Contribution to the achievement of SDGs
by Furukawa Electric GroupMulticore Fiber Manufacturing Technologies
Using Modified Cylinder Method
Masanori Takahashi *It is expected that the increase in transmission capacity in optical communication systems using conventional Single-Mode Fibers (SMFs) will reach its limit,
and that the early commercialization of Space-Division Multiplexing (SDM) transmission technology using Multicore Fibers (MCFs) is forecasted. In this report, we introduce the research anddevelopment trends of MCF manufacturing technology reported so far from the viewpoint of
improving the manufacturability for the practical use of MCF. In addition, we report on a new
method that we are developing as a new manufacturing method in order to solve the issues (drilling cost, limitation of preform size) in the drilling method, which is currently the most widely used
manufacturing method. In principle, the modified cylinder method has the advantages of larger
size and lower cost of the preform, and we have fabricated MCFs from the preform by optimizing
the design of the modified cylinder method, and have achieved a low attenuation that is comparable to that of the MCF fabricated by the drilling method.

ABSTRACT1.

INTRODUCTION

In recent years, Internet traffic has increased rapidly inresponse to the development of video distribution services and various Web services. Facing the rapid increase inInternet traffic, optical communication systems are
required to have larger transmission capacity. Various
technological innovations have enabled the transmission
capacity of optical communication systems to be
increased. However, in recent years, it has been suggested that the increase in transmission capacity per fiber in
optical communication systems using conventional SMFs
has reached its limit due to the nonlinear phenomena,etc. occurring in the optical fiber. Therefore, in recent
years, SDM transmission technology has been attracting
attention as a technology to break through the limit of theincrease of transmission capacity per fiber. Various
research and development activities have been conducted on optical fibers for SDM transmission. Among them,the development of SDM transmission technology using
MCF has attracted much attention, and many researchcases have been reported. For example, a high-capacitytransmission using 19-core fiber 1), 2) has been reported.
Furthermore, transmission experiments in which the number of SDM channels exceeds 100 have been reported by
combining mode multiplexing technology in each core of
the MCF, and transmission capacities exceeding 10 Pb/s
per fiber have been achieved 3), 4). In the SDM transmission
experiments using these MCFs, a significantly larger*

capacity was achieved compared to transmission systems using conventional SMFs, while the cladding diameter of MCFs was 200 µm or more, which is much larger
than that of conventional SMFs (125 µm). When fibers
with significantly larger cladding diameters are used in
actual transmission systems, there are significant issues
from the viewpoint of ensuring mechanical strength reliability due to bending, etc. In recent years, the use of
MCFs in actual transmission systems has been accelerated. Recently, MCFs with a cladding diameter of 125 µm,
which have good connection affinity with conventional
SMFs, have been reported 5), 6) with the goal of early application to actual transmission systems. The use of MCFs
with standard cladding outer diameter is a great advantage because it allows the use of existing technology interms of tools and techniques used for cutting and splicing fibers, fusion splicing technology, and cable construction.
In order to actually realize a transmission system using
MCF, in addition to using MCF with a standard diameter,
improvements in MCF manufacturing technology are
required. In particular, cost reduction and mass production by increasing the size of the preform are necessary.
In this report, we introduce research and development
trends in MCF manufacturing technology from the viewpoint of improving manufacturability for the practical useof MCF. In addition, we report on the manufacturing technology of MCF using a modified cylinder method that isunder development.

Communication and Energy Laboratories, R&D Division

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Multicore Fiber Manufacturing Technologies Using Modified Cylinder Method

2.
2.

VARIOUS MANUFACTURINGTECHNOLOGIES FOR MCF PREFORM

This chapter describes the MCF manufacturing technologies that have been reported so far. They are in the process of being developed for practical use, and eachmethod has its own characteristics. Table 1 shows themain MCF manufacturing methods under investigation.
2.1 Drilling Method
In the drilling method, a quartz rod is perforated with adrill to produce a clad rod with multiple holes, and a corerod containing a core is inserted into the holes of the cladrod to produce the MCF preform. The advantages of thedrilling method include a high core positioning accuracy
and a high degree of freedom in core placement. The
issues are the high cost of drilling and the limited lengthof drilling. Currently, the drilling method is the most widelyused method for manufacturing MCFs (Figure 1).
2.2 Stack and Draw Method
The stack and draw method 7) has been reported as amanufacturing method for MCF preform different from thedrilling method. In the stack and draw method, core rods
are inserted into a quartz pipe, and by appropriately

