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.
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.
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 small-diameter bends, particularly when applied in data center jumpers/modules and for high-performance computer applications, which really makes sense.