Development of Optical Fiber Cables for Access Networks
by Masanori Hattori *, Fumiki Hosoi *, Tetsuya Yasutomi *, Eiji Konda *,Ichiro Kobayashi * ,Ryuji Takaoka *2 and Daisuke Iwakura *3

In an effort to promote implementation of FTTH (Fiber To The Home), the authors
have been developing optical fiber cables for access networks. The development
at this time includes: 1) Optical fiber cables and terminations that help improve the installation
efficiency by enabling multi-cable laying, 2) Low-cost cables that permit one-step manufacturing
after fiber ribbon, and 3) Flame-retardant optical fiber cables with environment-friendliness. The
development program thus provided the optical fiber cables that had been developed three years
ago with additional improvements in terms of cost and function. It can be said, as the result, early
implementation of FTTH has been promoted.

ABSTRACT1.

INTRODUCTION

The authors have been developing optical fiber cables foraccess networks since three years ago, making a significant contribution to the construction of optical fiber accessnetworks that is aimed at coping with the rapid growth of
the Internet accompanied by an increasing volume of data
traffic.
The number of Internet users in Japan is estimated toreach 27 million at the end of 1999 (a 59.7% increaseover the previous year), rising to 77 million at the end of2005 1).
Data communication is predicted to keep growing,
expanding to the areas of not only web content accessand electronic mail exchange in these days but also integration of digitized broadcasting and telecommunications
together with circulation of such mass-media contents asmusic, computer games, and broadcasting in the future;
which leads to an explosion of information distribution inaround 2005, encompassing the SOHO, local community,and home information contents. In order for such a developing optical access network to expand economically andrapidly, it is essential that optical access cables developed
heretofore should be improved in terms of performance,
function, and cost.
This paper reports on the study of upgrading the installation efficiency of multi-fiber cables, investigation on the
new cable structures with small fiber counts, and thedevelopment of subscriber cables with environmentfriendliness.

* Development Sec., Engineering Dept., Opto-comm Div.
*2 Optical Fiber Transmission Line Development Dept., FITELPhotonics Lab.
*3 2nd Gr., Engineering Dept., Opto-comm Div.

2.

DEVELOPMENT OF CABLES WITH
IMPROVED INSTALLATION PERFORMANCEWith the increase of communication traffic, there is a needfor optical access networks to be constructed at an economical cost. To meet this requirement, not only cable
cost reduction but also total cost reduction should be considered including such cable installation and connection
technologies as long-length laying and multi-cable laying.
When it comes to the cost reduction of cable installation,
both shortening working hours and lessening working
materials contribute greatly to this target, and one of themost effective measures to achieve this is to decrease thelaying tension of optical fiber cables, involving reduction of
the friction coefficient of the cable sheath as well as lightening cable weight and upgrading cable flexibility. The
results of investigations on these measures will be reported below.
2.1

Investigation on Reducing Friction Coefficient ofCable Sheath
Possible techniques for friction coefficient reduction comein two ways: changing the constituting material to the one
with a lower friction coefficient and using lubricant todecrease the friction.
2.1.1 Investigation on Sheath Material
The sheathing materials suitable for multi-cable laying
were investigated. Various polyethylene (PE) materials
were compared in terms of requisite properties of cablesheath including low friction coefficient against cable duct,abrasion resistance, rigidity, and cost; and the results areshown in Table 1.
Referring to the above mentioned results, linear low
density PE (hereafter called L-LDPE) was selected for its
excellent wear resistance and low friction coefficient.

33

Comparison of various sheath materials' properties

Material

Low friction co-

Wearefficient against duct resistance

Rigidity

Cost

Low-density PE

Present

Linear low-

Comparable

density PE
Medium-density

High

PE
High-density PE

High

6

1.4

5

1.2
1

4

0.8
3
0.6
2

F: trial
F: conventionalµ: trialµ: conventional

1

0.4
0.2
0

0

Long-Term Reliability of Sheath Material andLubricant Characteristics
The reliability of the new sheath material was verified tomeet the performance requirements of sheath material
with regard to basic characteristics, aging characteristics,
ultraviolet-ray deterioration, oil resistance, etc.
It was also confirmed that when the bled lubricant was
wiped off using alcohol, it re-bled within about two weeks,
thereby recovering an original friction coefficient needed
for cable laying; and that the bled lubricant did not go
moldy when tested in accordance with JIS Z 2911 (1922).
Moreover, the inwardly bled lubricant was found to havelittle influence on the cable core, exhibiting a core pull-out
force equivalent to that of the cable in current use.