Table 1

designing the inner diameter of the pipe and the outer
diameter of the core rods, the quartz pipe and the outer
diameter of each core rod are constrained to place thecore on the lattice points of the hexagonally-most-dense
structure. The advantages of the stack and draw method
are the ease of processing the components and the small
constraints of the preform size. One issue is that thecores can only be placed on the lattice points of the hexagonally-most-dense structure. In addition, the positioning accuracy of the core depends on the clearance
between the inner diameter of the pipe and the outer
diameter of the core rod, but in the actual fabrication process, a clearance is required when the core rod is inserted into the pipe. Since this clearance leads to core misalignment, a poor core positioning accuracy becomes anissue. In addition to the MCF manufacturing process, thestack and draw method has been used in many otherapplications as a manufacturing process for hollow corefibers (Figure 2).
2.3 Over-cladding Bundled Rods Method
Recently, an over-cladding bundled rods method 8) has
been proposed as a method for manufacturing MCF preforms. In the over-cladding bundled rods method, a silica

MCF manufacturing technologies.
advantages

issues- High core positioning accuracy- Arbitrary core placement- Drilling cost
- Restriction of drilling length- Easy assembly- Smaller size restrictions- Core Positioning Constraints- Low core positioning accuracy

Over-cladding bundled rods
method- Existing equipment can be used
- Smaller size restrictions- Machining man-hours for rods
(When using polygonal rods)- Fewer applications

Slurry casting method- Relatively high degree of freedom
in core location- Optical Properties- Few applications

Drilling method
Stack and draw method

Clad rodCore rod
Quartz pipeCore rod

Figure 1

Structure of MCF preform using the drilling method.

Figure 2

Structure of MCF preform using the stack and draw
method.

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Multicore Fiber Manufacturing Technologies Using Modified Cylinder Method

glass layer, which serves as the cladding, is deposited ona bundle of core rods by an outside deposition method to
obtain the MCF preform. The advantages of the overcladding bundled rods method are that existing manufacturing facilities can be used for outside deposition of thecladding and that there are few restrictions on the size ofthe preform. The issue is that it is difficult to deposit cladding between the core rods when circular core rods areused. In order to solve this problem, a technique has
been reported in which core rods are fabricated into
polygonal shapes and targeted by bundling them together 8). This method has the advantage that the cladding canbe deposited uniformly around the core rods, but there isa concern regarding the man-hours required to process
the core rods because polygonal core rods are used. In
addition, since the over-cladding bundled rods method isa relatively new manufacturing method, there is an issue
that examples of its application to MCFs are limited.
(Figure 3).

Figure 3

Schematic illustration of the over-cladding bundledrods method.

2.4 Slurry Casting Method
The fabrication of MCF preform by the slurry casting
method 9) has also been reported. In the slurry casting
method, a liquid slurry is made by adding a curable resin,
etc. to quartz powder, which is then poured into a mold toform the MCF cladding material. One advantage of theslurry casting method is that it allows a relatively highdegree of freedom in the positioning of the core. The easeof processing makes this method suitable for the fabrication of preforms with many holes. However, the issue isthat there are currently only reports of applications to therelatively small preform, and no reports of applications to
the large preform. In addition, there are few examples ofapplication to MCF preforms and it is reported that optical properties such as transmission loss are inadequate
till now.
2.5 Modified Cylinder Method
In recent years, we have been developing a new manufacturing method for MCFs using a modified cylinder
method 10). In the modified cylinder method, since thecladding material is manufactured using the same manufacturing method as the one that is used for quartz tubes
typically in the manufacture of conventional optical fiber
preforms, it is possible to increase the size of the preform
and to reduce the cost. On the other hand, as with theover-cladding bundled rods method, there are few examples of applications at present, and the technology maturity has to be improved. In the next chapter, we will discuss the modified cylinder method in detail.
3

3.

MODIFIED CYLINDER METHOD

Various methods of manufacturing MCF preform havebeen studied, but the drilling method is currently the mostwidely used. As described in Section 2.5, we are developing a new modified cylinder method for MCF preform
suitable for reducing processing costs and increasing thesize of the preform, which are issues common in the drilling method.
3.1 Design of MCF Preform Using the Modified
Cylinder Method
Figure 4 shows the structure of the MCF preform fabricated with the modified cylinder method. In the modified cylinder method, a quartz tube having a cross-shaped
vacancy (cross-shaped cylinder) was used as the cladding rod, and four core rods were inserted into the crossshaped cylinder, and a cladding rod was inserted in thecenter to form a four-core MCF preform. The crossshaped cylinder of the modified cylinder method can be
fabricated in the same way as quartz tubes used in the
preforms of conventional optical fibers, so in principle, it
is possible to increase the size of the preform and to
reduce the cost. While this design is considered to haveexcellent manufacturability because it uses fewer components and simplifies the assembly process, there are concerns regarding core misalignment and core non-circularity after drawing the MCF preform because of the large
gap between the core rod and the cladding rod in thevacancy of the cross-shaped cylinder.
Figure 5 shows a design that reduces the gap in the
MCF preform made by a cross-shaped cylinder. In thisdesign, filler rods of different diameters were newly inserted in order to reduce the gap between the cross-shaped
cylinder, the core rod, and the cladding rod inserted in thecenter. As a result, the gaps are significantly reduced, andthe core pitch deviation and the core non-circularity are
expected to be reduced. On the other hand, it is expected that manufacturability will be compromised due to theincrease in the number of components used and thecomplexity of the assembly process. Figure 6 shows thedesign of MCF preform using a cross-shaped cylinder
with an optimized vacancy shape (modified cross-shaped
cylinder) to achieve both reductions of gaps and of manufacturability. In this design, four core rods and one cladding rod are used to construct the MCF preform, which
has excellent manufacturability due to the small numberof components used and the simple assembly process.
As a result of optimizing the shapes of the modified
cross-shaped cylinder, core rods, and cladding rods
inserted in the center, the gaps are significantly reduced,
and the core pitch deviation and the core non-circularity
are expected to be reduced.
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Multicore Fiber Manufacturing Technologies Using Modified Cylinder Method

