Letters

Flexible Optical Fiber Curl Cord
Katsumi Hiramatsu†, Shigeru Tomita, Toshio Kurashima,
and Eiji Araki
Abstract
We have developed an optical fiber curl cord that makes it easy to construct various wiring configurations for optical fiber networks in the home. It is flexible and fire retarding and has a connector at eachend.

1. Introduction
Optical fiber curl cord

Although the core network has already been converted to optical fiber, the end user side network still
consists of combinations of metal, wireless, and optical fiber technologies. Of these, optical fiber isspreading rapidly among home and business users
through the popularity of fiber to the home (FTTH)
because of its superior speed, bandwidth, and security. This rapid spread of optical fiber networking ispredicted to continue. Less-expensive and easier-towire networking components are now needed to support the conversion from existing metal and wirelessnetworks to optical fiber networks and expand theavailable service area. Currently, only highly trainedengineers can perform optical fiber installation andgreat care is required in handling the cable. One of themain reasons for this is that optical fiber suffers fromattenuation if bent with a small radius and is difficult
to connect. This article introduces an optical fiber
curl cord that has been developed to overcome these
difficulties and enable rapid expansion of FTTH [1].
2. Development targets
Our aim is to produce an optical fiber cord for connecting an information technology device (terminal)
to a wall outlet in the home as shown in Fig. 1. For† NTT Access Network Service Systems Laboratories
Tsukuba-shi, 305-0805 Japan
E-mail: hiramatu@ansl.ntt.co.jp

Vol. 3 No. 5 May 2005

Optical outlet

TerminalFig. 1. Example of the use of an optical fiber curl cord.

this purpose, there are several requirements. The fiber
cord must be robust at temperatures between –10 and60°C and humidity of up to 90%. It should have a 20year lifetime, and its optical and mechanical properties must be as good as those of existing optical fiber
cord. It should withstand being accidentally trodden
on or tripped over, have good fire retardant characteristics, not cause any attenuation change when moved
a short distance, and be easy to connect. Our opticalfiber curl cord, shown in Fig. 2, meets all theserequirements.
2.1 Cord core
We specified a minimum bending radius of 30 mm
because the optical loss would be too large if thecable core of the present optical fiber cord were bentwith a small radius. Recently, however, optical fibersthat have excellent bending characteristics have beenproposed. These are holey fiber (photonic crystal
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Curled part(length 100–500 mm, curl diameter 10–20 mm)

MU-connectors

Fig. 2. Optical fiber curl cord.
100
SM fiber

Loss-increases (dB)

10

Holey fiber

10.1
0.01
0.001
0.0001024681012

Radius of bend (mm)Fig. 3. Bending characteristics of holey fiber.
Table 1. Comparison of fibers with low bending loss.
Type

LossConnection

Cost

Holey fiber (PCF)

Excellent

Poor

Poor

Holey fiber (HAF)

Excellent

GoodGood

Fiber with highrefractive index core

GoodFairGoodfiber (PCF) and hole-assisted fiber (HAF)) and fiberwith a high refractive index core. The features ofthese fibers are compared in Table 1. They all havegood loss characteristics. We chose to use HAF for
our work because of its excellent compatibility withexisting fiber and its low cost. Although the attenuation is slightly greater than that of widely used optical fibers, this is not a serious problem if the fiber islimited to home use. Its bending loss is superior tothat of existing single-mode (SM) optical fiber, as
shown in Fig. 3.
2.2 Tensile strength
To increase the cord’s resistance to mechanicalstress, we attached a longitudinal high-tensile58

strength body fiber made of aramid fiber, which
exhibits excellent resistance to mechanical stress andtemperature.
2.3 Covering
In a conventional optical fiber cord, general-purpose plastics such as polyvinyl chloride (PVC) andpolyethylene are used as covering materials. However, considering environmental issues, we decided touse materials free of halogens and organic phosphorus. The goals were to ensure that the fiber cord isprotected against deterioration of its mechanicalproperties by trampling and to make certain that it isfire retardant. After examining various plastics, wechose thermoplastic elastomer for the covering.
2.4 Shape
With existing optical fiber, bending must be controlled because it affects both attenuation and longterm reliability. However, holey fiber features anextremely small radius bending loss, so we can ignore
attenuation caused by bending in home-use installaNTT Technical Review

