Development of New Field-Installable Connectors for FTTH
by Mitsuhiro Iwaya *, Koji Seo *, Hideki Miyazaki *, Naoya Nishimura *,Nobuhiro Nanri *, Naoko Shimoji *2 and Masahiro Hirao *2

Fiber-to-the-home (FTTH) services have achieved wide penetration in recent
years, and seem likely to grow even faster in the future. Thus, in the process ofservicing subscribers’ homes with fiber to provide optical internet access, there
is a growing need for shorter work times and lowering requirement for expertise in installation
personnel. Specifically in the splicing process--whether mechanical or fusion--it is necessary tohandle the excess lengths of fiber that remain after splicing and this has been a time-consuming
process. Another problem arises from the need, at the time of service changeover, to cut fibers
and then repeat the installation process. In an effort to solve these problems we have developed
optical connectors that can be joined directly to drop cables and indoor cables at any point fromthe aerial closure to inside the subscriber’s premises, in several types according to the locationin which they are used.

ABSTRACT1.

INTRODUCTION

In recent years Japan has seen a dramatic spread ofbroadband service. The number of FTTH subscribers has
already exceeded 7 million, and is increasing at a rate of
300,000 households per month. This rate of increase is
expected to further accelerate, creating a growing needfor shorter work times and a lower level of expertise ininstallation personnel.
Figure 1 shows an example of a conventional network
for providing subscribers’ homes with access to opticalservices. The optical fibers laid as a cable from the local
station are separated into single optical fibers at an aerial
closure near the subscriber’s premises, and connected toa drop cable to reach under the eaves of the subscriber’s
premises. Then in an under-eaves cabinet the drop cable
is connected to an indoor cable, which leads into therooms in the subscriber’s premises and is joined to an
optical network unit (ONU). In the conventional opticalwiring configuration for subscribers’ premises, the joiningof optical fibers at these connection points (aerial closure,cabinet, etc.) has been accomplished by mechanical or
fusion splicing. This has meant that when the splicing isdone, excess length of fiber is required and care is needed to make sure that bending, tensile or other strain is notapplied, so that the work needed a high level of expertise
and was time-consuming.
There was also the problem that during service changeover or interruption, the fibers had to be cut at the connection points and the installation work repeated.
Accordingly, with a view to improving maintenance by* FITEL Photonics Laboratory, R&D Division*2 FITEL Products Division, Telecommunications Company

introducing the use of connectors at aerial closures andcabinets and achieving a new optical access configuration (Figure 2), we have developed field-installable opticalconnectors and can report that their various characteristics have been confirmed.

Aerial closure

CabinetFigure 1

ONU

Layout of conventional access network.

Aerial closure

CabinetFigure 2

OutletONU

Layout of new access network.

Furukawa Review, No. 31

2007

7

Development of New Field-Installable Connectors for FTTH

2.

TYPES OF FIELD-INSTALLABLE
CONNECTORS FOR FTTH

2.1 FA Connector
This was developed for cable connection at under-eaves
cabinets in subscribers’ premises and for outlets of newly
built housing. It has a unique connection interface comprising a plug and a socket, and the connector can bedirectly joined to drop cables or indoor cables (i.e., iscable-holding). Specifically, the socket has a uniquecurved housing structure, and can accommodate JIS C
8435 compliant switchboxes and shallow SM36, and canbe connected directly to SC connectors without interposing an adapter, thereby achieving optical connection at
the outlet.
2.2 Field-Installable Connector for Aerial ClosureThis is field-installable connector in which the cross-sectional area is reduced to achieve a high-density arrangement in aerial closures. It comprises a unique plug-socketinterface. With respect to the connection configuration at
the aerial closure, the plugs are of two types: one “for thedrop cable” that can be connected to a drop cable andone “for the fiber” that holds the optical fiber; there is only
one type of socket, “for the fiber”.
2.3 Field-Installable Cable-Holding SC Connector
Applying the cable-holding technique of FA connectors,
we developed a field-installable cable-holding SC connector that can be used around subscriber station equipment. It can be directly connected to drop cables orindoor cables and has a connection interface equivalent
Plug

Figure 3

SocketAppearance of FA connector.

Plug (for drop cable)

to that of conventional SC connectors.

3.

