Compatibility of Bend Optimized Multimode Fibers
White Paper
Issued September 2009
Optimized Low Bend for the Use of Fibers In The Home
The current migration of optical fibers closer to and even into the home has increased
service providers interests in new fibers with improved bending performance. After the
successful introduction of low bend SM fibers, several leading fiber manufacturers have
recently launched bend optimized versions of their premium high bandwidth (OM3,
OM4) multimode fiber products, so as to reduce or even eliminate challenges encoun-
New fibers need
improved bending
performance for the use
in the home
tered during fiber installations. To confirm their promises, we examined bend optimized
OM3 fiber performances of different companies in real world conditions when compared
to standard multimode fibers. The investigations proved that these bend optimized fibers
are good candidates for loss and bend critical applications (such as indoor wiring) because of their higher immunity to bending losses, without loosing performances or compatibility to other standard high bandwidth multimode fibers.
Roger Krähenbühl, Ph.D.
Hanspeter Schiess
Claudio Cecchin
R+D Technology
Fiber Optics Division
Product Unit Manager
Fiber Optics Division
Product Unit Manager
Fiber Optics Division
HUBER+SUHNER AG
9100 Herisau, Switzerland
[email protected]
HUBER+SUHNER AG
9100 Herisau, Switzerland
[email protected]
HUBER+SUHNER AG
9100 Herisau, Switzerland
[email protected]
2
New Challenges Created from new Applications
Introduction
Optical fibers have already been used for a long time in environments where small bends were
not a major requirement. Recently, indoor and data center applications with tighter restrictions
have led to investigations in the bending performance of optical fibers. First, the optical network
market has seen a flurry of introductions of various grades of low bend radii singlemode fibers [1], with the consequence of recent updates to the low bend fiber standard (ITU G.657).
Despite the absence of a bend insensitive multimode fiber standard, recently, extended band
width (ISO/IEC 11801 OM3, OM4) multimode fiber (MMF) with enhanced macro bending
performance were introduced [2][3], designed to reduce and, in some cases, eliminate the cable bending challenges encountered in installations in local area networks (LAN), data centers
and other enterprise applications.
Compared with singlemode optical fibers, multimode fibers have a larger core that guide
multiple modes (or rays) of light simultaneously. Modes traveling at the outside edge of the
core have a longer distance to go than modes that travel near the center of the core (modal
dispersion). To minimize this modal dispersion and thereby maximize bandwidth, special care is
taken in the profile design of the refractive index [3]. Proper design (parabolic shape) will slow
down modes with a shorter distance so that all modes arrive at the end of the fiber as close in
time as possible. Such improvements resulted in products that fulfill bandwidth requirements as
given in the OM3 and the just recently published OM4 standards.
When you bend a traditional optical fiber, the tendency is for the light to keep going straight
and escape from the fiber. As the bend radius decreases, the amount of light that leaks out of
the core increases. This escaped light causes signal degradation and increases the potential
for transmission errors. To enhance bend performance in multimode fibers a unique approach
is required (as compared to singlemode fiber) to influence the many modes of light traveling
down the core without adversely affect fiber bandwidth (modes arriving simultaneously). After
attempts to address bending in multimode fibers reaching only low bandwidth performance,
new bend optimized multimode fibers now are able to deliver superior bandwidth performance in an ultra-bendable package without requiring any adjustments to standard field installation, termination monitoring, or maintenance procedures. The bend improvement in these
new multimode fibers is achieved through the use of a specially engineered optical barrier
(trench) that traps the many modes within the fiber core. This optical trench ensures that the outer
modes, which traditionally have a higher tendency to "leak" out of a multimode fiber when it
is subjected to bends, stay put [4]. By keeping these modes within the core of the fiber, much
less of the information carrying signal is lost and more information gets to the end user without
dropped packets or corrupted data.
Tighter restructions for
the bending performance due to the use of
optical fibers in indoor
applications
Important profile design
Preventing the escape
of the light
3
New Challenges Created from new Applications
Such bend optimized fibers claim to obtain extremely low bending loss at both 850nm and
1300nm, while keeping the advantages of existing OM3/OM4 fibers. These fibers should
be bendable down to a radius of 7.5mm with less than 0.2dB added loss at 850nm. At
a 15mm radius the added loss is less than 0.1dB. This is up to a 10x improvement in bend
loss compared with traditional multimode bending standards (Table 1). To verify these
claims, prove their usability and interoperability to standard MMF, we examined the most
widely used products. We took the bend optimized fiber type MaxCap® -BB-OM3/OM4
of Draka®, ClearCurve® OM3 fiber of Corning® and LaserWave® FLEX of OFS®, to execute
different tests and simulate daily use while comparing them to standard OM2 and OM3 fibers.
