By BERNHARD DEUTSCH and DAVID R.

VELASQUEZ

Interest in fiber and cable bend performance for multidwelling unit (MDU) FTTH cabling has steadily increased. Rightly so, because making fiber cable installation as easy and fast as copper cable installations while main-taining the optical performance and reliabil-ity advantages is a requirement for wide-scale MDU fiber deployments. The high density of households in MDUs can reduce the capital cost of installation per subscriber while dra-matically increasing the speed with which potential subscribers can be accessed and connected. As a consequence, MDUs often offer a stronger business case for FTTH de-ployment than single-family units.

Despite such economies of scale, the fact that only 5% of all homes connected with fi-ber in the U.S. are MDUs speaks to the de-ployment challenges carriers face here and the need for innovative MDU cabling op-tions. Space is at a premium and the aes-thetics of new installations are a major consideration when dealing with building owners. Carriers also need low-cost, reliable cabling options that are fast to install.

Recent advances in fiber technology that offer significantly improved bend perfor-

mance have brought the industry closer to its MDU goal: cable as fast and easy to install as copper that delivers the high performance expected of optical fiber.

Bend profilesRecognizing changes in fiber installation re-quirements, in late 2006 the ITU-T devel-oped Recommendation G.657 to define and standardize two classes of singlemode fiber with different levels of bend performance:• Recommendation G.657.A defines “bend-

improved” fibers. These fibers are required to be backward compatible with standard low-water-peak singlemode fibers as speci-fied in ITU-T G.652.D; the associated fiber design tradeoff limits the minimum bend radius specification to 10 mm. Bend-im-proved fibers have enabled major density improvements for FTTH hardware. They also enable a reduction in weight and size of local convergence cabinets by approxi-mately 40% and 75%, respectively, when combined with improved fiber manage-ment.

• G.657.B-compliant fibers are called “bend-tolerant” fibers and not required to be back-ward compatible. This is a big tradeoff but enables a reduction in the specified min-

imum bend radius to 7.5 mm. However, this mod-est further reduction in minimum bend radius is not enough to enable fiber cable installation practices on par with copper instal-lation in terms of speed, ruggedness, and flexibil-ity or to enable further density improvements for fiber hardware.

Several “new” fi-ber products have been launched in the past two years based on one of the following well established

fiber design options that are known to im-prove bend performance:• Reducing the core diameter and/or increas-

ing the refractive index of the core.• Depressed cladding fibers where a circular

zone around the core is doped (typically with fluorine) to achieve a lower refractive index than silica glass.

• Trench-assisted fibers similar to depressed clad fibers, but the ring of doped silica with a lower refractive index is further away from the core.Since these enhancing options are incre-

mental modifications of existing approaches to process and design, the achievable mac-robend improvements are limited and only incremental as well.

Completely different approaches also have been developed, primarily hole-assisted fiber (HAF) and photonic band-gap fiber (PBGF). These fibers have very different waveguide profiles compared to the profiles of legacy fibers that leverage the technology of chem-ical-doped waveguide fabrication. While these HAF and PBGF designs produce very bend-insensitive fibers, they are very costly to make in large quantities and long lengths, difficult to connect, and not backward com-patible with the aforementioned standards. In addition, HAF and PBGF fibers are not universally compatible with existing termi-nation and field procedures.

Figure 1 shows the bend performance re-quirements of fibers in compliance with ITU-T recommendations G.652.D, G.657.A, and G.657.B and illustrates which fiber designs are capable of what bend performance.

While providing excellent bend perfor-mance, the manufacturing and compatibility challenges of HAF and PBGF were consid-ered too tough to overcome—until recently, when a nanostructure-based fiber technol-ogy was developed that enables bend radii specification down to 5 mm while enabling backward compatibility with industry stan-dards and existing installation processes.

■ APPLICATIONS

New technology provides breakthrough for MDU fiber installation

Photo 1. Examples of fiber-optic cable installations in an MDU application, clockwise from upper left: building access, distribu-tion cable routing, drop cable routing in decorative molding in a hallway, drop cable routing through steel studs without any protection, and drop cable slack storage inside a raceway.

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Reprinted with revisions to format, from the October 2007 edition of LIGHTWAVECopyright 2007 by PennWell Corporation

Engineered nanoscale structures within the fiber create a bend-insensitive fiber with a unique “trapping” mechanism that confines the light in the core better than what is possi-ble by changing the material composition of the glass by fluorine doping. It is totally syn-thetic and does not require any mechanical work on the glass during manufacturing.

