What You Need to Know About Testing BIMMF

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What You Need to Know About Testing
BIMMF-Seymour Goldstein
www.flukenetworks.com 2006-2017 Fluke
Corporation
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What You Need to Know About Testing BIMMF
Developed for use in data centers and enterprise
network applications with a very tight loss
budget, bend insensitive multimode fiber (BIMMF)
is able to withstand tighter bends with
substantially less signal loss than non-BIMMF. A
BIMMF design tightly confines the higher-order
modes that are more likely to escape the fiber
core during bending. The design achieves this by
adding a specially engineered optical trench
between the fiber core and cladding. While BIMMF
allows cabling installers to deploy a network
with less worry about inducing bend loss due to
workmanship, there are some considerations when
it comes to testing. Lets take a closer look.
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What You Need to Know About Testing BIMMF
Mitigating Leaky Modes
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 propagate within the
trench are referred to as leaky modes and can
affect splice and connector loss. While accurate
testing of standard non-BIMMF fiber can be
achieved with a dual wavelength source and a
common test cord and mandrel, a standard 25 mm
mandrel does not do a good enough job of
stripping out the higher modes as the 850 nm
wavelength, resulting in pessimistic losses.
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What You Need to Know About Testing BIMMF
Mitigating Leaky Modes
While more turns around a smaller, uniquely
shaped 4 mm mandrel has been shown to strip out
higher order modes at 850 nm, the same mandrel
cannot be used at the 1300 nm wavelength.
Different loss readings between the two
wavelengths will be measured. Furthermore,
requiring a very small and uniquely shaped
mandrel design for each wavelength adds
complexity and confusion to the testing
process. Instead of mandrel testing, a more
tightly controlled launch condition such as
encircled flux (EF) is therefore required when
testing BIMMFit mitigates the problem of an
overfilled launch and leaky modes while enabling
consistent and accurate measurements at both
wavelengths.
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What You Need to Know About Testing BIMMF
Mixing and Matching
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. However, this also requires a more tightly
controlled launch such as EF. There is also
evidence that connector incompatibility and fiber
geometry differences (core diameter) may cause
direction dependence regardless of fiber type.
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What You Need to Know About Testing BIMMF
Launch Cord Selection
When testing multimode fiber links, attenuation
measurements are greatly influenced by the launch
condition of the light source. Because the use of
a BIMMF launch cord with a common mandrel for
both 850 nm and 1300 nm wavelengths causes
significantly different results, only non-BIMMF
fiber should be used in launch cords. To
complete a permanent link test using the
recommended 1-cord reference method, a receive
cord must be used to provide link attenuation
measures that include the installed fiber and the
two connections at each end of the link. While a
well-designed BIMMF test cord that matches the
installed cabling core diameter and numerical
aperture will work, it is prudent to also use a
non-BIMMF as the receive cord to avoid the
possibility of an overly optimistic test result.
This is an area that is still being studied.
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What You Need to Know About Testing BIMMF
So What Do You Need to Do?
Testing BIMMF is really no different than
testing other fiber types it requires an
encircled flux launch and non-BIMMF launch cord
attached to the light source, the recommended
1-cord reference method, and a non-BIMMF receive
cord when measuring a permanent link. The good
news is that with Fluke Networks
EF-compliant CertiFiber Pro, there is no extra
work at all. To take a closer look at test
methods for BIMMF and view the complete steps for
measuring and validating the attenuation of test
cords and permanent links, download the Fluke
Networks white paper, The Ins and Outs of
Testing Bend Insensitive Multimode Fiber (BIMMF)
The Need for Encircled Flux.
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