0

2.1.3

2.1.4 Verification of Installation Performance
The prototype of a 640-fiber cable was manufactured andexperimentally installed in order to verify the investigations
described above. The results about the cable laying tension are shown in Figure 1.
From the results shown in the Figure, it is seen that thecable using sheath material comprised of L-LDPE withcompounded fatty acid amide can reduce the cable laying
tension by about 30 % in comparison to conventional
cables using LDPE. Thus, the effectiveness of the new
sheath material composition was demonstrated.
2.2 Investigation on Lightweight Cable Structure
An alternative technique to reduce the cable laying tension is to reduce the cable weight itself, including suchmeans as using a slotted rod of decreased diameter
resulting in a reduced designed weight, using a foamed
slotted rod leading to a reduced cable weight, and using atension member of reduced diameter or of nonmetallic
material. The results of these investigations will be
described below.

34

Coefficient of friction(relative value)

Table 1

Pulling force(relativevalue)

2.1.2 Application of Lubricant
Lubricants to be applied onto cables and ducts for
decreasing friction coefficient should be optimally selectedconsidering such factors as the method of application,
lubricant type, effectiveness, and long-term reliability. As
for the method of application, both compounding at thetime of manufacturing and surface application at the timeof cable laying were tested for the sheath material and theduct material, respectively; and comparing the obtained
results in terms of reduction effectiveness, workability, andreliability, it was determined to use the compounding
method for the sheath material. As for the type of lubricant, fatty acid amide was selected for the following reasons.
(1) It is used in processed foods, so that health safety isensured.
(2) It is readily available, effective with a small amount ofusage, and thus is economical.
(3) By adjusting the compounding concentration, the friction coefficient can be adjusted.

20

40

60

80

100

120

140

Installation length (m)Figure 1

Relationship between cable length and pulling tension at installation

2.2.1 Reduction of Slotted Rod Diameter
Reduction of the slotted rod diameter results in also the
reduction of the design weight of slotted rod, wrapping,and sheath material, thus enabling the reduction of costs.
Viable measures to reduce the diameter include reexamination of the slot dimensions, the tension member thickness, the slot bottom thickness, and the rib thickness.
Among these, the former three were discarded because ofconcerns about large influence on cable's optical performance, so that the reduction of the rib thickness wasinvestigated. Conventionally, the slotted rod was fabricated by extrusion, and the minimum thickness of the rib waslimited to about 1.3 mm due to engineering reasons.
Accordingly, we changed the method to shaving, which
was able to decrease the minimum thickness by half.
Since the rib thickness decrease was expected to incur
deterioration in various mechanical characteristics of thecable, we simultaneously carried out mechanical tests toconfirm structural integrity. It was confirmed that, althoughsome dependence on the rib thickness was seen, themechanical characteristics were equivalent to those of
conventional cables, thereby meeting the performancerequirements with a sufficient margin.
2.2.2 Reduction of Tension Member Diameter
Due to the enhancement of fiber screening level from 0.5% to 1.0 %, the allowable elongation during cable laying
has been increased from 0.2 % to 0.3 %. This relaxationof requirements permits the reduction of the tension member diameter by 60 % in Furukawa's case or the use ofaramid fiber FRP in place of bluing-steel stranded wire
now in use. It was confirmed that the cables using the two

Furukawa Review, No. 20

2001Table 2

Comparison and breakdown of weight reduction in
1000-fiber cables (kg/m)