IEC 60793-2-10
ITU G.651.1
Bending radius
37.5 mm
15 mm
15 mm
7.5 mm
Number of turns
100
2
2
2
Max loss @ 850 nm
0.5 dB
1.0 dB
0.1 dB
0.2 dB
Max loss @ 1300 nm
0.5 dB
1.0 dB
0.3 dB
0.5 dB
Extremely low
bending loss
Bend optimized
Table 1Multimode bend standards and specifications given by the fiber manufacturers for the new bend optimized fibers.
For the following tests these types of fibers and standard OM2/OM3 of the same manufacturers were used to fabricate properly but neutrally marked 0.9mm semi-tight tubes (Figure
1, left) and 2.0mm indoor type simplex cables (Figure 1, right).
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Figure 1Bend optimized fibers (BO1-BO3) and standard OM2/OM3 fibers of different manufacturers in 0.9 mm semi-tight
tubes (left) and in 2.0 mm indoor type simplex cables (right).
4
Bending Performance
Bending performance
To determine the bending performance of all 0.9 mm type tube samples, standardized bending loss tests (IEC 60794-1-2 E11A) were carried out. Thereby the induced fiber attenuations for different number of turns, on mandrels with different radii at selected wavelength were
monitored. The non-linear behavior of the results obtained for consecutive number of turns
around a mandrel with r = 5 mm at λ = 850 nm (Figure 2, left) is an indication of the mode
filtering (outer modes are less guided) effect of MMF bending. Together with the bending
radius dependency (Figure 2, middle), it is apparent that bend optimized fibers (reddish lines)
present a well improved bending loss behavior compared to standard OM2 and OM3 fibers (bluish lines) and clearly fulfill the performance specification (0.1 dB for two turns around
r = 7.5 mm at λ = 850 nm) as given in Table 1. The wavelength dependences (Figure 2, right),
show a variation between the bend optimized fibers at longer wavelengths (e.g. BO3 shows
worst transmission at 850 nm and best at 1625 nm), this could be an indication of slightly different bend optimized refractive index profiles. All of these bending measurements clearly indicate that the bend optimized fibers of all chosen manufacturers could make fiber installations
less critical, and more flexible as they allow bending around tighter corners.
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Figure 2Change of optical transmission for the different variants of fibers in 0.9 mm tubes depending on number of turns (right), mandrel radius (middle) and on wavelength
(right).
5
Mechanical Characteristics
Mechanical characteristics
In real world installations fibers are often squeezed and crushed during and after installation through staples and other supporting features and they might have to endure temperature changes. To check and compare the mechanical and environmental reliability of the different fiber samples, crush tests (according IEC 60794-1-2 E3) and temperature cycling tests
(according to IEC 60794-1-2 F1) using the 0.9mm tubes and the 2.0 mm cables were
executed. Selected results are given in Figure 3.
Crush tests
Temperature cycling test
In contrast to the case for SM fibers [1], bend optimized MM-fibers demonstrated no loss
improvement during crush tests. Crushing the optical cable will not introduce macro bending
into the fiber but micro bending. It is believed that micro bending has a random mode mixing
effect (in contrast to mode filtering), resulting in the chaotic loss curves. The temperature change
test showed a more similar behavior as in the macro bending tests. For this case, it is assumed
that micro bending and macro bending are produced within the cable due to mantel material
shrinkages during the low temperature cycles.
As a result, the use of bend optimized fibers for manufacturing of optical cables improves only
to a certain degree the optical performance under variable environmental conditions.
1.0
BO1
BO2
BO3
0.5
0.0
200
150
100
50
0
Figure 3
λ =850nm
2
4
6
0
8 10 12 14 16 18 20 22
Time [min]
5.0
Temperature Cycling Cables: Average
OM2 BO1
OM3 A BO2
3.0 OM3 B BO3
4.0
2.0
1.0
0.0
0
λ =850nm
10
20
80
60
40
20
0
- 20
- 40
30
40
Time [h]
50
60
70
Temperature [C]
OM2
OM3 A
1.5
OM3 B
2.0
Change of
Attenuation [dB]
Crush Test: 100–200 N/cm; 3x3x1 min
Compressive
Stress [N/cm]
Change of Attenuation [dB]
2.5
Change of optical transmission for the three types of fibers in 0.9 mm tubes during short-term crush tests (left) and in 2.7 mm cables during temperature cycling (right).
6
Splicing Investigations
Splicing investigations
All indoor optical communication systems have to be connected to the legacy world, which
typically operates using standard MMF. Sometimes this connection is performed by fusion splicing. However, bend optimized MMF may not splice seamlessly to other standard
MMF due to dissimilar index profiles and number of modes within the fiber. To confirm the
spliceability of the investigated fibers a systematic splicing field test (Figure 4, upper left, power
meter loss measurements using encircled flux launch conditions according to IEC/PAS 62614)
was carried out between all bend optimized to the standard OM3 fiber at 850nm wavelength.