Cross tube

Core rod

MCF preform

Clad rod
Figure 4

Design of MCF preform using the cross-shaped
cylinder.

Cross tube

Core rod

Clad rod
Figure 5

Core rod

Cross-sectional picture of MCF fabricated using thecross-shaped cylinder.

Figure 8

Cross-sectional picture of MCF fabricated using thecross-shaped cylinder with filler cladding rods.

Figure 9

Cross-sectional picture of MCF fabricated using themodified cross-shaped cylinder.

MCF preform

Filler rod

Design of MCF preform using the cross-shaped
cylinder with a low gap ratio.

Improved cross tube

Figure 7

MCF preform

Clad rod
Figure 6

3.2

Design of MCF preform using the modified crossshaped cylinder.

Characteristics of MCFs Fabricated Using theModified Cylinder Method
Four-core MCFs were fabricated using the preform processed with the modified cylinder method with thedesigns shown in Figures 4-6. Pictures of the end facesof the fabricated MCFs are shown in Figures 7 to 9. As
shown in Figure 7, the MCF fabricated using the crossshaped cylinder MCF preform had a large core non-circularity. On the other hand, as shown in Figures 8 and 9, thecore non-circularity was significantly reduced in MCFs
fabricated from MCF preform with the reduced gaps. The
core pitch deviation and the core non-circularity of the
MCFs in Figures 7 to 9 were 9.2 µm, 2.3 µm, 1.7 µm,
20.0%, 2.1%, and 1.1%, respectively, when the corepitch deviation from the design value and the core noncircularity were calculated.

Table 2 shows the characteristics of the prototype
MCFs. In the cross-shaped cylinder, the transmission loss
increased significantly due to the large core non-circularity. Both the reduced gap cross-shaped cylinder and theimproved cross-shaped cylinder used cores equivalent toG.652, but the reduced gap cross-shaped cylinder
showed an increase in attenuation compared to the normal SMF. The increase in attenuation can be attributed tothe effect of stress applied to the core during fabricationin addition to the non-circularity of the core. On the other
hand, the attenuation in the modified cross-shaped cylinder with optimized preform design was similar to that of
the conventional SMF. These results indicate that the useof the modified cross-shaped cylinder can improve theproperties of MCFs without compromising the manufacturability of the preform.
Table 2

Characteristics of MCFs fabricated.
Cladding
diameter(µm)

Core pitch(µm)

Attenuation(dB/km)Cross-shaped cylinder

124.9

35.8

0.853

Reduced gap
cross-shaped cylinder

124.8

42.7

0.412

Improved cross-shaped
cylinder

124.9

38.2

0.190

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Multicore Fiber Manufacturing Technologies Using Modified Cylinder Method

44.

CONCLUSION

In this report, we introduced the research and development trend of MCF manufacturing technology being considered from the viewpoint of improving manufacturability
for the practical use of MCF. Also, we stated that thedrilling method is currently the most widely used manufacturing method because of its high degree of freedom
in core positioning and positioning accuracy, and about
the associated issues that need to be addressed in the
drilling method (drilling costs and restrictions on preformsize) for the practical application of MCFs. In addition, we
reported on the manufacturing technology of MCF using
the modified cylinder method, which we are developing
as a new manufacturing method in order to solve theproblems of the drilling method. The cross-shaped cylinder for the modified cylinder method can be manufactured with the same manufacturing method as the quartz
tube used in the manufacture of conventional opticalfibers, so it has the advantage that the size of the preform
can be increased and the cost can be reduced. On theother hand, there are still few examples of application to
the MCF fabrication because of the new manufacturingmethod, and the process that satisfies the characteristicsof the MCF is still in development. We have optimized thedesign of the MCF preform using the modified cylinder
method and reduced the core pitch deviation and thenon-circularity of the MCF. We plan to further develop
technologies to improve the manufacturability of MCFs
for practical use in the future.

ACKNOWLEDGEMENT

These research results were supported in part from thecommissioned research (No.20301) by National Instituteof Information and Communications Technology (NICT),Japan under “R&D on Innovative optical Fiber and communication technologies toward Standardization”.

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