Conventional structure 0.76
Light-weight structure

0.49

Reduced weight

0.27 0.15: Tension member diameter reduction

0.10: Slotted rod diameter reduction
0.02: Reduction by foaming

tension members mentioned above showed an elongationof 0.3 % or less for the slotted rod under a load of 3980 N,
thus demonstrating sufficient performance in terms of reliability.
2.2.3 Slotted Rod with Foamed Inner Layer
Foamed polyethylene is a cable insulation material thathas established a substantial track record in the PEC andthe CCP cables. This technology was applied to reduce
the weight of the slotted rod whereby a cable consisting ofa slotted rod with a 40 % foamed inner layer was investigated. The cable was confirmed to provide sufficient performance with regard to the temperature and mechanicalcharacteristics.
2.2.4 Summary of Weight Reduction
Table 2 compares the weights of two 1000-fiber cables;
one is the conventional and the other is the new one inwhich various weight-reduction measures hereto mentioned are applied. It is seen that the reduction in both theslotted rod diameter and the tension member makes amajor contribution.
2.3

Summary of Cables with Improved Installation
Performance
Reduction of both the friction coefficient of the sheathmaterial and the weight of the cable was investigated,demonstrating that the both techniques made a major
contribution in reducing the cable laying tension.
Moreover, the cable characteristics were confirmed to beequivalent to those of conventional cables, thus giving
ample performance.

3.

DEVELOPMENT OF AERIAL CABLESWITHOUT SLOTTED ROD

In order to promote implementation of fiber-optic accessnetworks economically, there is a need for optical fiber
cables with compactness, lightweight, and better installation performance. Although single-tube cable and loosetube cable represented optical fiber cables for access network heretofore, these cables had difficulties in handlingdue to the use of jelly and so forth.
In Japan in the meantime, SZ slotted rod cables havebeen developed and put into practical use for aerial distribution. While cables with three slots are used up to 24
fibers and five slots up to 100 fibers, there is an increasingneed for cable downsizing toward the cables with lessthan 40 fibers so as to promote FTTH. Accordingly, we
have developed new types of optical fiber cables in thiscable category at this time, which feature compactness,lightweight, and ease of handling.
3.1

Required Characteristics for Optical Fiber Aerial
Distribution Cables
3.1.1 Cable Structure Using Optical Fiber Ribbon
Aerial distribution cables are required to use optical fiber
ribbons so as to facilitate jointing with trunk line cables
and other distribution cables.
3.1.2 Long-Term Reliability in Aerial Environments
Whereas aerial distribution cables are installed on telephone poles, the cable-supporting member undergoes
tensile strain due to temperature change and wind load
under such aerial environments. Thus, the cable structure
has to be such that the fibers contained would not sufferfrom degradation when the supporting member elongates.
3.1.3 Mid-Span, Post Branching Capability
Aerial distribution cables are required to permit mid-span,post branching at arbitrary points easily, allowing subscriber drops at any point after installation.
3.1.4 Cost Efficiency
Construction of fiber-optic access networks should be
economical so as to promote implementation of these networks, requiring also aerial distribution cables to be low incost by downsizing.
3.2 Cable Design
Aerial distribution cables with new structure that meets theabove mentioned requirements have been designed as
described below.
3.2.1

Self-Supporting Structure with Excess Cable
LengthSelf-supporting structure with excess cable length relative
to the suspension member was adopted so that opticalfibers would not be subjected to excessive strain due tosuspension member elongation caused by cable laying
tension, temperature change, and wind load under aerial
environments. Two tension members were provided in thecable for reinforcement.
3.2.2

Reduced Diameter Structure by Optimizing Fiber
Count
Cable structures consisting of optical fiber ribbon stackand two tension members were designed, in which two
types were investigated: Type 1 prevents rotation of theribbon stack within the cable cross section, while Type 2
permits rotation. Letting n be the fiber count within thefiber ribbon and N be the number of ribbons within thecable, the cross sectional area of the ribbon stack S1 for
Type 1 and S2 for Type 2 can be calculated as follows.
S1 = W × H
S2 = π(W2 + H2)/4

35

Cross sectional area (mm2)9Suspension wire

8

Type1
(4-fiber ribbon)

7
6
5
4
3
2
1
0

0

8

16

24

32

40

Type1
(8-fiber ribbon)

Tension member

Type2(4-fiber ribbon)

4-fiber ribbon

Type2(8-fiber ribbon)

Buffer layer
Type 1 cable
Cable height: 11.0 mm
Cable weight: 0.10 kg/m

48

Fiber count

Cross sectional area of ribbon stack vs. fiber count

0.50

5

0.45

Fiber strain (%)