The measured loss distribution of the acquired splice loss data (80 connections per set) are
summarized in Figure 4 (right).
Splicing field test
Although to some degree a splice boundary is clearly visible (Figure 4, pictures in the lower
left), the spliceability of the three bend optimized fibers to standard OM3 fibers was very well
confirmed, as only one or less splice among the 80 splices per combination failed. Furthermore, the measured splice loss was well within the allowed values and the small mean loss difference (0.02 dB) between the different configurations can be explained by the limits (+/-0.01
dB) given by the measurement setup.
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Test setup (upper left), photograph of the splices(lower left) and distribution of the measured loss distribution at λ=850 nm for the different splice configurations (right).
7
Optical Connectivity
Optical connectivity
The outer world can, besides fusion splicing, also be reached by optical connectors. Due to
the different manufacturing process of the bend optimized fiber types, the question has arisen,
if during the assembly process the endface will behave in a similar manner as for the standard
MMF and if it will match the geometrical and quality criteria’s. The manufactured 0.9mm tubes
containing the three types of bend optimized OM3 fibers were assembled with LC-PC and
SC-APC connectors using standard assembly processes (blind test, not knowing what type of
fiber they were using). Fortunately, during the assembly process no special care was taken nor
had to be. Closer examination under a high performance microscope (using 400x magnification) using white light for backside core illumination showed the slight difference of the modal
distribution (see Figure 5). The variations of the visible faint light rings ("halo") around the 50m
core indicate the slight different index profiles (trench location) for the bend optimized fiber
among the three different manufacturers.
Blind test
Figure 5
Microscope picture of the fiber endfaces in the assembled ferrules (400x magnification).
The measured endface geometries (radius, apex offset, fiber protrusion) of the fiber core were
not distinguishable between the different samples and they stayed well within the tolerance
range given by IEC 61755-3-1/2.
To prove the compatibility of the connectorized fiber assemblies to standard MMF their loss
distributions were investigated at λ = 850 nm in a similar way as the splicing (encircled flux
launch conditions). Mating loss results (LC-connector) for all combinations are given as box
plots in Figure 6. Here the individual distributions resemble each other nicely and it can be
stated that the connector performances are well compatible to standard OM3 fibers.
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98 % (vertical line) range of measurement data.
8
Bandwidth Investigations
Bandwidth investigations
As mentioned it is very challenging to control bend loss and other waveguide properties simultaneously. Especially challenging is to keep the bandwidth performance, as even minor
change from the optimum shape of the ideal graded index profile leads to degradations.
To confirm the bandwidth behavior, the different fibers are characterized using differential
mode delay (DMD). The DMD technique [5] incorporates stepping the output of a singlemode,
fiber pigtailed 850 nm pulsed laser across the core of the fiber under test (FUT) in small
(1–2 µm) increments. Exit pulses from the fiber at each radial launch position are then collected
by a detector and sampling oscilloscope (blind test results for 300 m of OM3 and the three
bend optimized fibers are shown in Figure 7 on the left). From these curves, using well defined
weighting sets and transformation algorithm, the effective modal bandwidth (EMB) and overfilled modal bandwidth (OMBc) can be calculated. The calculated results for all measured
fibers are given in the table in Figure 7.
Calculation of the
effective modal
bandwith
The measurements confirmed the specifications given by the fiber manufacturer. The difference
(ripple) for high offset launch might be due to deviations in the index profile at the outer core
range.
To confirm the usability of these fibers in real system applications, 10Gb/s transmission bit error
rate (BER) measurements for different configurations were made. A variable optical attenuator
is inserted in the fiber loop of the optical data link to induce transmission degradation. Bit error
rate are then measured for 300 m fibers on standard transport spools (FUT) at different attenuation levels.
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9
Bandwidth Investigations
As BER test depend heavily upon applied components (transceivers) in the setup (Figure 8,
upper left), two different transmitter/receiver (Tx/Rx) sets were used. Results (Figure 8, left) for
these "worst" (lines) and "best" (dots) case show an 3 dB power penalty for the different pairs.
Whereas all bend optimized and standard OM3 fibers can transmit almost failure free 10Gb/s
even at low optical power levels, the OM2 fiber fails (as expected since not rated for 10Gb/s
transmission).
A further parameter that normally influence the optical bandwidth is the launching condition. To
simulate general use a simple but effective means, i.e. a cascade of 5 patchcords (PCs) using
standard (low cost) MMF connectors was introduced in front of the FUT. The major effect of this
cascade is increasing the mode power in higher order mode groups propagating more in the
outer core region. The influence on the result is shown in Figure 8, (right) indicating that apart
from fiber BO2, all the BER tested on the fibers are sensitive to change in launch condition (dot
to dash-dots). BO1 shows the worst penalty increase of about 0.3 dB, which is in line with the
observed larger outer DMD of this fiber.