STRUCTURE OF FIELD-INSTALLABLE
CABLE-HOLDING CONNECTOR

The basic structure of a field-installable cable-holding
connector is describes as follows: First, by means of the
housing the connection interface is formed and the components and fiber inside are protected. Inside the housing
are accommodated a ferrule-combined mechanical splice
and a cable-holding block, and, depending on the type of
connector, it may incorporate a spring to press the ferrule
toward the direction of the connection and a split sleeveto achieve axial alignment of the ferrule. The cable-holding
block is a plastic part having a slit with tab-shaped protrusions, with the function of holding the cable sheath andfastening the cable to the connector. From the leading
end of the cable the optical fiber is exposed, and is joined
to the connector by a ferrule-combined mechanicalsplice.
As Figure 7 shows, the ferrule-combined mechanicalsplice consists of a V-groove base integrated with the ferrule and lids for pressing on the fiber. These are compressed by clamp springs. The V-groove base has agroove to carry an optical fiber, and by means of the twolids holds the bare fiber and the coated part. Within theferrule the pre-installed fiber is fastened by bonding, andthe leading end is polished together with the ferrule, theother end is cleaved, and the fiber is set on the V-groove.
Figure 8 shows the mechanism of a mechanical splice.
There is a slit formed between the V-groove base and theside of the fiber-pressing lids, and by inserting a wedge atthat point it is possible to make a slight opening between
the meeting surfaces of the base and the lids and introduce the optical fiber along the V-groove. Here the opticalfiber that has been cleaved in the field is inserted, andwhen the end of the fiber butts against the pre-installedfiber the wedge is withdrawn and both fibers are clamped
between the base and the lids, creating a stable splice. In
Housing

Fiber

Cable-holding block

Socket (for ﬁber)

Ferrule-combined mechanical splice

Figure 4

Appearance of field-installable connector for aerial
closure .

Figure 6

FerruleV-groove base

Figure 5

Appearance of field-installable cable-holding SC connector.

Figure 7

Cable

Structure of cable-holding connector.

Pre-installed ﬁber

Lids

Index-matching material

Clamp spring

Ferrule-combined mechanical splice.

Furukawa Review, No. 31

2007

8

Development of New Field-Installable Connectors for FTTH

order to obtain satisfactory values for insertion loss andreflection characteristics, the area around the splice isfilled with an index-matching material.
The optical fiber protruding from the rear of the ferrulecombined mechanical splice is deflected slightly inside
the housing, and, at the rear of the housing is fixed,
together with the sheath, to the cable-holding block. In
those field-installable connectors for fiber, however, there
is no cable-holding block, and the optical fiber comes
directly out of the housing with no deflection.

4.

SPLICE MECHANISM OF FIELDINSTALLABLE CABLE-HOLDINGCONNECTORSThe splice mechanism of a field-installable cable-holding
connector is designed in a way that differs structurally
from that of ordinary factory-installed connectors. In ordinary cord-attached connectors, the optical fiber is fastened to the ferrule with a bonding agent and the fibers
are “loose”, that is to say the fibers do not stick to thecord sheath and can move inside comparatively freely.
Accordingly even if, for example, ferrule push-back occurs
when the connector is connected, the optical fibers arepushed back in the cord to the same degree that the ferrule recedes, and the excess length for the push-back canbe absorbed smoothly over the whole length of the cord.
Drop cables and indoor cables, by contrast, have a“tight” structure in which the sheath and the fibers stick
fast together so that the fibers cannot move inside. And
since, as has been explained above, the cable sheath isclamped inside the housing by the cable-holding block, it
is not possible to secure sufficient length for the absorption of the excess fiber length for the receding of the ferrule. For this reason when connection is made in thesame way as with a conventional connector and ferrule
push-back occurs, the fiber is sharply bent between the
trailing end of the ferrule-combined mechanical spliceand the edge of the cable sheath, leading to bending loss
and fiber breakage.
In order to overcome this problem, we will describe astructure that newly adopts the field-installable cableholding connector.

was of major importance. For this reason in the connector
socket, the housing itself is of a unique shape, curving at
90 degrees, and the optical fibers inside are bent to aradius of approximately 10 mm. If in this situation the ferrule-combined mechanical splice is pushed back, excessive bending is applied to the fiber and significant losses
occur. For this reason the socket is designed so that the
spring is eliminated and push-back does not occur. In
other words the socket is so constructed that the ferrulecombined mechanical splice and the cable-holding block
are fixed to the housing after the connector is assembled
so as not to move, thereby preventing variations in fiber
bending radius. See Figure 9.
In the connector plug that is connected to the socket,
on the other hand, there is a need for a spring to exert apressing action on the ferrule. Accordingly in the connector plug a joint element is provided to the rear of the ferrule-combined mechanical splice, coupling it to the cableholding block (Figure 10). By this means even when ferrule push-back occurs, the cable-holding block always
moves back together so that there is no change in opticalfiber excess length.
4.2 Field-Installable Connector for Aerial Closure
In the case of the field-installable connector for aerial closure, there are two types of plug: for drop cable and for

Ferrule-combined mechanical splice(ﬁxed to housing)
Fiber

Cable-holding block(ﬁxed to housing)

Split sleeve

Figure 9

Connecting mechanism of FA connector socket.