0.40

4

0.35
0.30

3

0.25
0.20

2

0.15
0.10

1

0.05
0.00

Transmission loss increase (dB)Figure 2

0
0

1

2

3

4

5

6

7

Figure 4

Type 2 cable
Cable height: 17.0 mm
Cable weight: 0.15 kg/m

Cross sections of Type 1 and Type 2 cables

fiber. Figure 3 also shows the results of transmission loss
measurements in which a prototype cable using 4-fiber
ribbon was kept in a temperature range of -30°C~70°C
with a bending radius of R = 150 mm. A loss increase due
to fiber bending was seen to arise with the number of fiber
ribbons over five.
Thus, it was shown that Type 1 was most suitable forcable downsizing and was practicable for cables with 16
fibers or less, while Type 2 was suitable for cables with 20
fibers or more.

Number of fiber ribbon

Figure 3

Fiber strain vs. the number of fiber ribbon for Type 1
cable

where, W and H are respectively,
W = 0.25 × n + 0.1
H = 0.3 × N(in mm)(in mm)Figure 2 shows the calculated results up to 40 fibers. It
can be seen that, in the entire range of fiber count, Type 1
gives smaller cross sectional areas of the ribbon stack,enabling downsizing of the cable. As for Type 2, the 4fiber ribbon can downsize the cable up to 28 fibers, while
8-fiber ribbon above 32 fibers.
3.2.3

Fiber Strain and Loss Increase under Cable
Bending
In Type 1 cable, fiber strain ε at the outermost layer of theribbon stack under cable bending can be represented by
the following equation, letting R be the cable bending
radius.

ε = (0.3 × (N - 2)/2 + 0.15)/R(in mm)Figure 3 shows the fiber strain ε with increasing numberof fiber ribbon at a cable bending radius of R = 150 mm.
When fiber ribbon increases in number, the fiber strain in
the outermost ribbon stack increases, resulting in an
increase in the force working to rotate the ribbon stack so
as to relax the strain. If the force exceeds the constraining
force due to the buffer layer, there arises bending in the36

3.3 Cable Structure
Based on the cable design described above, two types ofprototype cables were manufactured as described below.
It should be noted that the both cables enable one-stepmanufacturing whereby the fiber ribbon and the tension
member are incorporated by PE extrusion during thesheathing process.
3.3.1 Type 1 Cable
Figure 4 shows the cross section of Type 1 cable developed at this time. The 4-fiber ribbon stack is bulk
sheathed together with the buffer layer and two tension
members. The cable portion is intermittently connectedwith the suspension wire to form the cable neck, andexcess cable length is provided between the two.
3.3.2 Type 2 Cable
Figure 4 also shows Type 2 cable. The structure is basically the same as that of Type 1 cable.
3.4 Cable Characteristics
The characteristics of the two prototype cables have been
evaluated and the results are shown in Table 3. The
results show that the cables meet the requirements forgeneral optical fiber cables satisfactorily.
3.5 Summary of Aerial Cables without Slotted Rod
New types of aerial distribution optical fiber cables havebeen developed featuring compactness and lightweight.
To be more precise, the cables are characterized by:• One-step manufacturing process is permitted due totheir simple structure• Due to their compactness and lightweight, they

Furukawa Review, No. 20

2001enable economical implementation of access networks• Due to their dry cable structure eliminating jelly, they
provide superior installation performance in terms of
mid-span, post branching and so forth• The prototype cables showed excellent mechanicalcharacteristics, thus verifying their practicability

Table 3

Optical and mechanical characteristics of prototype
cables (Wavelength is 1.55 µm unless otherwise stated)
Test item

Results

Test conditions

Transmission characteristics
Attenuation (dB/km)

Max./Ave. at 1.31 µm

0.33/0.32

Max./Ave. at 1.55 µm

0.22/0.20<0.05

Temp. dependence (dB/km) -30~70°C
Mechanical characteristics

4.

