by John George, Peter Weimann, Ole Suhr, and Jesper Steenstrup
The current explosion in FTTx deployments has increased carrier interest in bend-capable fibers (BCFs) that greatly reduce macro-bending loss. Traditional singlemode fibers have been limited in fiber-to-the-home (FTTH) and FTTx applications by high macro-bending loss. A 1,550-nm video signal that encounters a single 10-mm radius turn will typically lose more than 60% (4 dB) of its power, likely turning off service and customers. Recently, new types of BCF have been introduced to support installation of optical drop cables in multiple-dwelling units (MDUs) and within homes without conduits or bend radius management. Understanding the various types of fibers (see Fig. 1), applications, and benefits is therefore more important than ever.
In 1984, AT&T introduced the most widely deployed BCF, known as depressed cladding fiber or protected core fiber. Protected core fibers were installed extensively in long haul, metro, access, and LANs in the 1980s and 1990s, and evolved in 2006 to a zero-water peak version that supports the full spectrum from 1,260 to 1,625 nm. In the 1980s AT&T also patented a bend-capable trench-assisted design for undersea cable applications. Trench-assisted fibers with similar designs are still used today for connectivity and in-building applications. Hole-assisted fibers (HAFs) are used by NTT for in-home applications. HAFs provide the lowest bending loss available but have substantial limitations in splicing and connectorization.
The fundamental AT&T approach of creating a circular region around the core having a lower refractive index than the cladding is still used today in most BCF designs, including the newly announced random void structure and solid glass ring-protected fiber designs. These two designs target MDU and in-home wiring (IHW) applications. Both designs promise macro-bending loss of <0.1 dB per turn at 5-mm radius, while maintaining some degree of compatibility with the installed base of conventional singlemode fiber.
However, there are significant differences in these designs and the resulting performance in splicing, connectorization, and performance uniformity. Splice and connection compatibility between a BCF and itself, and a BCF and the installed base of fibers, is critical to help simplify installations and achieve the high optical performance required to support video and other mission-critical services. BCFs that are standard compliant to ITU-T G.652D, for example, may not necessarily splice seamlessly to other G.652D fibers due to significant mode field diameter mismatches, holes or voids in the glass, or dissimilar types of glass within the fiber structure. Designs using voids or holes may also complicate mounting and optical performance of connectors if dirt and debris stick in holes to confound polishing and cleaning processes. Another variable is potential nonuniformity of the fiber�s properties along its length.
The International Telecommunications Union Telecommunication Standardization Sector (ITU-T) has created two recommendations for BCFs targeting broadband access networks; a portion of those are shown in Table 1.
The first, G.657A, describes fibers that are compliant to the G.652D standard yet offer macro-bending loss of 0.75 dB maximum for one 10-mm radius turn at 1,550 nm, about 3� to 6� lower bending loss than conventional G.652 fiber at this radius. Compliance with G.652D is intended to ensure that properly designed G.657A fibers can be seamlessly spliced and connected to the installed base of G.652D fibers without excessive added loss. However, as noted previously, this is not always the case. For some G.657A-compliant fibers, special splicing software and/or use of cladding-aligned splicing equipment may be necessary to ensure efficient splicing. End users of fiber must take care to ensure that the fiber they deploy is compatible with their splicing equipment and procedures.
The second recommendation, G.657B, describes the performance of fibers with macro-bending loss of 0.5 dB maximum for one 7.5-mm radius turn at 1,550 nm. G.657B fiber has about 15Ã� to 30Ã� lower macro-bending loss than conventional G.652 fiber at this radius. Yet even G.657B performance may not suffice for some in-residence optical drop applications, where installers may wish to staple and route drop cables around sharp corners without conduit or management elements. These "beyond class B" applications in MDUs and in-home drop cables will require a macro-bending specification on the order of 0.1 dB maximum for one 5-mm radius turn at 1,550 nm—about 100Ã� lower macro-bending loss at this radius than standard G.652D singlemode fiber.
With such standards in place, BCFs can enable installations not possible with conventional singlemode fibers. They can help decrease space requirements and improve network reliability in central offices, cable TV headends, data centers, splitter cabinets, and crossconnect cabinets. BCFs may also help carriers accommodate new, high-density equipment and apparatus that lessen the need for cable management and/or enable safe violation of traditional rules for bend radius management. More recently, service providers have desired fiber cable that can be cost-effectively and inconspicuously installed in residential environments. Given the varying requirements for bending loss, reliability, and backward compatibility for these key applications, a simple question arises: Which bend-capable optical fiber fits best with each application?