Letters

Accumulated fracture probability F

10–2
Proof distortion
10–4

εp = 1.0%

–6

10

10–8

1.5%

10–10

2.0%

10–12

2.5%

10–14

3571020

Radius of bend (mm)Fig. 4. Fracture probability of optical fiber (with 250 turns after 20 years).

tion. Therefore, we only need to be concerned withlong-term reliability. Figure 4 shows the calculated
reliability of optical fiber [2]. This shows that fiberwith a bending radius of 10 mm has a fracture probability of 10–4 or less over twenty years of use, which
is our design target. Considering the intended usage
we selected a cord diameter of 2.0 mm (Fig. 5), a curl
diameter of 10 to 20 mm, and a curled part length of
100 to 500 mm (see Fig. 2).
3. Actual characteristics

Optical fiber

High-tensile-strength fiber
Covering

2.0 mmThe properties of the manufactured optical fiber
curl cord are given in Table 2.

Fig. 5. Cross-section of optical fiber curl cord.

3.1 Optical properties
The manufactured optical fiber curl cord has an MU
connector at each end. Its total attenuation is 0.73 dB
on average, which satisfies the attenuation loss
requirement for the connector. There is no increase inloss in the curled part.

perature cycle test in the range from –20 to +60°C.
The results show that the loss variation was less than
0.1 dB and very stable.

3.2 Characteristics when used with Ethernet
We tested the compatibility of the optical fiber curl
cord with Ethernet by connecting it to a gigabit Ethernet circuit tester. No bit errors occurred during the24-hour test. Moreover, there were no errors when thecord was pulled or moved a short distance.
3.3 Thermal properties
The optical fiber curl cord was subjected to a temVol. 3 No. 5 May 2005

3.4 Fire retardation
The optical fiber curl cord met the requirements ofthe thermal burning test at an inclination of 60° (JISC3005). The covering consisted of non-halogen
materials to make the cord environmentally friendly
and it was made fire retardant by using a non-organic
phosphorus compound as an additive.
3.5 Elasticity
The relationship between pulling force and stretching length is shown in Fig. 6. In Region A, little force
was required to stretch the cord and when the force
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Table 2. Properties of optical fiber curl cord.
Property

Test conditions
With MU-connector at each end≤1 dB

Circuit

Ethernet bit errors

No errors

Temperature–20 to +60°C cycle

Loss variation ≤0.1 dBCombustion

JIS C3005 60° inclination combustion

Self reduction

Elasticity

1000% pull (Region A)

Loss variation ≤0.1 dB
Stress ≤200 g

Instantaneous pulling

Pulled 1000% in 0.5 s

Loss variation ≤0.1 dB

Pressure

100 kg applied radiallyNo breakage
Loss variation ≤0.1 dB

200% extension and release:one cycle per second for 24 hours≥90% of curl remained
immediately after test

Pulled until failure≥30 kgInsertion loss

Curl retention
RupturePulling power (N)ValueRegion ARegion B

Region CExtension (%)

Fig. 6. Stretching characteristics of curl cord.

was released, restoration was instantaneous andspring-like. When pulled with greater force, the cord
became almost straight. However, in Regions B andC, after being pulled with great force, the cord did not
regain its shape instantaneously. Within Regions Aand B, the loss variation was very small: less than 0.1
dB. When stretched to Region C, the attenuation
increased just prior to failure. For example, if wewish to manufacture a curl cord that is 3 m long and2 mm in diameter with a curl diameter of 15 mm, thecurled section will be around 13 cm long. For this
optical fiber curl cord, Region A will be up to about
200 cm (around 1500% of the curled part) and Region
B will be around 200–250 cm (1900% of the curledpart).
3.6 Robustness
We assumed that a curl cord may get accidentallycaught and tugged during normal use. We measured
the attenuation and stress caused by instantaneous(about 0.5 s) pulling to a distance of about 1 m andrelease. The measurement revealed no change inattenuation, and the stress was small (under 2 N), as
shown in Fig. 7. This means that even if a cord is acci60