The launching condition
To simulate the fiber layout in a datacenter environment with many in- and outlets in fiber
cabinets, an extended lay-out was made consisting of a cascade of 10 r = 5 mm full turns in
the launching end of the FUT. Between each of the loops a fiber distance of about 1 m was
present. The effect is usually positive as the added loss due to the heavy bending attenuates
preferentially the higher order modes. This clearly shows (Figure 8, right, dash-dots to line) for
fiber BO1 where the power penalty improves by about 0.3 dB due to the inserted bends.
Note that this is about equal to the added power penalty originating from the insertion of the
launching configuration. This behavior of Fiber BO1 in contrast to BO2 (little influence due to
inserted bends) can again be understood from its larger outer DMD performance.
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10
Reliability Considerations and Conclusion
Reliability considerations
So far we’ve only seen the benefits of using bend optimized MMF for optical cable installations. However, signal transmission reliability of an installed optical link must be viewed in
terms of both optical loss and mechanical stability. With traditional rules for fiber management at large bend radii, fiber breakage has not been an issue. However, as fiber bend radii
drop, fiber lifetime becomes an important consideration. Just take into account, that the stress
on the fiber at 5mm bend radius is 2 times higher than at 10 mm bend radius and more than
6 times higher than on a fiber following the traditional 30 mm bend radius. Similar as for low
bend SM fibers [1], the stress behavior can be calculated according to IEC 62048 leading
to the expected failure probability during the lifetime of a bend fiber. If we consider a rack
in a datacenter having 1000m of fiber bend at r = 30 mm, one out of 10 million racks will
fail during a 20 year lifetime due to fiber break. If the fibers in the same racks are bent at
r = 10 or even r = 5 mm we would see two fails among 100 or one among 10 racks, respectively.
Conclusion
Handling and installation testing has confirmed that no differences exist between standard 50-µm fiber and the bend optimized 50-µm OM3 fiber (of three different manufacturers) in terms of termination and splicing methods. Therefore, new products enabled with
the bend optimized fibers are able to provide the standard performance that network designers expect and the added benefit of enhanced bend performance without sacrifices.
The new bend optimized fibers have been extensively tested and compared in respect to the
optical properties of standard multimode fibers. They have clearly proven better performance
under bending, to some extent better mechanical and environmental behavior, as well as their
compatibility to standard MMF. Normal splicing and manufacturing processes can be used
to assemble connectors with these fibers while reaching similar optical performances. Taking
reliability considerations into account, it can be concluded that bend optimized multimode
fibers are recommended for bend and loss critical applications as they are a real advance
to current standard multimode fibers, but they should be handled with appropriate care as all
optical glass fibers.
Viewing both optical
loss and mechanical
stability
Bend optimized fibers
guarantee the expected
performance
Note: For all of the above described investigations we used for standard: OM3 of
Draka®, for the bend optimized OM3 fibers: MaxCap® -BB-OM3 of Draka®, ClearCurve®
of Corning®, and LaserWaveFlex® of OFS®; for 0.9 mm tubes: 01-G50/CW-E9-F of
HUBER+SUHNER®, and for 2.0 mm cables: 01-G50/CWJH-E20-F of HUBER+SUHNER.
Acknowledgement: The authors would like to thank the three fiber manufacturer for providing
their bend optimized fibers, Draka Communications(G. Kuyt) for taking the DMD measurements, and HHI Berlin for making the BER test
References:
[1]
R. Krähenbühl, et al., "Usability of low bend fibers for optical connectivity", HUBER+SUHNER white paper, 2009.
[2]
L. Molin, et al., "Low bending sensitivity of regular OM3/OM4 fibers in 10GbE applications", OFC 2010, JThA55.
[3]Y. Sun, et al., "Specialty high bandwidth multimode fiber for optical interconnection", SPIE, vol. 7631,
ACP 2009, TuZ3.
[4]D. Donlagic, "A low bending loss multimode fiber transmission system", Optics Express, vol.17(24),
p. 22081-22095, 2009.
[5]
"Differential mode delay measurement of multimode fiber in the time domain",TIA/EIA-455-220, Dec. 2001.
11
WAIVER
It is exclusively in written agreements that we provide our
customers with warrants and representations as to the technical
specifications and/or the fitness for any particular purpose.
The facts and figures contained herein are carefully compiled
to the best of our knowledge, but they are intended for general
informational purposes only.
Bend optimized-Fibers/4291/4743/08.2011
HUBER+SUHNER is certified according to
ISO 9001, ISO 14001, ISO/TS 16949 and IRIS.
HUBER+SUHNER AG
Fiber Optics Division
Degersheimerstrasse 14
9100 Herisau/Switzerland
Tel. +41 71 353 4111
Fax +41 71 353 4444
[email protected]
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