As we all know, when optical fiber exceeds a certain bend radius, some amount of light can be lost, causing signal loss. This can happen during installation or anytime during fiber handling, and is often a concern within the tight spaces of high-density fiber patching areas in the data center. Today, a bend insensitive multimode fiber (BIMMF) was introduced, which can withstand tight bends, or even kinks, without suffering significant loss or any loss in a lot of cases.

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Fiberstore (FS.COM) | Does Bend Insensitive Multimode Fiber Make Sense? Page 1
As we all know, when optical fiber exceeds a certain bend radius, some amount of light can be lost,
causing signal loss. This can happen during installation or anytime during fiber handling, and is often
a concern within the tight spaces of high-density fiber patching areas in the data center. Today, a
bend insensitive multimode fiber (BIMMF) was introduced, which can withstand tight bends, or
even kinks, without suffering significant loss or any loss in a lot of cases. However, there are no
standards around BIMMF and there are concerns about compatibility between BIMMF and
traditional fibers. Besides, there are also questions around bandwidth measurements in the factory
and actual performance in the fields. So, does BIMMF really make sense? Let’s find the answer
together.
What Is Bend Insensitive Multimode Fiber?
Bend insensitive multimode fiber, first introduced in 2009, is quickly becoming the fiber of choice
for high-performance enterprise LANs and data centers. With the introduction of BIMMF, installers
were finally able to deploy fiber networks without fear of over-bending the fiber and degrading
performance. Compared with standard fibers, BIMMF has a specially engineered optical “trench”
added between the core and cladding. This trench contains the propagating modes within the fiber
core, even in an extreme bend. It retains more of the light that would have escaped the core of a
traditional multimode fiber. BIMMF enables more compact fiber management systems and to
improve space utilization in modules, enclosures, cabinets and patch fields. Today, BIMMF is widely
deployed in data centers and much has been published about its design and benefits.
Does Bend Insensitive
Multimode Fiber Make
Sense?

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Is BIMMF Compatible with Non-BIMMF?
The preceding core diameter and numerical aperture discussions revealed that there are mode-field
shape differences between traditional MMF and BIMMF. These differences fundamentally reduce
the match between these fiber types and can lead to elevated connection loss. However, modeling
and testing on BIMMF has shown that an optimized BIMMF is backward compatible and can be
mixed with non-BIMMF without inducing excess loss. There is
also evidence that connector incompatibility and fiber
geometry differences (core diameter) may cause direction
dependence regardless of fiber type. In fact, according to
most fiber manufacturers, BIMMF is fully compatible with
OM2, OM3 and OM4 standards for laser-optimized multimode fibers and is also backward
compatible with the installed base of non-laser-optimized 50µm multimode fibers.
What Are the Issues?
Except the compatibility of BIMMF, there still exist some other issues. All BIMMF designs exhibit a
length dependency if an overfilled launched is used. Higher-order modes that get launched into the
trench can remain there for some distance until they attenuate. These modes that are captured and
propagate within the trench area are referred to as “leaky modes.” This phenomenon affects splice
and connector loss. On the other hand, non-BIMMF does not have a length dependency. An
encircled flux launch mitigates the core diameter and numerical aperture length dependency for all
BIMMF designs. Further, an encircled flux launch accurately depicts the system performance.
Conclusion
BIMMF allows cabling installers to deploy a network with less worry about inducing bend loss due
to workmanship. Besides, it is also comparable and compatible with other non-bend insensitive
multimode fiber such as OM3 and OM4. For proper operation of BIMMF links, either homogenous
or mixed with legacy fiber, it is important to use a more tightly controlled launch—encircled flux. An
overfilled launch will trap more high-order modes in the trench and performance will be
compromised. With more and more fibers are being installed in smaller areas, requirements for a
higher bend radius become crucial. BIMMF helps mitigate link failures when optical cables undergo

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small-diameter bends, particularly when applied in data center jumpers/modules and for
high-performance computer applications, which really makes sense.
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Notice: This document is for informational purposes only and does not set forth any warranty, expressed or
implied, concerning any equipment, equipment features, or service offered or to be offered by Fiberstore.
Fiberstore reserves the right to make changes to this document at any time, without notice, and assumes no
responsibility for its use. This information document describes features that may not be currently available.
Contact a Fiberstore sales team for information on feature and product availability.
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