There are four critical elements of fiber cable management: bend radius protection; cable routing paths; cable access; physical protection. All four aspects directly affect the network’s reliability, functionality, and operational cost.
This blog post is going to talk about the importance of bend radius protection.
There are two basic types of bends in fiber—microbends and macrobends.
As the names indicate, microbends are very small bends or deformities in the fiber, while macrobends are larger bends (see Figure 1).
The fiber’s radius around bends impacts the fiber network’s long-term reliability and performance. Simply put, fibers bent beyond the specified minimum bend diameters can break, causing service failures and increasing network operations costs. Cable manufacturers, Internet and telecommunications service providers, and others specify a minimum bend radius for fibers and fiber cables.
The minimum bend radius will vary depending on the specific fiber cable. However, in general, the minimum bend radius should not be less than ten times the outer diameter (OD) of the fiber cable. Thus a 3 mm cable should not have any bends less than 30mm in radius.
Telcordia recommends a minimum 38 mm bend radius for 3 mm patch cords.
This radius is for a fiber cable that is not under any load or tension. If a tensile load is applied to the cable, as in the weight of a cable in a long vertical run or a cable that is pulled tightly between two points, the minimum bend radius is increased, due to the added stress.
There are two reasons for maintaining minimum bend radius protection: enhancing the fiber’s long-term reliability; and reducing signal attenuation. Bends with less than the specified minimum radius will exhibit a higher probability of long-term failure as the amount of stress put on the fiber grows.
As the bend radius becomes even smaller, the stress and probability of failure increase. The other effect of minimum bend radius violations is more immediate; the amount of attenuation through a bend in a fiber increases as the radius of the bend decreases. The attenuation due to bending is greater at 1550 nm than it is at 1310 nm—and even greater at 1625 nm.
An attenuation level of up to 0.5 dB can be seen in a bend with a radius of 16 mm. Both fiber breakage and added attenuation have dramatic effects on long-term network reliability, network operations costs, and the ability to maintain and grow a customer base.
In general, bend radius problems will not be seen during the initial installation of a fiber distribution system (FDS), where an outside plant fiber cable meets the cable that runs inside a central office or headend. During initial installation, the number of fibers routed to the optical distribution frame (ODF) is usually small.
The small number of fibers, combined with their natural stiffness, ensures that the bend radius is larger than the minimum. If a tensile load is applied to the fiber, the possibility of a bend radius violation increases.
The problems grow when more fibers are added to the system. As fibers are added on top of installed fibers, macrobends can be induced on the installed fibers if they are routed over an unprotected bend. A fiber that had been working fine for years can suddenly have an increased level of attenuation, as well as a potentially shorter service life.
The fiber used for analog video CATV systems presents a special case. Here, receiver power level is critical to cost-effective operation and service quality, and bend radius violations can have different but equally dramatic effects.
Analogue CATV systems are generally designed to optimize transmitter output power. Due to carrier-to-noise-ratio (CNR) requirements, the receiver signal power level is controlled, normally to within a 2dB range. The goal is for the signal to have enough attenuation through the fiber network, including cable lengths, connectors, splices and splitters, so that no attenuators are needed at the receiver.
Having to attenuate the signal a large amount at the receiver means that the power is not being efficiently distributed to the nodes, and possibly more transmitters are being used than are necessary.
Since the power level at the receiver is more critical, any additional attenuation caused by bending effects can be detrimental to picture quality, potentially causing customers to be dissatisfied and switch to other vendors.
Since any unprotected bends are a potential point of failure, the fiber cable management system should provide bend radius protection at all points where a fiber cable makes a bend.
Having proper bend radius protection throughout the fiber network helps ensure the network’s long-term reliability, thus helping maintain and grow the customer base. Reduced network downtime due to fiber failures also reduces the operating cost of the network.
Bending of singlemode fiber has everyone talking these days. The idea that you can bend a fiber around a pencil without a dramatic increase in attenuation is a concept that has everyone considering new fiber applications and design possibilities.
Today, industry standards for traditional singlemode jumpers typically specify a minimum bend radius of ten times the outside diameter of the jacketed cable or 1.5” (38 mm), whichever is greater.
This breed of flexible singlemode optical fiber has the potential to significantly reduce these minimum bend radius requirements to values as low as 0.6” (15 mm), depending on the cable configuration, without increasing attenuation. There are many names for optical fiber that can endure a tighter bend radius—“bend insensitive,” “bend resistant” and “bend optimized” are several that come to mind.
As mentioned above, reduced bend radius fiber is able to withstand tighter bends within frames, panels and pathways. In general, reduced bend radius optical fiber is designed to perform with low loss across the spectrum of wavelengths, from 1285 nm to 1650 nm, using all the channels available on those wavelengths to maximize bandwidth.
Current designs include low water peak or zero water peak so that high attenuation is avoided at 1383 nm. Many reduced bend radius optical fiber products meet ITU-T recommendation G.657, meaning they work well at 1550 nm for long distance and voice applications and at 1625 nm for video applications.
Despite the improved bend radius, the reality of this fiber is that bend radius protection is still a concern—just not to the extent of regular fiber. There is still a mechanical limit on how tightly an optical fiber can be routed before the structural integrity of the glass is violated.
The assumptions about improved performance are not accurate either, at least beyond the exceptional bend radius performance. In reality, the performance of reduced bend radius optical fiber—or any optical fiber—depends upon many factors, not just bend radius properties. By itself, reduced bend radius optical fiber does not offer improvements in attenuation. True, it bends more tightly without causing additional attenuation.
Yet laid out on a long, straight run next to a standard optical fiber, there is no difference in performance that can be attributed to the cables’ construction. It is inaccurate to believe that reduced bend radius optical fiber is the end-all solution when, in fact, there are many other factors that determine optical fiber link performance, including durability, connector pull-off resistance and connector performance.
When it comes to an optical fiber network, success may be measured in one or many ways— maximum system uptime, minimum operational and material costs, no lost revenue due to outages. Achieving these goals requires a complete cable management system that includes cable routing paths, cable and connector access, physical protection and, of course, bend radius protection.
In recognition of the increasing importance of fiber optics, we offer a SP4420 fiber certified training course.