Performance vs. requirementsTo evaluate the new technology’s poten-tial benefits, users must consider two major questions. First, what kind of performance do MDUs really require—i.e., how low does the bend radius have to be to achieve attrac-tive deployment cost savings while main-taining an adequate power budget? Second, what mechanical reliability and field prac-tices should installers consider?

The answer to the first question lies in an understanding of the application space. In many field trials and deployments with car-riers across the globe, it has become evident that bend radii down to 5 mm are required to be on par with copper cable installation and to maximize the potential labor and material cost savings. Bending cables around corners at the building entrance; in hallways, wiring closets, and wall outlet boxes; and around doors or other obstacles is common practice in premises installations.

Photo 1 provides several examples of fi-ber-optic cable installation in MDUs. The drop cable routing shown in Photo 1 along the hallway required the cable to negotiate

15 bends to pass only four living units. In ad-dition, some-times a heavy mechanical load is in-duced where a well-in-tended effort to organize a bundle of ca-bles results in a single cable carrying the burden of the whole bun-dle.

Mean-while, sta-pling cables

to studs, molding, and dry wall is common practice to accelerate the installation process and avoid the high material and installation cost of microducts. Photo 2 shows the effect on the optical fiber of stapling cable to studs. From these images, it is clear the bending in-duced by this practice definitely creates bend radii in the 5-mm range.

It seems clear that to maximize the potential for lower deployment costs, faster rates of de-ployment, meeting optical power budgets, aes-thetically acceptable cable designs, and optical capability at a bend radii of 5 mm will be re-quired and specified. Discussions with carriers internationally indicate they understand and are comfortable with technology that meets this requirement. As an example, NTT has al-ready deployed products with performance tar-geted at bend radii below 5 mm.

Naturally, when thinking about bends to these tight radii, customers can and should investigate the long-term mechanical consid-erations—part of our second question. Over the years, extensive investigation and mod-eling of optical fiber mechanical reliability have been performed. As a result, standard tests have been developed to ensure fibers meet the reliability requirements for opti-cal networks. The two main test parameters are proof and dynamic tensile strength. It is important to note that mechanical reliability is a statistical parameter with a probability distribution.

Standard singlemode fibers have bend ra-dii specifications to ensure virtually “zero

breaks” over a 20-year period. The latest ITU-T G.657 standard for fibers with im-proved bend performance implies a level of “managed reliability” where modeled failure rates will slightly exceed “zero” over this life-time.

The outer surface of a bent fiber is under stress just as if the fiber was under tensile load. The stress decreases with radial posi-tion, through a “neutral axis” along the cen-ter of the fiber, with the inner surface of the bend actually in compression (see Fig. 2). The likelihood of any fiber breaking depends on bending stress (i.e., bend radius), the length under bend (i.e., the number of bends), the strength distribution (i.e., length spacing be-tween flaws that will be expected to break), and the fatigue (i.e., flaw growth) during service. As a result, the probability of fiber component failure for bends down to 5 mm in radius is still in the low single-digit parts per million. At these levels, the fiber is still the most reliable optical component in the network.

These modeling results can be extrapo-lated to a carrier’s network configuration to estimate the probability of a link failure per household, per kilometer of fiber deployed, or other relevant metrics.

Finally, what are the considerations in us-ing this nanostructure-enabled technology in the field? Let’s walk through the three key processes required to connect an optical fi-ber when installing a link in the network and see what needs to be done.1. Access the fiber in the cable and pre-

pare for the splice. This step is the same for nanostructure-enabled fibers and stan-dard fibers. A technician uses lint-free wipes to clean the optical fiber after enter-ing the cable, accesses the right fiber, and mechanically removes the coating; some-times a cleaning fluid (e.g., isopropanol) is used with the wipes. The technician then cleaves the fiber for an optimum fiber end-face to achieve the desired splice loss with an automatic fusion splicer. If the fiber endface comes into contact with any ma-terial (dust, liquids, hands, surfaces, etc.) before the splice is completed, it should be cut back an appropriate length (typically 2 to 4 inches) and the cleaning and cleav-ing process must be repeated. If fibers have been cleaved and stored in terminals or wall outlets for future use, standard procedures require the installer to cut back, clean, and cleave again before connection.

