Bend-Insensitive Fiber – What Is It?

Written by Ben Hamlitsch, trueCABLE Technical and Product Innovation Manager RCDD, FOI

One of the important considerations when looking at optical fiber for installation is bending concerns. This is because fiber optic cable is sensitive to stress, particularly bending. When optical fiber is stressed by bending, the light that is transmitted through the core will start to scatter and escape causing some of the light to be lost from the core into the cladding and ultimately escaping as shown in the image below. Bending creates an even higher loss in the stressed section of the fiber. If you put a visible laser or (VFL) in a fiber and stress it, you can see the light lost by the bending stress. There are a number of things that fiber cable manufacturers will do to help with bending issues.

Bending losses are an inherent characteristic of fiber both in singlemode fiber (SMF) and multimode fiber (MMF) with fiber optic cabling showing more sensitivity and stress at longer wavelengths. In this case we can think about 1310nm and 1550nm which are the wavelengths used in singlemode fiber optic cables.

Optical fiber manufacturers knew that something needed to be done to improve structural characteristics of the optical fiber. In 2007, a new type of fiber called "bend-insensitive" singlemode fiber was introduced, which was followed by multimode fiber in 2009. Manufacturers began demonstrating this fiber by bending it around impossibly small bends or stapling it to a piece of wood, something no one would even think of doing in the early days of fiber. This was something unheard of and somewhat unbelievable. But the demonstrations showed that these fibers could be bent in ways that seemed impossible with very little light loss.

What did they do differently? Manufacturers were more willing to explain what they had done to make these fibers so tolerant to tight bends once they had all the patents filed. An additional material with a lower index of refraction was used between the core and cladding. This was built into the fiber to basically reflect the lost light back into the core of the fiber. It was first tested and proven in singlemode fiber. Once that proved to work well in 2009, manufacturers introduced multimode fibers, which showed using a similar technique could also improve bending loss in multimode fiber.

Check out the video below for a more visual explanation about Bend Insensitive Fiber!

Examining the design of bend-insensitive multimode fiber

Bend-insensitive fiber cables are special types of cables designed to keep light inside the cable even when the cables are bent more than usual. Unlike regular fiber cables, these have a special layer—a kind of "ring" around the core—that stops light from escaping. This design allows the signal to stay inside, reducing any data loss.

These cables are especially helpful in places with limited space, like data centers, where cables often need to be bent tightly to fit. They come in two main types: single-mode for long-distance connections and multi-mode for shorter, faster connections. In simple terms, bend-insensitive fiber cables are more flexible and durable, making them ideal for tight spaces where regular cables might struggle.

So now you know in simple terms what is happening in bend-insensitive fiber. Read on to get more of the technical details if you wish to know more. Or continue on to the next subheading.

In regular graded index multimode fiber, there are about 400 modes (or rays of light) being transmitted down the fiber. The inner modes are "strongly guided" which means they have little sensitivity to bending stresses. But the outer modes are "weakly guided" which means they can be stripped out of the core when the fiber is bent.

Bend-insensitive fiber adds a layer of glass around the core of the fiber which has a lower index of refraction. This literally "reflects" the weakly guided modes back into the core when stress normally causes them to be coupled into the cladding. Some early singlemode fibers used a similar technology to contain the light in the core of the fiber, but this design has a much stronger effect.

The trench, or moat as some people call it, surrounds the core in both bend-insensitive SMF and bend-insensitive MMF to reflect any lost light back into the core. The trench is just an annular ring of lower index glass surrounding the core with very carefully designed geometry to maximize the effect.

Bend-insensitive fiber (or BI fiber as it is now called, even BI MMF or BI SMF) has obvious advantages. In patch panels, it should not suffer from bending losses where the cables are tightly bent around the racks. In buildings, it allows fiber to be run inside molding around the ceiling or floor and around doors or windows without inducing high losses. It's also insurance against problems caused by careless installation.

Bend-insensitive fibers are available in 50/125 MM (OM3 and OM4) and SM versions. Considering the advantages of bend-insensitive fiber, and the small incremental cost to manufacture it, some manufacturers have decided to make all their 50/125 MM fiber bend-insensitive fiber.

Many applications for bend-insensitive singlemode fiber are in premises installations like apartment buildings or for patch cords, where it simplifies installation and use. Bend-insensitive singlemode fiber is also used in OSP cables since it allows fabrication of smaller, lighter, high fiber count cables.

Another application for bend-insensitive fibers is micro-cables and high fiber count cables. By reducing the fiber's sensitivity to stress, one can make the buffer diameter smaller (200 instead of 250 microns) and squeeze more fibers into smaller cables. This has led in two directions. Micro-cables are simply like loose tube cables shrunken to small sizes, about half the size of conventional cables. This 144-fiber cable is about the size of a pencil.

What does the TIA standards say about bend-insensitive fiber?

