AN-125
Bend-Insensitive Multimode Fiber (BIMMF)
Over view
It is no secret that when standard optical fiber is bent, twisted, or otherwise contorted, it will exhibit increased
loss. Until recently it was an accepted fact that in certain situations, such as in patch panels, cabinets,
enclosures, and other places where space is limited or sharp corners or bends exist, there was a trade-off
between how small these spaces could be and how long the cables would have to be without significant bend
loss due to bending, twisting, or stretching. In 2009, however, a more durable, stress-resistant type of multimodecable was developed and it is revolutionizing fiber optics by allowing enclosures, patch-cabinets, and other typesof cable hubs to be designed much more compact than was previously possible without introducing loss.
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Bend Loss of Bend-Insensitive Multimode Fiber (BIMMF)
Standard MM Fiber
22mm diameter

BIMMF
22mm diameter

Loss = Medium 850nm
Medium 1300nm

4mm diameter

Loss =Loss = Low 850nm
Medium 1300nm

4mm diameter

High 850nm
High 1300nm

Loss =Medium 850nm
High 1300nm

Figure 1: Bend Loss of Bend-Insnesitve MultiMode Fiber (BIMMF)

Bend-Insensitive Multimode Fiber, or BIMMF, as its name suggests, is more impervious to bend loss thanstandard 50μm multimode cable. In fact, BIMMF is so resistant to bend loss that measurements taken with a
22mm mandrel produce essentially the same results as without a mandrel. Furthermore, a 4mm mandrel willproduce the results that would typically be exhibited by a 22mm mandrel on standard 50µm fiber.
Unfortunately, BIMMF cables are only optimized for 850nm and not for 1300nm. While the bend loss at 850nm is
as described above, the results at 1300nm are not much different than with standard 50µm fiber.

1OptoTest
Corp.
SANTEC CORPORATION

4750
Quetzal5823 Calle
Ohkusa-Nenjozaka,
Komaki,
Camarillo,
CA 93012
Aichi 485-0802, Japan
Doc: AN-125 Rev. A

7/15 /13+81-568-79-3536

8 05.987.170 0
SANTEC sales
USA @CORPORATION
optotest.com
USAToll Free +1-800-726-8321
engineering@ optotest.com

www.santec.com

SANTEC EUROPE LIMITED
Tel. +44-20-3176-1550

AN-125

Bend-Insensitive MultiMode Fiber (BIMMF)

How It Works
BIMMF is structurally the same as standard multimode cable but there is an added layer of glass between thecore and the cladding. This layer of glass, often referred to as the trench or moat, has a much lower refractive
index than the cladding. While the innermost modes are strongly guided to the end of the fiber, the modes
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that propagate towards the edge of the core are less strongly guided. Normally when the fiber undergoes
more and more stress, these modes are lost into the cladding and only the innermost modes reach the end ofthe cable. However with the addition of this extra layer of glass, the modes that are less strongly-guided getreflected back into the core and are more likely to propagate through the length of the cable. With such highNE W
signalfidelity, it is certainly unsurprising that BIMMF is quickly replacing standard 50μm multimode cable inapplications around the world.

Standard MM Fiber
n0

BIMMF

Refractive Indexn0

Refractive Index

Core RadiusCore Radius

n1

n1aa

Radial Position

50 µm

50 µm

125 µm

125 µmRadial Position

Figure 2: Basic differences between Standard MMF and BIMMF refractive index profiles.

Testing BIMMF

Numerous experiments have concluded that BIMMF can be tested and implemented in applications just as
any other 50μm cable could be used. One application where the use of BIMMF is strongly advised against is
as a reference cable. Testing insertion loss and return loss on BIMMF cables should be handled as though thecables were not bend-insensitive. In general, reference cables should always be non-BIMMF cables, the core
diameter of the reference cables should always match that of the cables under test, and the fibers should neverbe over-filled, but rather be conditioned to meet EF standards or be slightly under-filled.
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2OptoTest
Corp.
SANTEC CORPORATION

4750
Quetzal5823 Calle
Ohkusa-Nenjozaka,
Komaki,
Camarillo,
CA 93012
Aichi 485-0802, Japan
Doc: AN-125 Rev. A

7/15 /13+81-568-79-3536

8 05.987.170 0
SANTEC sales
USA @CORPORATION
optotest.com
USAToll Free +1-800-726-8321
engineering@ optotest.com

www.santec.com

SANTEC EUROPE LIMITED
Tel. +44-20-3176-1550