dentally caught and tugged, it will not break, the connectors will not be disconnected, and the information
terminal will not be moved.
3.7 Pressure resistance
We assumed that a curl cord may get accidentally
stepped on when it is on the floor. We tested the cords
under such conditions and found that they could withstand a pressure of 100 kg.
3.8 Curl retention
The cord will undergo many cycles of expansion
and contraction in daily use. We looked for any loosening of the cord that might occur under such conditions by pulling and releasing it. The test we usedmeasured the slackness resulting after we had
stretched the cord to 200% by pulling and releasing it
once per second continuously over a 24-hour period.
The cord exhibited a shape recovery rate of over 90%
immediately after testing, and the rate increased to100% after a rest period.
3.9 Rupture properties
The curl cord was not broken even when it was
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Letters4
3.5

Stress (N)

32.5
2
1.5
10.5
00.2

0.4

0.6

0.8

11.2

Time (s)Fig. 7. Instantaneous pulling.

pulled from one end (with the other end fixed) until it
became straight. The force required to break the cord
was greater than 30 kg, which is almost the same asthat for a non-curled optical fiber cord.

and develop additional novel optical fiber components.
References

4. Future plans[1]

The development of the optical fiber curl cord is
now complete. We are proceeding with commercialization to make this cord available as a home network
wiring component. We plan to continue our studies[2]

K. Hiramatsu, T. Kurashima, E. Araki, and S. Tomita, “Developmentof the Optical Fiber Curl Cord,” IEICE, B-10-3, Mar. 2004 (in Japanese).
Y. Mitsunaga, Y. Katsuyama, H. Kobayashi, and Y. Ishida, “StrengthAssurance of Optical Fiber Based on Screening Test,” IEICE, Vol. J66-B, No. 7, 1983 (in Japanese).

Katsumi Hiramatsu

Toshio Kurashima

Research Engineer, Media Utilization Group,Access Media Project, NTT Access Network
Service Systems Laboratories.
He graduated in electrical engineering andchemical engineering from the Technical JuniorCollege of Ibaraki University, Ibaraki in 1971and 1974, respectively. He joined NTT Laboratories, Ibaraki, Japan in 1967, where he worked
on metallic cable, optical fiber cable, and opticalfiber sensing techniques. He is currently developing future home networks.

Senior Research Engineer, Supervisor, Media
Utilization Group, Access Media Project, NTT
Access Network Service Systems Laboratories.
He received the B.E., M.E., and D.E. degrees
in electronic engineering from the University ofElectro-Communications, Chofu, Tokyo in
1986, 1988, and 1997, respectively. He joined
NTT Transmission Systems Laboratories in1988. He has been engaged in research on opticalfiber distributed sensing techniques. Since 2001,
he has been engaged in R&D on broadband homenetworking. He received the IEICE excellent
paper award and young engineer award in 1991
and 1992, and the outstanding paper award of theInternational Wire and Cable Symposium in1995. He is a member of the Institute of Electronics, Information and Communication Engineers (IEICE) of Japan.

Shigeru Tomita
Senior Research Engineer, Supervisor, Media
Utilization Group Leader, Access Media Project,NTT Access Network Service Systems Laboratories.
He received the B.S. and M.S. degrees in electronic engineering from Nihon University, Chibain 1981 and 1983, respectively. He joined Nippon Telegraph and Telephone Public Corporation (now NTT) in 1983 and was active in developmental research on optical fibers and cables
until 1999. He managed research affairs from1999 to 2003. Now he is engaged in research onbroadband home networking. He received theoutstanding paper award of the 40th International Wire and Cable Symposium. He is a memberof IEEE.

Vol. 3 No. 5 May 2005

Eiji Araki
Research Engineer, Media Utilization Group,Access Media Project, NTT Access Network
Service Systems Laboratories.
He received the B.S. degree in civil engineering from Kumamoto University, Kumamoto in1986. He joined NTT in 1986. He has been
engaged in R&D of customer service systems
since 1998. He is a member of IEICE.

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