Based on the TIA-568.3-E standard for optical fiber cabling, bend-insensitive fiber cables are designed with greater flexibility and are less prone to signal loss when bent at sharper angles compared to standard fiber. This makes them ideal for high-density environments where tighter cable management is needed, such as in data centers or office spaces with limited routing space. Here are some details from the TIA-568.3-E for bend-insensitive fiber below.

Key Points on Bend-Insensitive Fiber Cable (BIF) in TIA-568.3-E

Minimum Bend Radius: There are two types of factors when stating cable bend radius. These are in an installed condition and under load. These are import to differentiate because these two conditions are looking at 1. Installing the physical cable (under load) and 2. In the installed condition once it is in place and in its final location in the cabling installation.

Installed Condition: Bend-insensitive fiber cables generally have a minimum bend radius as low as 7.5 mm for some single-mode fibers.

Under Load: Under tension during installation, bend radius recommendations can still vary, but many BIF cables can handle a radius of 10 times the fiber diameter.

It is always important to verify you are not exceeding the TIA-568.3-E standard but also or even more important is to verify with the manufacturer's data sheet since different BIF cables may have unique specifications.

When reviewing the TIA standard it will include guidance on singlemode and multimode bend-insensitive fibers. Singlemode bend-insensitive fibers (like G.657 fibers) are specifically optimized to perform with minimal attenuation under tight bending scenarios. Multimode BIFs, typically OM3 or OM4 types, are also designed for greater and more improved bending tolerance.

One of the reasons Bend-insensitive fibers perform better in a tighter bend application is because they have special cladding or coating structures that reduce attenuation, even in high-bend scenarios. This ensures they maintain lower signal loss and better performance in installations with tighter bends, which may otherwise cause high attenuation in traditional fibers. This was discussed previously in this article. 

TIA Standards Recommended Usage

TIA-568.3-E recommends bend-insensitive fibers for high-density installations, particularly for patch cords and cables that need frequent handling or tighter coiling.

They are also recommended where space constraints make standard bend radii impractical, such as in cable trays, racks, or enclosures.

While bend-insensitive fibers are more resilient, TIA-568.3-E emphasizes adherence to manufacturer-specific bend radius guidelines to avoid unnecessary stress and maintain long-term network reliability.

What is the standard bend radius of optical fiber?

Below we have a general fiber optic bend radius chart. Keep in mind that specific bend radii can vary based on the fiber type and manufacturer guidelines, so always refer to the specific cable’s data sheet when possible. It is also important to note that these are the minimum allowable bending during installation and after installation. Both of these minimums need to be followed in order to ensure the fiber optic cable will perform to its maximum performance. Once these minimums are exceeded it can drastically decrease the performance of the fiber cable. This can have drastic effects as the bandwidth required increases above 10 Gigabit applications.

Bend Radius During Installation: This is the minimum bend radius when pulling or placing the cable, as it will experience additional stress.

Bend Radius After Installation: This is the minimum bend radius for the cable in its installed state with minimal external forces. Some of these external forces to watch out for and keep in mind when installing fiber cable are:

1. Cable ties: Ensure if cable ties are used they are not tightened down too much causing it to pinch the cable. (The recommended use for cable straps are velcro tie wraps. These are far better in eliminating macro bending of the fiber cable.)
2. Staples: Do not use staples to secure your fiber optic cables. Staples will almost always cause issues in a couple ways. One way is by causing excessive macro bending and the other is the cable accidentally piercing the jacket and potentially hitting the fiber strands.

Exceeding the minimum bend radius can lead to signal loss or damage to the fibers.

What are some singlemode types of bend-insensitive fiber?

Let’s take a closer look at the G.657 Cat A and Cat B Singlemode fiber types.

Historically, the fiber optic industry has standardized on the widely used ITU-T G.652 Singlemode fiber (SMF) cable for network alignment, installation, and operation. Nonetheless, SMF has been optimized for future deployments where low macrobending loss is necessary because of the density of fiber in the drop component of our network and because of the lack of room in many other parts of the network. While several ITU-T standards are at work in our optical networks, the most recent edition of G.657 for SMF cable provides additional optimizations for fiber rollouts.

Existing networks that use the G.652 SMF cable standard get a boost from G.657. For full optical spectrum functioning, the most up-to-date subcategory, G.652D, is preferred because of its lower water peak. The bending radius of G.652D, however, is restricted. According to G.657 Edition 4, there are several installer-friendly improvements that benefit the fiber installation performance. G.657's reduced attenuation in bends makes it a good fit for fiber access networks. The downsizing of enclosures, cabinets, pedestals, and terminations makes G.657 more suitable for usage in the access network, even in confined spaces. FTTH MDU deployments are another successful area for G.657.