Before connecting
Cable-holding block Joint

Ferrule-combined
mechanical splice

4.1 FA Connector
Let us first describe the case of the FA connector. As has
been stated above, the FA connector was developed witha view to providing service to outlets, so that reducing thesize of the connector socket accommodated in the box
SpringFiber being inserted

Fiber clamped
When connected

Wedge

Ferrule pushed backPushed back together

Figure 8

Mechanism of mechanical splice.

Figure 10 Connecting mechanism of FA connector plug.

Furukawa Review, No. 31

2007

9

Development of New Field-Installable Connectors for FTTH

fiber, whereas the sockets are only for fiber. Accordingly aspring is used only in the sockets, whereas in the plugs,
which may possibly be connected to a drop cable, a system similar to the FA connector socket is adopted, inwhich the ferrule-combined mechanical splice and cableholding block are fixed to the housing. In other words inthe socket, even if push-back of the ferrule-combined
mechanical splice occurs, the fiber is simply pushed outside the housing without being bent, and in the plug nopush-back will occur. Note that in plugs for fiber there isno cable-holding block, but, as in the plugs for drop
cable, the spring is eliminated.
4.3 Field-Installable Cable-Holding SC Connector
The field-installable cable-holding SC connector, unlike
the FA connector or the field-installable connector for aerial closure, does not make connection by bringing together a plug and socket, and it therefore requires a spring.
Accordingly, as for the FA connector, it is provided with ajoint element at the rear of the ferrule-combined mechanical splice, coupling the cable-holding block.

5.

INSTALLATION METHODS AND TOOLSIn field-installable cable-holding connectors the distance,
after the connector has been installed, between the position at which the cable-holding block is accommodated
and the position at which the leading end of the opticalfiber is fixed (the position of the fiber pre-installed in theferrule-combined mechanical splice) is pre-determined,
irrespective of the connection configuration (jointed or
fixed), by the dimensions of the constituent parts of eachconnector. For this reason it is extremely important when
installing the connector to control the length from thecable-holding block to the leading end of the fiber.
That is to say if the fiber is too long, the deflection of thefiber inside the housing after connector assembly increases giving rise to bending loss, whereas if it is too short,the fiber will not reach the pre-installed fiber and it is
impossible to achieve a proper mechanical splice.
Nevertheless, fiber length is determined by the termina-

Socket (for ﬁber)tion dressing operation in the field, and from the standpoint of simplifying the operation and reducing the timerequired, it is impossible to leave precise adjustment ofdimensions up to the operator.
Accordingly we have developed dedicated fiber preparation tools that allow anyone to carry out proper adjustment of fiber dimensions easily and quickly. And by making fiber length the same for every connector it is possible
for the preparation tools to be used in common for allconnectors.
5.1 Holder and Stripper
We have established the dimensions shown in Figure 12
as the optical fiber length common for all of the fieldinstallable cable-holding connectors. And to achieve thiseasily, we developed a dedicated holder and a dedicatedstripper. See Figure 13. By using these tools and conventional fiber cleaver, the proper fiber length is obtained
easily and precisely.
Control of fiber dimensions is also required with respect
to the field-installable connector for fiber, and we havealso developed a fiber holder that can be used in combination with the drop cable holder 5).
5.2 Wedge
The field-installable cable-holding connector that we havedeveloped has a wedge inserted at the factory in themechanical splice portion, so that the process of wedge
insertion need not be performed in the field. This bothprevents connector installation errors due to faulty inser-

10 mm
25 mm

Figure 12 Proper fiber length.
Drop cable holder

Cable-holding block

SpringFiber

Plug (for drop cable)

Drop cable

StripperDrop cable holder

Fixed to housing

Figure 11 Structure of field-installable connector for aerial closure.

Figure 13 Tools for fiber preparation.

Furukawa Review, No. 31

2007

10

Development of New Field-Installable Connectors for FTTH

tion of the wedge and contributes to shortening the timerequired. After the fiber termination has been introduced,
the wedge can be easily withdrawn in a single action.
Figure 14 shows the procedure for wedge removalusing the example of a field-installable connector for aerial closure. By pressing on the protrusion at the rear of thewedge (to the right in the photo), the wedge is withdrawn
by leveraged action from the ferrule-combined mechanical splice. The connector housing is sandwiched by thewalls on the wedge, so that even when the wedge is withdrawn it does not separate from the connector preventing
unintended drop down at the site.
5.3 Assembly Support Tool
Since all of the field-installable connectors that we havedeveloped make connection by mechanical splice, it isnecessary to insert the optical fiber into the ferrule-combined mechanical splice base. To simplify this operation
we have produced a fiber assembly support tool for eachof the connectors. Since with respect to the FA connector
the installation process is different for the plug and thesocket, we have designed fiber guide structures that are
optimized for each, and have consolidated them into asingle tool. Similarly for the field-installable connector for
aerial closure, the fiber guides for fiber and for drop cable
are consolidated into one 2), 5).