Central offices (COs), headends (HEs), and data centers (DCs) typically contain hundreds and more commonly thousands of cables, and each fiber may support hundreds or thousands of businesses and residences. Thus these applications require very high reliability.
Optical fiber reliability must be viewed in terms of both optical loss and mechanical reliability. A view of mechanical reliability is shown in Fig. 2, based on classical modelling equations used by the International Electronics Commission (IEC) recommendations for optical fiber systems. With traditional rules for fiber management at large bend radii, fiber breakage has not been an issue. As fiber bend radii drop, based on improved low-bending-loss fiber designs, fiber reliability becomes an important consideration for all applications.
All of the applications discussed here share the same need for low fiber bending loss, regardless of the minimum acceptable bending radius. Additional loss beyond a few tenths of a decibel for most applications might impair or shut off video and other mission-critical services. In general one should expect the maximum macro-bending loss of a bend-optimized cable assembly to be less than 0.2 dB for a single turn at 1,550 nm at 10-mm radius for the CO/HE/DC/cabinet applications. Below a 10-mm radius there is risk of random fiber breaks for a length of 20 cm (about three turns at 10-mm radius).
An additional concern in COs and HEs is the high optical power transmitted over fiber to support RF video or DWDM systems. Optical cable assemblies used in these applications should be able to support such high powers under bending conditions without degrading, burning, or excessive signal loss, while maintaining mechanical reliability. Again, for these applications the 10-mm minimum radius is recommended to balance these needs.
Service providers have recently requested BCF drop cables using "beyond class B" fibers that can be routed down hallways and inside residences, will conform to corners, and can be stapled in place. This type of installation promises to be less conspicuous to the customer and can help lower the cost of deployment for the service provider. The material cost savings per unit passed, for example, could be �4 (US$5.9) to �7 (US$10.5) per unit in addition to significant labour savings of �7 (US$10) to �15 (US$22.2) per unit.
Unlike traditional applications, MDU/IHW drop installation can involve intentionally bending the fiber to radii down to somewhere between 5 and 7.5 mm. All things being equal, mechanical reliability of the glass in sharply bent MDU/IHW cable will inherently be much lower than that in the CO/HE/DC/cabinet applications: The stress on the fiber at 5-mm bend radius is 4� higher than at a 10-mm bend radius and more than 40� higher than fiber following the traditional 32-mm bend radius fiber management upon which historical optical fiber reliability experience is based.
The optical cable design is a key to preserving optical and mechanical reliability in these demanding deployment conditions. Deployment of optical cables in a manner similar to that used for copper telephone wire leads to new questions for the industry: Is the reliability risk associated with these intentional sharp bends acceptable in MDU/IHW applications, given the first-cost benefits of using ultrabendable cables? One consideration with the MDU or single-family residential application is that only one subscriber is supported for each optical drop cable. Is it acceptable in this case to lower the reliability expectation? Is the reliability of nanovoid types of fibers the same or lower than that of solid glass fibers? This is unexplored territory in practice and one that the industry will study in the coming months as the market desire for ultrabendable cables intensifies. Preliminary studies in OFS laboratories suggest that the combination of bending strain and installation tension in these aggressive installations may have a significant impact on fiber lifetime.
New low-diameter cables, known as micro-cables, can pack powerful fiber-optic bandwidth in a small package. The benefit of this micro-cable override approach becomes apparent as providers increasingly attempt to place cables in existing duct instead of disrupting both budgets and the environment by digging to bury new ducts. Micro-cable overrides can reduce installation costs by more than 50% compared to direct burying of new duct.
A benefit of some types of BCFs is further reducing the diameter of micro-cables. Protected core fiber has bending loss properties that enable the packaging of 96 fibers in the same diameter that can hold only 48 or 72 fibers of conventional G.652D full-spectrum fiber. As a result, a further reduction of up to 50% in installation cost is possible by using protected core fiber micro-cables compared to conventional micro-cables.
BCFs provide significant value to many applications (see Table 2). These benefits include lower labor installation, smaller packaging (for example, plug-and-play terminals are now available for distributing up to 12 drop cables from a backbone cable to subscriber units), and protection of the many long-wavelength applications used today and coming in the future for video, data, and voice applications. Three bend-capable technologies were described that are optimized for key application spaces. By choosing the fiber optimized for the application, one can achieve the best balance of reliability and optical performance in bending challenged systems.
John George is director, systems and applications engineering; Peter Weimann is senior R&D engineer; Ole Suhr is SAE, EMEA fiber; and Jesper Steenstrup is senior account manager, EMEA fiber, at OFS (www.ofsoptics.com).