What is Reduced Bend Radius
Optical Fiber?


As mentioned above, reduced bend radius fiber is able to withstand tighter bends within frames, panels and pathways. To understand how this is achieved, it is important to understand that all fiber types rely on principles of Total Internal Reflection, which allows a light signal to travel from one end of the fiber to another (see Figure 1). By improving the bend radius of optical fiber, light entering the core is effectively reflected by the cladding back into the core. Instead of using a matched clad profile, some constructions of reduced bend radius optical fiber use a depressed clad profile with a lower index of refraction than the core, causing light to stay within this core.

To achieve tighter bend radii, some constructions change the Mode Field Diameter (MFD)- the area across the core of the fiber that fills with light. Typical MFD for standard singlemode optical fiber is about 10.4μm; reduced bend radius optical fiber may exhibit MFD of between 8.9μm and 10.3μm
Regardless of the type of construction, all reduced bend radius fiber products do one thing very well—they can perform under a tighter bend radius where macrobends occur. Examples include a CO application, where fiber exits from a panel into a vertical cable route; or in an FTTX deployment, within the confines of an optical network terminal (ONT).

The fiber’s performance is definitely impressive. For example, in ADC tests a standard singlemode optical fiber with one turn around a 1.26″ (32 mm) diameter mandrel shows induced attenuation of less than 0.50 dB at 1550 nm. This same test on a reduced bend radius singlemode 1550 nm optical fiber shows less than 0.02 dB of attenuation.

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.

Does it Improve Performance?

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 any 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.


Reduced bend radius optical fiber offers the same crush resistance and tensile strength as the same cable with standard singlemode fiber. As with standard optical fiber, excessive weight will crush reduced bend radius optical fiber and excessive pulling tension will damage the cable, both of which affect attenuation.


Connector pull-off resistance

Cable assemblies and connectors must meet Telcordia® (GR326) requirements for strength of the fiber termination connector. Reduced bend radius optical fiber does not improve connector pull-off resistance. Connectors that are easily loosened or disconnected increase attenuation and cause failures.


Connector performance

When it comes to connector performance, endface characteristics determines loss from the connector. Reduced bend radius optical fiber does not impact insertion loss from connectors, making termination and quality of connectors an important consideration in link performance.

Bend Radius Protection – Just
One Component of Proper Cable
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. 

Bend radius protection
Reduced bend radius optical fiber and patch cords that protect against macrobends are a good start—but only a start. First, there is a limit on the bend radius; it is smaller, but it still exists. Second, reduced bend radius optical fiber is still subject to microbends, which can permanently damage cable. Proper slack storage is still required to ensure optical fiber and patch cords neither exceed bend radius limits nor encounter sharp edges on
frames and throughout pathways.

Cable routing paths
It is essential to be proactive when it comes to cable handling. Vague routing paths create congestion that reduces density, confuses the next technician and strands capacity with haphazard use of precious space. Clear, intuitive cable routing paths are essential for management of fibers in cabinets, racks, raceways and panels


Connector and cable access

Front and rear connector access is essential for service turn-up, rearrangements and troubleshooting. All connectors need to be accessible without disturbing adjacent fibers and all connections must be easily identified to prevent pulling the wrong termination. There is a physical limit to density. When connectors cannot be properly removed or seated, service can be affected. When a configuration is so dense, chassis port can become inaccessible and are not fully utilized.


Physical protection

Once optical fibers are installed, they should be protected throughout the network from accidental damage by technicians and equipment. Fibers routed between pieces of equipment without proper protection are susceptible to damage, which can critically impact network reliability.
Robust cable management ensures that every fiber is wellprotected and designed to withstand daily wear and tear.


Proper Applications for Reduced

Bend Radius Optical Fiber
Singlemode reduced bend radius optical fiber offers benefits for applications that including the central office, FTTX deployments, the data center and OEM solutions.

Singlemode reduced bend radius optical fiber is best suited for environments where little or no bend radius protection is available. It is also ideal for applications where space is an issue. Specific applications that make sense for this type of fiber are described below: 

Space is tight For drop cable or termination of pigtails in multiple dwelling unit (MDU) and optical network terminal (ONT) boxes for FTTX deployments—where there is no space and often no cable management—reduced bend radius optical fiber offers less chance of increased attenuation during field installation and maintenance.


No fiber management is available

The front of frames and routers—where moves/adds/ changes occur—is ideal for use of reduced bend radius patch cords and multifiber breakout assemblies. Many OEM active components do not have bend radius limiters or protection on the front of the equipment.


Space is at a premium

Patch cords and multifiber breakout assemblies that can bend more tightly enable increasing density of active equipment in racks and cabinets without sacrificing access. For manufacturers of active equipment, reduced bend radius optical fiber can help reduce size of electronics, improving density and airflow. However, in these applications, even more consideration must be paid to the elements of proper cable management. Tighter bend radius also offers OEMs the chance to increase the functionality of active equipment by utilizing less chassis space. 

Of course, a key advantage of reduced bend radius optical fiber is use in high bandwidth applications. For standard optical fiber, the 1625 nm to 1550 nm wavelengths are the first to go when the cable is wrapped around a mandrel. Preserving these wavelengths around tighter bends offers benefits for OEMs seeking to improve functionality of network equipment or network managers looking for the efficiency of having all wavelengths available on a given optical link.