Bend loss (dB/turn) at 1,550 nm

0.01

0.1

1

10

5 10 15 20

Bend radius (mm)

Bend performance comparison

0.0001

0.001

ITU-T G.657.A

ITU-T G.652.D

ITU-T G.657.B

Figure 1. This bend-loss diagram illustrates different fiber specifications. Also shown is the typical performance for different fiber designs, clockwise from lower left: photonic band-gap, hole-assisted, trench-assisted, de-pressed clad, small core, and standard singlemode fiber.

Applications

2. Fusion splicing or field connectorization.Fusion splicing of nanostructure-enabled bend-insensitive fibers has been demon-strated with standard fusion splicers using only minor adjustments to the splice pa-rameters for both splice variations: bend-insensitive fibers with themselves and with standard singlemode fibers. Although the fiber does not couple out light under bend easily (by design) and has a different vi-sual appearance on the screen of the splicer,

settings are available on current splice equipment that support the splicing of nanostructure-enabled fibers just like standard fibers. Studies with thousands of splices have shown typical values below 0.1 dB, without splice machine optimization, for both homoge-neous (nanostructure-enabled to nanostructure-enabled fibers) and heterogeneous (nanostructure-en-abled to standard singlemode fi-bers) splicing, meeting Telcordia GR-20-CORE requirements. Light injection and detection (LID) sys-tems may have difficulties esti-mating the splice loss or detecting

the core for alignment purposes. However, field-proven and widely used v-groove and cladding alignment systems as well as core detection systems work perfectly well with nanostructure-enabled fibers.Field-installable connectors may be used

as well. When the fiber is prepared properly and the connector assembled per the recom-mended procedures, the installer will see no difference between currently available fibers and nanostructure-enabled fibers. Both con-nectivity options achieve splice/insertion and return losses in the same range expected with standard singlemode fibers. In addi-tion, nanostructure-enabled factory-termi-nated cables are available that can be used in a plug-and-play fashion. After testing thou-sands of connectors, the typical insertion loss for mating connectors with standard singlemode fiber and nanostructure-enabled fibers and nanostructure-based fibers with themselves is in the same range as today’s state-of-the-art connectors.3. Testing. All bend-improved and bend-tol-

erant fibers create some challenges for test equipment makers since certain fiber pa-rameters must be modified to achieve the bend performance (e.g., mode-field diam-

eter, cut-off wavelength). Even if the fibers are standard compliant, there is always a window of performance that the test equip-ment operator has to take into account. When using an OTDR to test a link where fibers with different mode-field diameters are connected, “gainers” or “exaggerated loss” effects can be seen. These phenomena are well known, and operating procedures calling for testing the link from both ends and averaging the results when seeing these are already in place. In MDUs, where com-paratively shorter lengths dominate, an al-ternative to bidirectional testing already is used for the drop section of FTTH net-works. Testing the entire link with power meters is an easier and less costly technique and gives the true link-loss measurement. Finally, live fiber testers have demonstrated sufficient sensitivity to detect light in nano-structure-enabled fibers.In summary, nanostructure technology

enables cabled fiber to be bent very tightly with virtually no signal loss while still main-taining backward compatibility with in-dustry standards and existing installation processes. The bend performance this new fiber achieves enables network planners to no longer worry about bend-induced link loss. Feedback from early adopters shows that by reinforcing existing operating procedures, network installers will see minimal impact on field termination and testing practices. They can now treat truly bend-insensitive optical fiber cable like copper cable—and provide the bandwidth they need today and tomorrow.

Bernhard Deutsch, PhD, is director, market-ing and market development, at Corning Cable Systems (www.corning.com/cablesystems). David R. Velasquez is global product line manager, telecom and premises fibers, at Corning Optical Fiber (www.corning.com).

Fiber stress

Tension

Compression

Bending forceapplied

Bending forceapplied

Allowed bend radius(mm)

5.0

7.5

10.0

Failure probability per turn(ppm)

3.0

1.0

0.5

Figure 2. Illustration of stress applied to optical fiber under bend and its impact on the failure probability per 360° turn in parts per million over 20 years.

Photo 2. An MDU drop cable stapled to wooden stud with a standard stapler using a standard flat staple (right). The left picture shows the relevant x-ray image demonstrat-ing the impact on the fiber bend radius.

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Applications

Corning Cable SystemsPO Box 489Hickory, NC 28603-0489Tel: 800-743-2675FAX: 828-901-5973www.corning.com/cablesystems

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