For G.657, we distinguish between two primary groups: category A (cat A) and category B. (cat B). They're complementary in that they both encompass the full range of optical frequencies from 1260 nm to 1625 nm (O, E, S, C, and L). Cat A complies with G.652D specifications for use in the transport and access networks across long distances. Since Cat A has the same transmission and connectivity properties as the other networks specified by G.652D, it can be utilized for them all. Cat A provides improved macrobending losses compared to G.652D. Cat B is not fully compliant with G.652, however, it is aligned to G.657A and G.652D in access networks. Cat B was designed to support very low macrobending losses at a low bending radius for short-reach (< 1000 m) applications. Cat B is designed for near building (e.g., riser) and in building deployments like MDUs or for business service applications.

Cat A has two subcategories, A1 and A2, for use in long-range transit and access applications. The G.657. The minimal design bend radius in A1 is 10 mm, but in G.657.A2 the bend radius is 7.5 mm. Both of these are enhancements above the 15-30 mm bend range of the G.652 standard.

Drop applications with a limited range can choose between the B2 and B3 subcategories of cat B. The G.657.B2 specifies a minimum design radius of 7.5 mm, while the G.657.B3 specifies 5 mm.

If you're a fiber installer, you'll do better if you're familiar with the features and classifications of G.657 Singlemode fiber. Below is a image and table showing the bend radius for each of the singlemode Category A and Category B fiber cables.

What about compatibility with conventional fiber cable?

Many questions often arise, such as: “Are these fibers compatible with regular fibers?” “Can they be spliced or connected to other conventional (non-BI) fibers without problems?” “How does the inclusion of higher order modes affect bandwidth?” The answers seem to be yes for all singlemode fibers. Since only one mode is guided in the core, the trench has a minimal impact on system performance and measurement. It seems you can mix and match regular and bend-insensitive singlemode fibers with no problems.

For MM fibers, it is less clear. Measurements of core size, numerical aperture, differential mode delay (DMD) and bandwidth, were developed prior to the introduction of bend-insensitive multimode fiber designs. For the most part, bend-insensitive multimode fiber can be made to be compatible to other non-bend insensitive fibers by modifying the core design slightly or careful engineering of the refractive trench surrounding the core. This may not eliminate all of the differences, but it does help.

Another potential factor is bend-insensitive multimode fiber may have a larger effective NA and core size than conventional multimode fibers since they propagate "leaky modes" that are attenuated in conventional fiber designs. This may affect splice or connector loss when mating BI MMF with conventional MMF but usually only in one direction, from BI MMF to conventional MMF, in a manner similar to the losses seen from mismatched fiber.

One approach to make BI MMF compatible with non-BI fibers is to modify the core index profile slightly to reduce the higher order modes. This makes them compatible to conventional fibers without otherwise materially affecting the performance of the fiber. A second approach is to leave the core index profile alone but carefully engineer the trench to produce the bend-insensitivity.

However, this is only an issue if you are installing bend-insensitive fiber cable with non-bend-insensitive stand fiber cables. If the entire installation is bend-insensitive, then these issues should not be a problem. Therefore, the testing and certification for the given installation should not have any real issue. In today’s fiber installations, more and more bend-insensitive fiber is being used and for good reason; it is much more tolerant to the effects of real-world cabling installations.

So what does it all Cost?

The big question at the end of the day is “How much does it cost.” this is typically the second most important question after performance. Cost drives many decisions when it comes to determining what type of cabling and connectivity is used in a project. When it comes to multimode cable today bend-insensitive fiber is standard and in most cases will be the go to options for OM3 or OM4 cable. Bend-insensitive fiber has minimal additional cost. The cost of installing bend-insensitive fiber cables is only slightly higher than standard fiber optic cables. 

But the benefits of Bend-Insensitive Fiber Optic Cable outweigh any small cost differences. Some of the benefits are: 

Flexible Installation Options: Bend-insensitive fiber optic cables are ideal for indoor setups. They allow for easy routing around walls, pillars, ceilings, ducts, and other irregular surfaces within buildings without concerns about excessive bending.

Reliable Performance: With bend-insensitive fibers, high-bandwidth applications can be implemented with confidence, as accidental bending does not significantly impact performance.

Enhanced Durability: These cables offer high resilience in conditions where they are secured to surfaces with clamps, tie wraps, or staples.

Compatibility with Standard Splicing: Bend-insensitive fiber cables can be spliced just as easily as standard fiber optic cables, using the same methods.

In simple terms, bend-insensitive fiber optic cables are a special kind of cable that works well even when you have to bend them a lot. These cables keep the light inside, even around tight corners, which helps them work reliably without losing any signals. They’re great for places where cables need to be squeezed into small spaces, like big data centers or buildings with lots of wiring. They’re also strong and don’t break easily when they’re bent or pulled, and they don’t cost much more than regular fiber cables. Because of all these benefits, people prefer using bend-insensitive cables in places that need sturdy, reliable networks that work and allow for upgrades for a long time.

HAPPY NETWORKING!

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