optic Bandwidth

Students in my fiber optic classes have been inquiring about new optical networking companies dominating the business news recently. Much of their interest is based on disbelief that a major telecommunications player would pay $1 billion for a brand new company that may not even be manufacturing a product yet! The communications industry has already been transformed by glass. The telephone companies and community antennae (or cable) television (CATV) industry have converted over to fiber optic backbones. The Internet is an all-fiber network. But telecommunications traffic has grown so fast that, in many locations, it has used up all the bandwidth of the installed fiber optic cables! The capacity of all the installed fiber optic cables can be increased considerably if they are converted to wavelength-division multiplexing (WDM). Dozens of start-up companies are developing products for just this application—probably more companies than real customers today—but the big guys in telecommunications are buying them up as fast as possible, just to hedge their bets on future technology. This article is not meant to educate you on a technology you are likely to install; rather, it is to explain why this technology is so “hot.” Why is WDM used? With the exponential growth in communications, caused mainly by the wide acceptance of the Internet, many carriers are finding that they have significantly underestimated their fiber needs. Although most cables included many spare fibers when installed, this growth has used many of them and new capacity is needed. Three methods exist for expanding capacity: installing more cables, increasing system bit rate to multiplex more signals, and WDM. Installing more cables is often preferable, especially in metropolitan areas, since fiber has become incredibly inexpensive and installation methods more efficient (like mass fusion splicing). But, if conduit space is not available or major construction is necessary, this may not be the most cost-effective option. Increasing system bit rate may not prove cost effective either. Many systems are already running at SONET OC-48 rates (Synchronous Optical Network systems at 2.5 Gbps) and upgrading to OC-192 (10 Gbps) is expensive and requires changing out all electronics in a network. It also adds four times the capacity; more than may be necessary. The third alternative, WDM, has often been proven to be more cost effective. It allows using current electronics and current fibers, but simply shares fibers by transmitting different channels at different wavelengths (colors) of light. Systems that already use fiber optic amplifiers as repeaters also do not require upgrading for most WDM systems. How does WDM work? It’s easy to understand WDM. Consider that you can see many different colors of light (red, green, yellow, blue, etc.) all at once. These colors are transmitted through the air together and may mix, but they can be easily separated using a simple device such as a prism, just as we separate “white” light from the sun into a spectrum of colors with the prism. This technique was first demonstrated with optical fiber in the early 1980s when telephone company fiber optic links still used multimode fiber. Light at 850nm and 1300nm was injected into the fiber at one end using a simple fused coupler. At the far end of the fiber, another coupler split the light into two fibers, one sent to a silicon detector more sensitive to 850nm and one to a germanium or InGaAs detector more sensitive to 1300nm. Filters removed the unwanted wavelengths, so each detector then was able to receive only the signal intended for it. By the late ’80s, all telecommunications links were single-mode fiber, and coupler manufactures learned how to make fused couplers that could separate 1300nm and 1550nm signals adequately to allow WDM with simple, inexpensive components. However, these had limited usefulness, as fiber was designed differently for 1300nm and 155nm, due to the dispersion characteristics of glass. Fiber optimized at 1300nm was used for local loop links, while long haul and submarine cables used dispersion-shifted fiber optimized for performance at 1550nm. With the advent of fiber optic amplifiers for repeaters in the late ’80s, emphasis shifted to the 1550nm transmission band. WDM only made sense if the multiplexed wavelengths were in the region of the fiber amplifiers operating range of 1520 to 1560nm. It was not long before WDM equipment put four signals into this band, with wavelengths about 10nm apart. The input end of a WDM system is just a simple coupler that combines all the inputs into one output fiber. These have been available for many years, offering two, four, eight, 16, 32, or even 64 inputs. The demultiplexer is the component that’s difficult to make. The demultiplexer takes the input fiber and collimates the light into a narrow, parallel beam of light. It shines on a grating (a mirror like device that works like a prism, similar to the data side of a CD), which separates the light into the different wavelengths by sending them off at different angles. Optics capture each wavelength and focus it into a fiber, creating separate outputs for each wavelength of light. WDM to DWDM Current systems offer from four to 32 channels of wavelengths. The higher number of wavelengths has led to the name dense wavelength division multiplexing (DWDM). The technical requirement is only that the lasers be of very specific, stable wavelengths, and that the DWDM demultiplexers be capable of distinguishing each wavelength without crosstalk. Advantages of WDM A WDM system has some features that make it very useable. Each wavelength can be from a normal link, such as an OC-48 link, so you do not obsolete most of your current equipment. You merely need laser transmitters chosen for wavelengths that match the WDM demultiplexer to make sure each channel is properly decoded at the receiving end. If you use an OC-48 SONET input, you can have 4 ¥ 2.5 Gbps = 10 Gbps up to 32 ¥ 2.5 Gbps = 80 Gbps. While 32 channels are the maximum today, future enhancements are expected to offer 80 to 128 channels! You are not limited to SONET: you can use Gigabit Ethernet, or you can mix and match SONET and Gigabit Ethernet or any other digital signals! About the only thing you can’t do is mix in analog channels such as CATV. Repeaters Another technology that facilitates DWDM is the development of fiber optic amplifiers for use as repeaters. Fiber amplifiers work totally in the optical domain; they do not have to convert the optical signal to an electrical signal to amplify it. They can amplify numerous wavelengths of light simultaneously, as long as all are in the wavelength range of the FO amplifier. They work best between 1520 and1560nm, so most DWDM systems are designed for that range. Now that fiber has been made with less effect from the OH absorption bands at 1400nm and 1600nm, the possible range of DWDM has broadened considerably. Technological development is necessary for wider range fiber amplifiers to take advantage of the new fibers. Applications Two obvious applications are already in use: submarine cables and extending the lifetime of cables where all fibers are being used. DWDM enhances the submarine cables’ capacity without adding fibers, which create larger cables and bulkier, more complicated repeaters. Adding service in areas where cable fibers are now full is another good application. WDM is even being considered for 10 gigabit Ethernet to allow use over multimode fiber. Further enhancements But this technology should also reduce the cost on all land-based long-distance communications links, and it may lead to entirely new network architectures. Imagine an all-optical network that uses DWDM, switches signals in the optical domain without converting signals to electronics, and can add or drop signals by inserting or withdrawing wavelengths at will. All this is being researched. Given the speed with which optical technology advances, an all-optical network may not be far in the future! HAYES, a frequent contributor to Electrical Contractor, is president of Fotec in Medford, Mass. He can be reached at jeh@fotec.com.

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