DEVELOPMENT OF HALOGEN-FREE
CABLES FOR SUBSCRIBER USE

Cables for subscriber use such as for office building andhousing comprise drop cables to subscribers' premises,indoor cables for indoor wiring, and termination cables forconnection with optical network unit, etc. See Figure 5.
While PVC was used for the sheath of these cables conventionally, the material is known to release toxic gases
when combusted thereby contaminating the environment
since it contains halogens. Accordingly, we have developed drop cables and indoor cables using halogen-freematerials.
4.1

Requirements for Halogen-Free Cables for
Subscriber Use
Common requirements for halogen-free cables for subscriber use are(1) Flame retardance, and(2) Halogen-freeness
Other requirements depend on the service environment.

TensionFiber elongation and loss

<0.001 %change at a suspension wire

<0.01 dBelongation of 0.2 %Bending

R=160 mm x 10 repetitions<0.01 dB

Lateral force

1960 N / 100 mm x 1 min<0.02 dB

Squeezing1960 N using R=250 mm gage,<0.01 dB

4 repetitions
Twist

98 N ±360° /m x 10 repetitions<0.01 dB

Dynamic load

1 kg x 1 m<0.01 dB

Using 10-m cable>58.8 N

Using 30-m cable>98 N

Core pulling force

Mid-span, post branching

Loss change due to core
withdrawal
Residual strain after cabling

Fiber strain

process, measured using BOTD

Sheath:Flame-retardant polyolefin
0.25 φ SM fiber<0.01 dB

<0.01 %

1.5 φ halogen-free
optical fiber cord

Steel wire

4.1.1

Requirements for Outdoor Specification Cables(Drop Cables)(1) Resistance against ultraviolet ray(2) Low-temperature resistance down to -30°C(3) Resistance against chemicals
Requirements for Indoor Specification Cables(Indoor Cables and Termination Cables)(1) Have to be available in pale colors such as cream soas to harmonize with indoor environments.

Cable Size: 1.6×3.0 mm(a) Indoor cable
0.25 φ SM fiber

4.1.2

4.2 Cable Design
4.2.1 Design of Aerial Drop Cables(1) Investigation on cable sheath material
In view of the requirements for outdoor use, the cables
were designed putting emphasis on the long-term reliability of the sheath material. Polyolefin was adopted as abase material and a flame-retardant agent of high flameretardance was selected so as not to degrade the intrinsic
characteristics of polyolefin by adding a large amount of
the agent. Thus, a sheath material with sufficient mechanical strength and ample environment resistance was developed.
4.2.2 Design of Indoor and Termination Cables(1) Investigation on cable sheath material
The sheath material for indoor and termination cables forindoor use was developed putting emphasis on harmo-

Steel wire

Cable Size: 2.6×3.8 mm(b) Termination cable
Suspension wire

Sheath:Flame-retardant polyolefin

Cable Size: 2.0×4.5 mm(c) Aerial drop cable

Figure 5

Structures of halogen-free, flame-retardant cables forsubscriber use

nization with indoor environments such that the material
would be available in pale colors such as cream.
Polyolefin was adopted for the sheathing material and apale-colored flame-retardant agent was selected to matchthe color of the sheath.
(2) Improvement in lateral load performance of indoor
cables
The flame-retardant agent used in the indoor cables had asmaller flame-retardance than that used in the outdoor
cables so that the amount of addition had to be increased.
This brought about a change in the material characteristics of polyolefin, whereby the Young's modulus, in particular, was decreased leading to degradation in the lateralload performance of the cable.

37

Conventional structure
Steel wireTransmission loss increase (dB/km)

Halogen-free sheath

0.25 φ fiber

Newly developed structure
Steel wireα

Halogen-free sheath

Figure 6

6.0
5.0
4.0

No. 2

3.0

No. 3

2.0
PVC

1.0
No. 5

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Shrinkage force (relative value)

Conventional structure

Figure 8

Relationship between shrinkage force of cord sheathmaterial and transmission loss increase (at -10°C)

Table 4

Mechanical characteristics test results for halogenfree, flame-retardant cables (at 1.55 µm)

0.30

Results
Test item and test conditions

0.25

Cable

Cord

1.31 µm≤0.22 dB/km≤0.22 dB/km

1.55 µm≤0.33 dB/km≤0.33 dB/km≤0.1 dB/km≤0.1 dB/km

R=30 mm≤0.02 dB

≤0.02 dB

Lateral force 1200 N/25 mm for cable≤0.02 dB

≤0.02 dB

≤0.02 dB

—
—

0.20

Transmission characteristics
0.15

Attenuation(dB/km)

