Why Not Use Bend Insensitive Fiber Optic Cable to Reduce Bend Radius?

Fiber optic market has witnessed the increased use of optical fiber cable assemblies. FTTx networks are the impetus for the adoption of fiber cables. During installation of these cables, more attention is focused on the effect of bend radius and the need to maintain a specified cable bend radius. So far, are you still worried about bending the fiber optic cables? Bend insensitive fiber optic cable can help you solve this problem.

Bend Radius of Fiber Optic Cable

Because optical fiber is sensitive to stress, light may leak out when the fiber is bent. As the bending becomes more acute, more light leaks out (shown in the picture below). Therefore, when installing fiber patch cables, especially in tight spaces of high-density fiber patching areas, you are not supposed to bend them beyond their bend radius. What is on earth the bend radius?

Figure 1: Bend Radius of Fiber Optic Cable

What Is Bend Radius and Minimum Bend Radius of Fiber Optic Cable?

Bend radius is the amount of bending that can occur before a cable may sustain damage or increased attenuation and limit bandwidth performance. When a fiber cable is bent excessively, the optical signal within the cable may refract and escape through the fiber cladding. Bending can also permanently damage the fiber by causing micro cracks. The result is known as bend loss: a loss of signal strength that may compromise the integrity of the data transmission.

Note: Excessive pulling tension and overly tight fasteners can also cause transmission problems and micro-bends in optical fiber cables.

Figure 2: Minimum Bend Radius of Fiber Optic Cable

The minimum bend radius is the smallest allowable radius for a given fiber optic cable to be bent around. The new standard ANSI/TIA/EIA-568B.3 sets performance specifications, minimum bend radius standards and maximum pulling tensions for 50/125-micron and 62.5/125-micron fiber optic cables. For inside plant cable, the fiber cable bend radius is 10 times the cable’s outside diameter under no pull load, and 15 times the cable’s outside diameter when subject to tensile load.

How to Calculate Bend Radius of Fiber Optic Cable?

In generally, the allowable bend radius varies based on cable type, outside diameter (OD), and the condition of the cable under stress both during installation (tensile load) and after installation when the cable is reset (no-load). The following formula is used to calculate bend radius:
Minimum Bend Radius = Cable Outer Diameter x Cable Multiplier

The cable multiplier is determined by industry standards and cable type. For fiber optic cables, the cable multiplier is six times for cables rated 5000 volts or less, eight times for cables rated over 5000 volts.

What Is Bend Insensitive Fiber Optic Cable?

Bend insensitive fiber patch cable is designed to transmit light with minimum loss even if they are bent beyond the bend radius. In these BIF optical cables, an optical “trench” – a ring of lower index of refraction material – is built into the fiber to basically reflect the lost light back into the core of the fiber, thereby minimizing data loss. Bend insensitive fiber cable offers greater flexibility in demanding environments than traditional fiber cable. It is typically used in data centers or any space constrained area where tight bends and flexibility are required. Bend insensitive fibers are available in 50/125 MMF (OM3 and OM4) and 9/125 SMF versions.

Figure 3: Bend Insensitive Fiber

Bend Insensitive Multimode Fiber Optic Cable (BIMMF)

Multimode fiber is popular in data centers and intra-building backbones. In the era of increased connectivity, more demands on the quality and performance of link components become more critical. New bend insensitive multimode fiber minimizes bend-induced attenuation, which helps maximize system reliability and minimize downtime. These BIMMF cables are available in all laser optimized grades, OM2, OM3 and OM4, and exhibits 10 times less signal loss in tight bend scenarios. The fiber can be installed in loops as small as 7.5mm radius with less than 0.2dB bending loss at 850nm and 0.5dB at 1300nm.

Figure 4: Bend Insensitive Multimode Fiber Optic Cable (BIMMF)

Bend Insensitive Single-mode Fiber Optic Cable (BISMF)

In 2007, a new type of “bend-insensitive” single mode fiber was introduced. It can withstand stress from bending, twisting, or stretching without suffering significant performance loss. ITU recommendation G.657 specifies two classes of single mode bend insensitive fiber patch cables: G.657 A and G.657 B. The minimum bend radius of G.657. A1 fibers is 10mm, of the G.657. A2 and G.657.B1 fibers is 7.5mm and of the G.657.B2 fibers is 5mm. Compared with minimum bend radius of the standard single mode G652 fibers which is usually 30 mm, G.657 single mode bend insensitive fiber patch cables are much more flexible. Thus, BISMF cables can be confidently installed with a variety of installation methods and in the increasingly high-density application spaces of today’s data center.

Figure 5: Bend Insensitive Single-mode Fiber Optic Cable (BISMF)

Advantages of Bend Insensitive Fiber Optic Cable

Flexible Installations: Bend insensitive fiber cables are very useful for indoor fiber cable installations as they can now be taken around walls, pillars, ceilings, ducts, and other uneven surfaces within the buildings without worrying about excessive fiber bends.

High Performance: Higher bandwidth applications can be confidently deployed using bend insensitive fibers as accidental excessive bending of fibers does not cause much of performance degradation.

Great Resilience: Bend insensitive fibers also show a great deal of resilience in situations where fibers are fixed to surfaces using clamping, tie-wrapping or stapling.

Small Incremental Cost: The cost of manufacturing bend insensitive fibers is not very high when compared to the cost of manufacturing normal fiber cables.

Same Splicing Methods: Bend insensitive fiber cables can be spliced using the same methods used for normal cables.

Compatibility of Bend Insensitive Fiber Optic Cable

Bend insensitive fiber optic cable has so many advantages. One question may arise: are these BIF optical cables compatible with regular fibers? The answer seems to be yes for all SMF cables. Since only one mode is guided in the core, the trench has a minimal impact on system performance and measurement. But for MMF cables, it is less clear. Measurement of core size, differential mode delay (DMD) and bandwidth was developed prior to the introduction of BIMMF designs. These measurements are in the process of being evaluated and updated so measurement results may depend on the manufacturer of the BIMMF.

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