Futureproofing with full-spectrum fiber

Sept. 1, 2001
SPECIAL REPORTS / Fiber and Cable

Fused silica provides a wide range of complex applications for the metropolitan market.

DAVID RUSIN, American Fiber Systems Inc.

Fiber-optic cable for communications networks comes in many types, each with its own characteristics, purposes, and applications. Using the wrong type of fiber in a network yields the same results as using the wrong equipment for any task. You can use a screwdriver to pound a nail, but the results are much less efficient than using a hammer. Similarly, the wrong type of fiber-optic cable dramatically diminishes the bandwidth-and thus the efficiency and effectiveness-of a fiber-optic network.

That's why many of today's fiber providers are adopting full-spectrum cable for their metropolitan dark-fiber networks. The fused silica (pure quartz) used in full-spectrum cable provides maximum efficiency and effectiveness for the full range of complex applications continuously being pulsed over any city's main communication pipeline.

Full-spectrum fiber is the only real means of "futureproofing" a network to support all current and future applications and equipment. After all, who knew just a few years ago that passive-optical-network (PON) technology would become one of the hottest technology advancements around? Just imagine if legacy fiber networks could not support new optical-switching equipment.

Similarly, new technology developments that take advantage of all portions of the fiber spectrum are emerging every day. Given the multimillion-dollar costs of deploying a metro network, prudent providers are installing fiber strands capable of carrying the full range of optical traffic and equipment rather than gamble that all the applications that will ever need to be transmitted through a city have been invented already.

The vast majority of U.S. communications traffic-75-85% by most estimates-originates and terminates in cities. Telephone calls, e-mail, financial transactions, interactive video games, videoconferencing, telemedicine applications, Internet traffic, Web hosting, enterprise networking, and many other applications all contribute to the deluge of megabytes being streamed daily over metro communication networks.

Because of the vast array of applications originating and terminating in cities, the metro backbone infrastructure must provide a medium capable of transmitting all traffic-voice, video and data-with the same efficiency and economy, and in a "bursty," rather than consistent, method across the entire span of the fiber.

The type of fiber typically used in metro fiber-optic networks is singlemode, as opposed to multimode fiber used in short-distance transmissions, generally within buildings. All optical fiber acts as a waveguide directing light over a discrete path. With singlemode fiber, a polarized beam is reflected coherently over the path with minimal absorption and dispersion. With multimode fiber, non-polarized light is reflected in multiple modes, like sunlight off water, and experiences considerably more absorption and dispersion. Thus, singlemode fiber is better suited for supporting communications traffic that travels across longer distances.

However, not all singlemode fibers are created equal. Like any waveguide, fibers operate optimally only in discrete wavelength ranges of the bandwidth spectrum. For example, so-called "legacy" fiber (any fiber deployed before 1995) is designed to operate over a spectrum range of 1,285 to 1,625 nm with a zero dispersion point at 1,310 nm and high positive dispersion at 1,625 nm. In addition, it is plagued by a high content of water ions (HO-) that absorb energy around and peak at 1,385 nm.
Shown here are the types of applications and the associated wavelengths achieved with full-spectrum fiber. This fiber uses the entire E-band, thus increasing the total possible bandwidth per fiber.

Some newer types of fiber are designed to operate best in the third (1,530-1,565-nm) and fourth (1,565-1,620-nm) bandwidth "windows," or the C- and L-bands.

In fact, the most popular optical glass deployed recently for high-speed DWDM applications in long-haul fiber-optic cable is nonzero dispersion-shifted fiber (NZDSF), with spectrum capability focused on the C- and L- DWDM bands-the third and fourth bandwidth windows. Unfortunately, this fiber doesn't operate well over the entire usable spectrum of the fiber, thus limiting capacity.

Specifically, this fiber wasn't designed to support legacy applications at 1,310 nm and is incapable of using the emerging 1,400-nm band due to considerable attenuation from its high water peak. Unfortunately, the higher the attenuation rate, the shorter the distance the signal can travel.

As good as this type of fiber is for some applications, (10-Gbit/sec DWDM, for example), it does not support access to the entire bandwidth spectrum required in metro regions. These applications include legacy leftovers such as 1,310-nm SONET as well as emerging applications like Ethernet, native IP over coarse wavelength-division multiplexing (CWDM), and cable TV.

Full-spectrum fiber, by contrast, was specifically designed to open the entire spectrum by eliminating the "water peak" that contributes to signal loss in important new bandwidth windows, yet maintain a dispersion characteristic that is not skewed for a particular application to the exclusion of others. Full-spectrum fiber enables the use of the entire spectrum band, supporting all possible applications, from "legacy" to DWDM through CWDM and high-band cable TV.

Full-spectrum fiber is capable of using the full bandwidth spectrum, from 1,285 nm to 1,625 nm, including the 1,400-nm band, providing more capacity with less attenuation and therefore less loss (see Figure). By enabling the use of additional wavelengths in the 1,400-nm band (from 1,350 to 1,450 nm), full-spectrum fiber can in crease capacity by as much as 50% and can provide as many as 120 more DWDM channels than conventional singlemode fiber.

By operating within the 1,400-nm E-band, full-spectrum fiber takes advantage of the low positive dispersion found in shorter wavelengths of standard zero dispersion fiber (which lessens four-wave mixing and enables DWDM at higher speeds), while also profiting from lower attenuation (thus, the ability to travel longer distances) as found in the longer wavelengths (i.e., the L- and C-bands).

The 1,400-nm window offers the ideal dispersion characteristics to support high-speed 10-Gbit/sec DWDM in tomorrow's metro network. Dispersion is the tendency of a light beam to spread out as it travels, becoming less exact. Before full-spectrum fiber, special dispersion-shifted fiber was required to support 10-Gbit/sec or higher DWDM. In fact, the 1,400-nm window is critical for supporting emerging technologies-where the action is in metro applications.

Put simply, the emergence of full-spectrum fiber makes many new fiber technologies more cost-effective, thus bringing many advantages to the metro network that were previously only available in long-haul networks.

For instance, DWDM in many cases is not cost-justifiable in metro networks because of the costly, closely spaced, very precise lasers required. Additional costs and engineering complications arise, such as the need to provide amplification, equalization, and dispersion compensation in the network. These costs stem from the fact that the nodes in metro networks are not as predictable and do not have the same distance requirements as long-haul networks.

By opening up the spectrum E-band, enabling and expanding CWDM using less precise, less expensive lasers to support just as much capacity as more precise alternatives, metro networks can now enjoy the benefits of DWDM technology without the associated costs. Because it squeezes more channels into one strand of fiber, full-spectrum fiber can better support new emerging technologies such as CWDM in a way that uses even less precise lasers to send signals. Because its wavelengths are spaced further apart, CWDM can decrease costs by as much as a factor of 10, compared to the DWDM supported on nonzero dispersion fiber in metro networks.

The ability to support one emerging technology leads to the support of others. For instance, full-spectrum fiber enables metro networks to incorporate 16-wavelength CWDM technology on 25-nm spacing, an emerging technology that allows real-time wavelength switching (i.e., photonics)-one of the hottest new network developments that will eventually lead to a new generation of less expensive lasers and filters.

Full-spectrum fiber is clearly the best choice for the complex demands of metro networks. Its ability to support legacy applications such as 1,310-nm SONET, DWDM, and emerging CWDM, by opening the entire spectrum for use and eliminating the water peak that previously limited fiber's capabilities, has positioned it as the premier choice in superior dark-fiber metro networks.

David Rusin is founder, president, and CEO of American Fiber Systems Inc. (Rochester, NY). He can be reached at www.americanfibersystems.com.

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