Letters

Development of Aerial Optical Cables with a Small Number ofFibers and Aerial Optical Closure for Connecting the Cables
Syusuke Kiyosue, Masahiro Hashiguchi, Naoshi Ogawa†,
and Masashi Awamori
Abstract
We have developed aerial optical cables with a small number of fibers and an aerial optical closure inwhich these cables can be connected. They will reduce the investment required for aerial optical facilities to meet the wide-spread demand for FTTH (fiber to the home) services. Installing access points nearcustomers’ houses in advance will allow us to complete construction promptly when faced with sudden
demands for FTTH.

1. Background
With the rise in the number of subscriptions toFTTH (fiber to the home) services such as the BFLET’S service provided by NTT East and NTT
West, there has been considerable investment in theconstruction of aerial optical facilities in recent years[1]. Aerial drop optical fibers and self-supporting
small-diameter optical fiber cables have already been
employed to meet the wide-spread FTTH demand,but the cost of these cables must be further reduced.
In addition, as the amount of construction increases,
we must find a way to shorten the construction time.

We have therefore developed more economical aerial
optical cables that contain a small number of fibers
and a new aerial optical closure to accommodate
these cables that is designed to be installed in advanceat access points near customers’ houses before service orders are received. In addition, we improved theexisting aerial optical closure so it can handle the newcables (Fig. 1).
In our previous reports [1], [2], which dealt with theadvanced technology of connecting cables and fibers,
we described the advantage of using connectors.
However, in this article, we focus on improvements tocables and closures without mentioning the methods

Access point
New aerial optical cables
with two 4-fiber ribbons

New aerial optical closures
to connect new cables

Access point

Access point

Existing aerial optical closures New aerial optical cables
with eight fibers

Existing aerial optical fiber cables

Fig. 1. New developments.
† NTT Access Network Service Systems Laboratories
Tsukuba-shi, 305-0805 Japan
E-mail: n.ogawa@ansl.ntt.co.jp

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61Lettersof connecting cables and fibers, so the conventional
connection methods are assumed here.
2. New aerial optical cables
Based on the following main requirements, wedeveloped two new cable designs. One has eightfibers and the other has two 4-fiber ribbons.
(1) The cables must have an economical design.
(2) They must be self-supporting between telephone poles.
(3) Their fibers must be easy to extract with existing tools.
The new cables are compared with conventional
ones in Fig. 2. Conventional 8-core aerial drop cablehas eight elements each consisting of a pair of tension
members and a 0.25-mm coated fiber. Each element
is enclosed in a sheath. The messenger wire* in thecenter is also sheathed. The self-supporting smalldiameter optical fiber cable has a pair of 4-fiber ribbons, which each has a nylon sheath, enclosed in asheath. The new design reduces the cable cost not
only by reducing the number of manufacturing
processes and the amount of materials required by* Messenger wire: a steel wire that takes up most of the tensile loadwhen the optical cable is installed and hung on the pole.

integrating the messenger wire and the cable along its
length but also by directly covering the optical fibers(and messenger wire) with the sheath. To enable thecable to withstand the tension when it is hungbetween poles, we carefully chose the diameter of thesingle steel wire. We were concerned that the fibers
might be difficult to extract, especially in the cablewith eight fibers, which has a tight structure due tothe optical fibers being directly covered with asheath. However, we successfully devised thearrangement of each fiber in the cable and optimizedthe load exerted by the sheath on the optical fibers inthe cable. As a result, the new cables have a lower
cost, weight, and diameter than conventional cables.
3. New aerial optical closure
The requirements for the new closure were to besmall and as light as possible to make it economical,
to reduce the tension in the messenger wire, and not
to appear unsightly. The conventional closure needed
to be a certain minimum size because it had to accommodate extra lengths of optical fibers for cutting andsplicing after the cable was installed. To enable thenew closures to accommodate extra lengths of opticalfibers, we partially detached the messenger wire section from the cable and cut off the messenger wire for

8-core aerial drop optical fiber cableSelf-supporting small-diameter
optical fiber cable

Messenger wire
SheathTension members
0.25-mm coatedoptical fiber
SheathConventional
structure

Messenger wire
SheathTwo 4-fiber ribbons
Nylon sheath
SheathWeight: 0.15 kg/m

Weight: 0.07 kg/m

Cross-sectional
area:
–80%
Weight:
–60%
Cost:
–70%

Cross-sectional
area:–40%
Weight:
–15%
Cost:
–70%

Aerial optical cablewith eight fibers

Aerial optical cablewith two 4-fiber ribbons
SheathMessenger wire
Tension members

Newstructure

Pair of 4-fiber ribbons
Eight 0.25-mm coatedoptical fibers
Weight: 0.06 kg/m

Weight: 0.06 kg/m

Fig. 2. Cross-sectional views of optical cables with a small number of fibers.