7.

The results and conditions for various reliability tests areshown for the field-installable connector for aerial closurein Table 1 by way of example. Measurements were madeat two wavelengths, 1310 nm and 1550 nm, and it was
confirmed that both mechanical characteristics and environmental characteristics showed outstanding reliability.
Representative of these is the graph showing the resultsof the long-term temperature and humidity test in Figure
16. With respect to the FA connector and the field-installable cable-holding SC connector, reliability was confirmed
to be equivalent to that of the field-installable connector
for aerial closure for drop cable.

CHARACTERISTICS

6.1 Initial Optical Characteristics
Here we present as an example in Figure 15 the random
insertion loss characteristics for the plug (for drop cable)
and socket (for fiber) connection of a field-installable connector for aerial closure. The results obtained were satisfactory: 0.20 dB (average) and 0.51 dB (maximum) measured at a wavelength of 1310 nm, and 0.19 dB (average)
and 0.51 dB (maximum) at 1550 nm. Good results werealso obtained for return loss, in excess of 40 dB in all

RELIABILITY

120
100
Count

6.

cases.
While not shown in a graph, the values for plug-socket
connection in FA connectors were also confirmed to besatisfactory: 0.25 dB (average) and 0.52 dB (maximum) at
1310 nm, and 0.25 dB (average) and 0.48 dB (maximum)at 1550 nm.
Similarly the insertion losses from field-installable cableholding SC connector to SC master connector weregood, at about 0.18 dB (average) and 0.30 dB (maximum)at 1310 nm, and 0.16 dB (average) and 0.25 dB (maximum) at 1550 nm.

Ave.
Max.
Min.
Std.

80
60

1310 nm0.20 dB
0.51 dB
0.08 dB
0.08 dB

40
20
0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Insertion loss（dB）

Figure 15 Random insertion loss characteristics of field-installable connector for aerial closure.

Table 1

Reliability tests of field-installable connector for aerial closure.

Test item
TensileFlex
Vibration

Conditions
For drop cable: 10.0 N
For fiber: 3.0 N
For drop cable: 4.9 N
For fiber: 0.05 N

Loss increase(max.)
0.2 dB
0.2 dB

1.5 mm, 10-55 Hz, 2 hrs, 3 axes

0.2 dB

Impact100 G, 6 ms, 3 times, 3 axes

0.2 dB

Durability

500 times, cleaning every 10 times

0.2 dB

Temperature-40~70℃, 6 hrs x 10 cycles
cycling
Humidity conden-10~65℃, 95%RH, 24 hrs x 10 cycles
sationFigure 14 Procedure for wedge removal.

1550 nm0.19 dB
0.47 dB
0.12 dB
0.07 dB

0.2 dB
0.2 dBHigh temperature

70℃, 240 hrs

0.2 dB

Low temperature-40℃, 240 hrs

0.2 dB

Long-term
environmental
reliability

85℃, 60℃, 95%RH, -40~70℃,
336 hrs for each

0.2 dB

Furukawa Review, No. 31

2007

11

Development of New Field-Installable Connectors for FTTH

8.

REFERENCES

CONCLUSION

0.50
0.40
0.30
0.20
0.10
0.00
-0.10
-0.20
-0.30
-0.40
-0.50

Temperature
温度

450
350
250
150
50

Temperature（℃）

Loss imcrease（dB）

To respond to installation issues and requirements
accompanying the recent penetration of FTTH, we haveundertaken developments aimed at use of connectors
from the aerial closure to the ONU in the subscriber’s
premises. We developed field-installable optical connectors suitable for each connection point, and have confirmed satisfactory optical characteristics and reliability.
We have also developed appropriate working tools toenable the work to be carried out easily and precisely.

1) Yasuo Oda: “An optical wiring technology for home network to realize a service-ready and low-cost FTTH service”, Proceedings of the2005 IEICE General Conference, B-10-9 (March, 2005). (inJapanese)2) Mitsuhiro Iwaya: “Novel field installable connector for optical fiberwire”, Proceedings of the 54th IWCS, 1-4, Nov. 2005.
3) Kuniaki Terakawa: “Aerial joint technology to realize large amountof optical construction”, Proceedings of the 2006 IEICE General
Conference, B-10-5 (March, 2006). (in Japanese)
4) Tatsuya Nakajima: “Development of the aerial optical connector to
realize large scale optical network construction”, Proceedings ofthe 2006 IEICE Society Conference, B-10-7 (Sep, 2006). (inJapanese)5) Koji Seo: “Development of field assembly connector for aerial closure”, Proceedings of the 2006 IEICE Society Conference, B-10-8(Sep, 2006). (in Japanese)-50

Figure 16 Result of long-term environmental reliability test of fieldinstallable connector for aerial closure (at 1550 nm).

Furukawa Review, No. 31

200712