Sometimes the future catches up with us before we know it. Consider the videophone, a technology that never quite caught on. In its place, Skype and similar services have been enabled by the availability of broadband service and have empowered users to make video calls over computers. Today, that is an ordinary part of many people’s daily lives.

Another example is occurring in the use of single-mode fiber optic networks. For years, we’ve been waiting for equipment and applications that will enable us to utilize the entire spectrum that fiber offers. Interestingly, new wavelengths in the previously unusable “E” band (defined as 1360 nm - 1460 nm) are being claimed at a speedy pace, fulfilling the promise of full-spectrum optical fiber.

Wavelengths Across the Spectrum
Original fiber networks installed a couple of decades ago operated only at 1310 nm in the “O” band. The 1550 nm wavelength in the “S” band became available soon after, and both wavelengths formed the basis for the explosion of traffic in the transport portion of the network in the 1980s and 1990s.

DWDM applications operating in the “C” and “L” bands came of age in the late 1990s and early 2000s. However, these networks were, and still are, mainly devoted to longer-distance, higher data-rate transport applications.

The early part of this decade brought fiber-based access networks, and introduced the PON wavelengths, including 1490 nm for downstream data, and 1590/1610 nm for RF return-path applications. Next-generation PON systems expand the trend further, using the 1577 nm wavelength (downstream) and 1270 nm upstream. The trend is continuing as WDM-PON technology and accompanying standards begin to take shape, claiming even more wavelengths up and down the optical spectrum.

See-E-ing the Future
Since the initial development of single-mode fiber in the 1980s, the “E” band has not been used for any significant applications. The “E” band includes the “water peak” found in all standard single-mode fibers, where absorption of the hydroxyl (OH-) ion historically has caused attenuation performance of up to and beyond 1 dB/km. (See Figure 1.)

Figure 1. Attenuation After ITU Specified Hydrogen Aging.

But finally, applications and equipment that use the “E” band are emerging. This is a major development in spectrum allocation.

What’s changed? Over the past 10 years, improvements in manufacturing processes have driven much better fiber attenuation performance in the “E” band. Most fibers deployed today are classified as either low water peak (LWP) or zero water peak (ZWP).

The ITU G.694.2 CWDM grid includes wavelengths spaced 20 nm apart starting at 1271 nm up to 1611 nm. As laser technologies have advanced, the availability of CWDM equipment, including the “E” band wavelengths, is giving operators new flexibility in matching wavelengths with appropriate services. Inexpensive CWDM SFPs, sources, and test equipment are now available for such previously unused wavelengths as 1371 nm and 1391 nm. Given all of the expected demands on the network, it’s none too soon.

Network Design Implications
What does this mean for the network operator? First, when choosing a fiber to deploy in the network, operators need to pay special attention to fiber performance in the “E” band. ZWP fiber performs a minimum of 12 percent better than LWP fiber, and depending on the specification supported by the manufacturer, sometimes as much as 22 percent better. Over a wide area network, that performance can make a significant difference in reaching customers.

In addition, although the majority of fibers on the market are classified as LWP or ZWP, some fibers that have very poor performance in the “E” band inevitably sneak into the system, especially in times of fiber shortage where customers look for supply wherever they can find it. Caveat emptor -- let the buyer beware.

From a network operation standpoint, the availability of “E” band sources gives the operator unprecedented options for deploying traffic and managing precious fiber resources. A typical network may have a wide variety of equipment deployed over it, including:
•  Internal traffic for video cameras or internal networks operated over SFP-driven switches.
•  Access PON equipment feeding internal and external customers.
•  Transport equipment operating on DWDM wavelengths.

Over the lifetime of a network, it’s almost inevitable that there will be one or more sections of the network with wavelength conflicts on a particular set of fibers. A wavelength allocation strategy can help to avoid those conflicts. One proposed strategy is to move non-revenue-generating applications to the “E” band, including wavelengths such as 1371, 1391, and 1411 nm, while keeping revenue-generating applications on the more traditional wavelengths, such as 1310, 1490, and 1550 nm.

For those networks with only non-revenue-generating applications, it’s still a good idea to operate that equipment in the “E” band, leaving the other wavelengths open for future revenue-producing applications that may be operated on the 1310 nm and 1550 nm wavelengths.

A sample wavelength allocation map with various applications is shown in Figure 2. Traditionally, much of this traffic would have been solely on the 1310 and 1550 nm wavelengths, or on separate fibers altogether. However, now that “E” band equipment is available, applications such as SCADA, internal communications, and cameras can be placed on the “E” band, leaving the 1310 and 1550 nm wavelengths open for future, potentially higher value, applications.

Figure 2. How to use the E-band.

While each network is different, these general guidelines show what can be done with a little planning.

See-E-ing Fiber’s Future
The future is never clear, but there are a few trends that support this approach. Bandwidth demands are only going to increase. This will inevitably lead to the need for new wavelengths. Paying attention to fiber performance in the “E” band and careful wavelength planning now can help save future outages or equipment forklifts in the future.

 Mark Boxer is the Applications Engineering Manager for OFS, where he assists customers deploying fiber in a wide variety of network design scenarios and analyzes market trends. Mark has more than 20 years in the fiber industry, with experience in fiber manufacturing, applications engineering for various fiber-based products, and assisting customers in FTTH deployments. For more information, email ofs@ofsoptics.com or visit www.ofsoptics.com.

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