Guide to Bend-insensitive Multimode Fiber

A common problem facing cabling installers during installation or other fiber handling is the light losses or weaker optical signal, which is cause by the fact that when a fiber bend radius exceeds the specified figure, the angle at which the light hits the cladding changes and some light will escape and result in power loss. It’s known that light losses is the commonplace found in fibers, like LC-SC multimode fiber patch cord. In 2009, bend-insensitive multi-mode fiber (BIMMF) has been introduced, which can withstand tight bends, or even kinks, or other withstand tough treatment without suffering significant light losses in many cases. It seems to be an ideal product. How much do you know about it? Have some ideas about its working principle, its compatibility, its testing issues? Follow this article and find answers.

An introduction to the working principle helps you to understand how BIMMF reduces signal losses with its bend-insensitivity and becomes the preferred choice in data center applications.

BIMMF technology prevents light from escaping. BIMMF has an innovative core design that incorporates a graded-index core profile combined with a specially engineered optical trench. A specially engineered optical trench (image below) is used to trap the light in the many modes which propagate within the fiber core. The trench, or also called moat, with low refractive index, surrounds the core in BIMMF to reflect lost light back into the core. It acts like a barrier for propagating light. Keeping the light in the core, even in the most challenging bending scenarios, significantly reduces the bend-induced. The trench is just an annular ring of lower index glass surrounding the core with very carefully designed geometry to maximize the effect.

One question with BIMMF is whether it’s compatible with conventional 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?

Measurement of core size, NA, differential mode delay (DMD) and bandwidth were developed prior to the introduction of BI MMF designs. These measurements are in the process of being evaluated and updated, so measurement results may depend on the manufacturer of the BIMMF. For the most part, it appears that BIMMF 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, but at this point it is left to the manufacturers to show their product will perform equivalently to the installed base of fiber.

When short lengths of BIMMFs are measured, they may have a larger effective NA and core size than conventional MMFs since they propagate "leaky modes" that are attenuated in conventional fiber designs. This may affect splice or connector loss when mating BIMMF with conventional MMF but usually only in one direction, from BIMMF to conventional MMF, in a manner similar to the losses from mismatched fibers.

Testing BIMMFs or using them for reference cables for testing is another matter. For testing, link set ups included both all BIMMF channels and mixed links using a combination of BIMMF assemblies and standard non‐BIMMF assemblies. 10G and 40G testing used full duplex traffic with two transceivers (e.g. E10GSFPLR or QFX-QSFP-40G-SR4), and this approach is necessary, since the trunk cables contain only 12 fibers and per the standard, and full duplex traffic at 100G requires 20 fibers. The channel configurations for the various active tests are shown below.

This kind of cable 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.

However, BIMMF profiles pose a complex design challenge. Profile parameters must be carefully selected to ensure all key attributes satisfy all relevant industry standards. Bandwidth is a key parameter for MMFs and the graded-index profile remains a key driver to achieve high bandwidth. Any deviation from the optimum profile results in reduction of bandwidth of the MMFs. Inappropriate trench location can result in delay errors of the higher-order modes, and this can significantly impair bandwidth performance. Locating the trench too far from the fiber core can result in failure to comply with fundamental industry standards. The fiber's trench location must be carefully engineered to ensure superior bandwidth and macrobend response while maintaining industry standard requirements for core diameter, numerical aperture and chromatic dispersion.

BIMMF is fully compliant with the OM3 and OM4 standards for laser-optimized fibers, and is also backward compatible with the installed base of 50-μm MMFs. With its improved bend -sensitivity, BIMMF allows for less light losses in the stressed section of the fiber, reducing the challenges encountered in installations in local area network (LAN) data centers.