Bend-Insensitive FiberOptical fiber is sensitive to stress, particularly bending. When stressed by bending, light in the outer part of the core is no longer guided in the core of the fiber so some is lost, coupled from the core into the cladding, creating a higher loss in the stressed section of the fiber. If you put a visible laser in a fiber and stress it, you can see the light lost by the stress as in this multimode fiber.  Fiber coatings and cables are designed to prevent as much bending loss as possible, but it's part of the nature of the fiber design. Bending losses are a function of the fiber type (SM or MM), fiber design (core diameter and NA), transmission wavelength (longer wavelengths are more sensitive to stress) and cable design. In 2007, a new type of "bend-insensitive" singlemode fiber was introduced, followed by multimode fiber in 2009. Manufacturers liked to demonstrate this fiber by bending it around impossibly small bends or stapling it to a piece of wood - demonstrations that made veterans of the business cringe at seeing fiber treated so badly! But the demonstrations showed that these fibers could be bent in what seemed like impossibly small radii without significant light loss, although skeptics still wondered about the long term effects of this kind of abuse on reliability. Once the patents were filed, manufacturers were more willing to explain what they had done to make these fibers so tolerant to tight bends. An optical "trench" - the term used for a ring of lower index of refraction material - was built into the fiber to basically reflect the lost light back into the core of the fiber. It turns out, the design was similar to a type of singlemode fiber called "depressed cladding" fiber that was first introduced in the late 1980s. And in 2009, manufacturers introduced multimode fibers that showed using using a similar technique could also improve bending loss in multimode fiber. Let's examine the design of bend-insensitive multimode fiber (which we will usually call by its acronym BI MMF) that shows the technique.  In regular graded index multimode fiber, there are many modes (or rays of light - about 400 of them) 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 that 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 (depressed-cladding 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 BI SMF and BI MMF to reflect 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. See the red ring around the core on this fiber drawing.  When you look at the end of a bend-insensitive fiber in a microscope with angled lighting, you can sometimes actually see the trench as a gray ring around the core.  File
courtesy of PanduitAnd here are two fiber comparisons from the field. The BI fiber is the more complex structure on the right.  50/125 MMF, regular (L) and bend-insensitive (R)  SMF, regular (L) and bend-insensitive (R) 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. BI fibers are available in 50/125 MM (OM3 and OM4) and SM versions. Considering the advantages of BI 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 BI SM fiber are in premises installations like apartment buildings or for patchcords, where it simplifies installation and use. BI SMF is also used in OSP cables since it allows fabrication of smaller, lighter high fiber count cables. Another application for BI fibers is microcables 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. Microcables are simply like loose tube cables shrunken to really small sizes, about half the size of conventional cables. This 144 fiber cable is about the size of a pencil.  A 144 fiber microcable compared to a pencil.The other use for BI fibers is is make cables with as many fibers as possible. This has led to cables with 1728 fibers like this one from Prysmian as well as 3456 and 6912 fibers.  
Compatibility With Conventional Fibers One question that was asked when these fibers were introduced is are they compatible with regular fibers. Can they be spliced or connected to other conventional (non-BI) fibers without problems? That answer seems to be yes for all SM fibers. Many manufacturers have modified their singlemode fiber to ensure compatibility. In fact the conventional G.652 fiber is now more like G.657.A1 BI SMF. With the introduction of BI singlemode fiber, new standards were written as G.657 fiber with several grades, each having a minimum bending diameter and loss specification.  G.652 fiber bend radius 30mm (The G.657 standard notes "ITU-T G.652 fibres deployed at a radius of 15 mm generally can have macrobending losses of several dB per 10 turns at 1625 nm.") G.657 fiber (bending loss specs at 1550nm) G.657.A1 bend radius 10mm, loss 0.75dB/turn G.657.A2 bend radius 7.5mm, loss 0.5dB/turn G.657.B3 bend radius 5mm, loss 0.15dB/turn (for special applications) Designing singlemode fibers requires tradeoffs. A smaller mode field diameter will have better bend performance but higher attenuation. Larger MFD provides lower attenuation, and the majority of G.652 fiber, which is much of the installed base, is a MFD of 9.2 µm. Simply reducing MFD for better bend performance leads to mismatch losses when splicing or connecting fibers and causes OTDR tests with gainers, requiring time consuming bidirectional testing. The right way to create a BI singlemode fiber is to redesign it to get BI performance while maintaining a larger MFD for compatibility and lower attenuation. And that's what has already happened at some fiber manufacturers with standard 250 micron and smaller buffer coating fibers. Here is what two manufacturers say: Corning: The industry is moving towards a G.657.A specification in fiber, because the industry is moving towards smaller denser cables in the network & the bend resilience is a requirement for the cable design. The industry will not move wholesale towards a G.657.A2 specification because this is not necessary in all cases. There is no need to compromise on the 9.2 um MFD to get a G.657.A fiber because Corning innovation delivers this, alongside the bend resilience in; SMF-28 Ultra and SMF-28 Contour fibers. Worth reading: Corning ap note AN2020 on splicing compatibility. OFS: The simple answer is most SMF is moving to G.657.A1. OFS AllWave+ and Corning’s Ultra fiber which are among the most deployed fibers in America right now are both examples of this trend. There have been some modifications to the G.657 specification that puts more stringent boundaries on MFD to assure compatibility of BI fiber with standard G.652 fiber. Further ITU has studied the full set of transmission parameters for G.657A1 and A2 fibers and has stated that the products are fully compatible. That said, smaller MFD’s have better macrobend performance and as a result many of the more bend insensitive G.657.A2 and G.657.B3 fibers on the market may show artifacts in one way OTDR traces due to the MFD change. See the FOA Newsletter, Features and Technical sections for July 2024 for more on the compatibility of G.652 and G.657.A1 fibers. So singlemode fiber is moving to being BI fiber, exactly what happened with 50/125 laser optimized fibers a decade ago. With most new fiber, compatibility is not an issue. But it is recommended to check with the cable manufacturer if you are not sure what fiber is being used in the cable you are purchasing. For multimode (MM) fibers, the answer was less clear. How does the inclusion of higher order modes affect bandwidth? Measurement of core size, NA, differential mode delay (DMD) and bandwidth were developed prior to the introduction of BI MMF designs. For the most part, it appears that BI MM fiber can be made to be compatible to other non-BI fibers by modifying the core design slightly or careful engineering of the trench surrounding the core 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 to make 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. Today, essentially all MM fiber is bend-insensitive and non-BI fiber is difficult to find. When the compatibility of BI and non-BI MM fiber was being questioned, testing standards for MM fiber were written to require using non-BI fiber for reference test cables. Once practically all MM fiber became BI fiber and that fiber was identical to non BI MM fiber, that became an issue for standards committees to correct. Today all MM testing standards are being rewritten to allow any type of MMF for reference test cables. If you want to read more on the past controversy about compatibility of BI MM fibers, see the FOA Newsletter for February, 2011. FOA thanks major manufacturers of optical fiber for their contributions to this page. Table of Contents: The FOA Reference Guide To Fiber Optics |
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