0.10
0.05

Temp. dependence(dB/km)

0.00
1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

-10~40°C for indoor

1.8

Length of straight section α (relative value)

Figure 7

No. 4

0.0
0.0

0.40

Transmission loss increase
due to lateral load (dB)Material No. 1

7.0

0.25 φ fiber

Cross sections of indoor cables

0.35

8.0

Improvement of lateral load performance by using acable cross section approximating a rectangle-30~70°C for outdoor
Mechanical characteristics
Bending

4.9 N/mm for cordTwist(a) Mechanism of indoor cables' loss increase due tolateral load
When indoor cables are trodden on by the leg of adesk or by human foot, their sheath will bend causing
the fiber inside to bend. This bend will increase transmission loss. Since the halogen-free material adopted
this time has a smaller Young's modulus than the conventional material of PVC, the bending rigidity of thesheath is lower than the conventional. Thus, the new
cable is easily bent resulting in large loss increase
when subjected to lateral load.
(b) Improvement in lateral load performance
The cross section of the cables was investigated so
as to improve lateral load performance by changing
the shape. In order to suppress small bending of thesheath, the cross sectional shape was changed toapproximate a rectangle as illustrated in Figure 6,
thus upgrading the bending rigidity of the cable.
(c) Design of cable cross section
The bending rigidity of the cable sheath can beincreased by making the cable cross section approximate a rectangle, i.e., by enlarging the straight section α shown in Figure 6. Figure 7 shows the dependence of loss increase due to lateral load on α. The
cable cross section was designed using an α that cansufficiently suppress a transmission loss increase due
to lateral load.

38

Dynamic load

±180°
300 g x 1 m≤0.02 dB
Extinguished

Flame-retardance JIS C 3005

Extinguishedspontaneously spontaneously(60° inclined)(level)(3) Investigation on termination optical cord
If the sheath of a termination optical cord shows a highshrinkage force at low temperatures, the fibers contained
become serpentine thus increasing in its transmission
loss. Five kinds of materials were used to manufacture
termination optical cords and their transmission losschanges at a low temperature were measured as shownin Figure 8. It is seen from the Figure that the materials
No. 1 to 3 that have a shrinkage force exceeding that of
PVC conventionally in use give a significant increase in
the transmission loss. Generally speaking, the shrinkage
ratio of a material is proportional to the Young's modulus,
so that the use of a material having lower Young's modulus is desirable to reduce shrinkage. However, too small a
Young's modulus will degrade lateral load performance.
Material No. 5 in the Figure actually gave a loss increaseof 0.1 dB and more at a lateral load of 4.9 N/mm.
Accordingly, material No. 4 was selected as the sheathmaterial for the cord, in which a good balance in performance between loss increase at low temperatures and lateral load resistance is achieved.

Furukawa Review, No. 20

20014.3 Cable Characteristics
Table 4 shows the results of mechanical tests for thecables. Each cable achieved excellent mechanical characteristics. In addition, the cables passed the flame-retardance tests in accordance with JIS C 3005, thereby realizing a high level of flame-retardance comparable to that of
PVC in earlier use.

5.

CONCLUSION

The results of development of fiber-optic access cables
including multi-fiber cables, small fiber-count cables, andsubscriber cables were described.
As for multi-fiber cables, it was verified that improvement in installation performance was possible throughdownsizing, weight reduction, and friction coefficient
reduction. As for small fiber-count cables, a new structureof slot-less type was proved to satisfy characteristic
requirements without using slot or jelly. These cables are
excellent in installation performance enabling constructionof optical access networks at an economical cost. In addition, halogen-free subscriber cables were developed, inwhich characteristics equivalent to conventional cables
were obtained and harmonization with environment was
satisfactorily achieved thus realizing the development concept of environment-friendliness with increasing importance in the future.
It is believed that these achievements lead to practicalapplication of the products for fiber-optic access networks
thereby contributing to early implementation and expansion of such networks.

REFERENCES
1) Ministry of Posts and Telecommunications; White Paper on Infocommunications in Japan, 2000 (in Japanese)

Manuscript received on November 14, 2000.

39