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Letters

the length corresponding to the extra length of opticalfibers. The extra lengths of optical fibers thus
obtained can be stored compactly in the closure, so
we were able to reduce the size and weight of the closure (Fig. 3). Moreover, the new closure enables optical fibers to be connected toward the terminal side sothat another customer on the terminal side can use afiber when a customer cancels a service order,
because the new closure can accommodate longer
extra lengths of optical fibers than the conventionalclosure. As a result, we also achieved a substantial

cost reduction.
Installing these new closures near customers’ houses in advance will allow us to complete construction
promptly at reduced cost when faced with sudden
demands for FTTH.
As Fig. 4 shows, the closure contains eight fiberpass trays that accommodate optical fiber passing
through the closure to the terminal side and six drop
trays to accommodate optical fiber to be dropped tocustomer’s houses.

Conventional closure
Conventional closure

Messenger wire

Conventional cable

Cable section of
the conventional cable

Telephone pole

New closure
Messenger wire section

New cable

New closure

Cable section of
the new cable

Fig. 3. Comparison of the closures.
New closure
Messenger wire

Wire clamp

Base
Sheet for fiber storage
Fiber-pass trays

Cable clamp base
Cable clamp
GrommetDrop trays

0.25-mm coatedoptical fiberFiber-pass tray: accommodates an optical fiber passing through the closure to the terminal side.
Drop tray: accommodates an optical fiber dropped to a customer’s house. (Right photograph: without drop trays.)
Sheet for fiber storage: stores eight optical fibers that go through the closure into a fiber-pass tray or drop tray.

Fig. 4. Structure of new aerial optical closure for connecting the new cables.

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Letters

4. Improvements to existing closure
Syusuke Kiyosue

We also improved the existing closure to enable itto accommodate both the existing cable and the newcable. Closures of the improved conventional design
will be applied to routes where the demand is estimated to be low enough to be handled by eight orfewer fibers. We developed metal fittings to clamp thenew cable and a spiral tube to protect a single 0.25mm-diameter fiber arranged in the closure.
References
[1]

[2]

H. Aoyama, H. Tanaka, Y. Hoshino, and Y. Oda, “Optical Wiring
Technology for Home Networks for a Service-ready and Low-cost
FTTH Service,” NTT Technical Review, Vol. 3, No. 4, pp. 33-37,
2005.
K. Ando, Y. Aoyagi, and M. Awamori, “Aerial Optical Fiber Interconnection Method for Connecting Fibers of Different Carriers,”
NTT Technical Review, Vol. 1, No. 2, pp. 71-74, 2003.

Senior Research Engineer, Second Promotion
Project, NTT Access Network Service Systems
Laboratories.
He received the associate degree in electricalengineering from Oita National College of Technology, Oita in 1985. He joined NTT in 1985 andmoved to his present position in 2001. He is
actively engaged in the development of opticalfiber cable technology.

Masahiro Hashiguchi
Research Engineer, Second Promotion Project,
NTT Access Network Service Systems Laboratories.
He joined Nippon Telegraph and Telephone
Public Corporation (now NTT) in 1981 andmoved to his present position in 2000. He isactively engaged in the development of opticalclosure technology.

Naoshi Ogawa
Research Engineer, Second Promotion Project,
NTT Access Network Service Systems Laboratories.
He received the B.E. degree in material engineering from Waseda University, Tokyo in 1991.
He joined NTT in 1991 and moved to his presentposition in 2003. He is actively engaged in thedevelopment of optical fiber cable technology.

Masashi Awamori
Research Engineer, Second Promotion Project,
NTT Access Network Service Systems Laboratories.
He received the associate degree in electricalengineering from Ishikawa National College ofTechnology, Ishikawa in 1993. He joined NTT in
1993 and moved to his present position in 2001.
He is actively engaged in the development of
optical closure technology.

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