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1002
IEICE TRANS. ELECTRON., VOL.E84–C, NO.7 JULY 2001
PAPER
Long-Term Reliability of Plastic Ferrules for Single-Mode
Fiber-Optic Connectors
Yoshito SHUTO†a), Shuichi YANAGI†, Masayoshi OHNO†, Hirotsugu SATO†∗,
Shin SUMIDA†,and Shunichi TOHNO†,Regular Members
SUMMARY We examined the creep properties and hazard
rates of plastic ferrules to ensure the long-term reliablity of op-
tical fiber connections. The endface deformation ∆Lhad to be
smaller than 3 µm to keep the insertion-loss and return-loss fluc-
tuation to acceptable levels in the worst case of random concate-
nation of similarly deformed plastic ferrules. From the fluctuation
data, we estimated the time-to-failure tfat which the ∆Lvalue
became 3 µm. We estimated the acceleration parameters, me-
dian lifetimes ξ, and hazard rates λby using tfvalues based on
Weibull statistics. The ξvalues decreased rapidly with increasing
temperature and relative humidity. We found we could expect
small λvalues of <0.1 FIT (FIT=10−9/hour) and of 1 FIT for
20 years in a normal atmosphere (25◦C/50%RH) and in a more
severe case of 25◦C/90%RH, respectively.
key words:
1. Introduction
Recent fiber-optic telecommunication systems have
been constructed mainly using single-mode (SM) opti-
cal fibers. Several optical connectors have already been
developed that are capable of connecting SM optical
fibers with low loss and low reflection [1]–[6]. Among
them, the single coupling (SC) and miniature unit cou-
pling (MU) type optical connectors were developed for
use as all-purpose connectors with which to couple to-
gether simplex or duplex SM optical fiber cables [3],
[5].
The SC- and MU-type optical connectors were de-
signed to have a push-pull coupling mechanism that
employed a split alignment sleeve and precise cylindri-
cal ferrules. The structure of an SM optical fiber con-
nection made with an SC or MU connector is shown in
Fig. 1. The combination of a zirconia or copper-alloy
split alignment sleeve and precise cylindrical zirconia
ferrules has been widely used.
Today, we need optical connectors with inexpen-
sive sleeves and ferrules to reduce the cost of optical
subscriber systems such as fiber to the home (FTTH).
Ceramic ferrules account for a major portion of the cost
of optical fiber connector parts. The use of plastic ma-
terials for the ferrules would be one way to reduce this
Manuscript received January 16, 2001.
†The authors are with NTT Photonics Laboratories, Ni-
ppon Telegraph and Telephone Corporation, Ibaraki-ken,
319-1193 Japan.
∗Presently, with NTT Electronics Corporation.
a) E-mail: shuto@iba.iecl.ntt.co.jp
cost. However, the ferrules of SM optical fiber con-
nectors must have submicron accuracy and it had been
difficult to achieve such accuracy with plastic molding
techniques [9]–[11].
One of the most important factors in terms of
achieving low connection loss characteristics is eccen-
tricity, which is the difference between the center of the
outer diameter and the center of the minute hole into
which the optical fiber is inserted. We have recently
injected molded plastic ferrules using an eccentricity
control system in the mold and a liquid crystalline poly-
mer (LCP) as the molding material [7]. By optimizing
the eccentricity control conditions, we realized injec-
tion molded plastic ferrules with a small eccentricity of
<1µm and excellent optical characteristics [8].
However, when plastic materials are subjected to
a constant force or stress, they exhibit the increasing
deformation with time known as creep. When we join
two connector plugs with plastic ferrules in an adaptor,
compressive contact forces are generated on the ferrule
endfaces. These compressive forces result from the dif-
ference between the spring compressive force of the plug
and the gauge retention force of the adaptor, and they
cause the plastic ferrule to experience creep. Therefore,
to ensure the long-term reliablity of optical fiber con-
nections, we must determine the creep properties and
plastic ferrule lifetime when we apply the compressive
forces needed for ferrule connection.
In this paper we examine these aspects of plastic
ferrule behavior in detail.
Fig. 1 Optical fiber connection mechanism.
SHUTO et al.: LONG-TERM RELIABILITY OF PLASTIC FERRULES
1003
Fig. 2 Side view of plastic ferrule.
2. Experimental
2.1 Plastic Ferrule Fabrication
We used an injection molding technique to fabricate the
plastic ferrules. We selected liquid crystalline polymer
(LCP) as the molding material because of its fine mold
replication ability. In addition, the use of thermoplastic
LCP resin prevents the problem of flash in the injection
molding process.
The ferrule mold we used in this work has four
gates, and molten LCP resin flows into the cavity
through four runners in the mold. We were able to
control the resin flow through each runner by using a
runner cross-section control mechanism. We have pre-
viously found that the eccentricity of the plastic ferrule
can be changed by changing the balance of the four
resin flows [7], [8]. This cross-section control mecha-
nism, therefore, allows us to control the eccentricity
during the injection molding process.
The molding process was as follows. First, we
made a test molding and measured the eccentricity.
Then, we adjusted the cross-sections of the four run-
ners in the mold and made another test molding. We
repeated this process until the eccentricity became
less than 1 µm, after which automolding began. We
used a 30-ton horizontal injection molding machine for
the thermoplastic injection-molding process. Figure 2
shows an injection-molded LCP ferrule.
We assembled an SC-type connector plug with a
plastic ferrule whose eccentricity was less than 1 µm.
The plastic ferrule endfaces were polished using appro-
priate materials. The radius of curvature of the pol-
ished endface was around 10 mm and the fiber with-
drawal was about 0.1 µm. We observed no significant
change in the eccentricity after endface polishing.
2.2 Measurement Procedures
We carried out creep tests on SC-type plastic ferrules
(20 samples), each of which we connected to an SC-
type zirconia ferrule. The plastic ferrules were sub-
jected to compressive forces Wof about 0.7 N, which
Fig. 3 Deformed endface of plastic ferrule.
Fig. 4 Scheme of endface deformation.
resulted from the difference between the spring com-
pressive forces (1.0 N) of the SC plug and average gauge
retention forces (about 0.3 N) of the SC adaptor. The
endfaces of the plastic ferrule were gradually deformed
by the compressive forces Was shown in Fig. 3.
We defined the endface deformation ∆Las the
length change measured at the top positions of the
initial convex surface and the deformed surface (see
Fig. 4).
In the creep tests we measured the fluctu-
ation in the ∆Lvalues as a function of time.
We performed these tests in five different atmo-
spheres; 65◦C/90%RH, 55◦C/90%RH, 45◦C/90%RH,
65◦C/75%RH, and 65◦C/60%RH. We determined the
∆Lvalues of the plastic ferrules by using a three-
dimensional-imaging surface-structure analyzer.
We used a 1.31-µm LD light source when mea-
suring the insertion and return losses of the SC-type
connector plugs incorporating the plastic ferrules. For
these measurements, we used an advanced physical con-
tact (AdPC) polished zirconia ferrule as a master plug
and zirconia split alignment sleeves. The experimental
details of the optical measurements have been described
elsewhere [13].
3. Results and Discussion
3.1 Allowable Endface Deformation of Plastic Ferrule
When a plastic ferrule is connected with another ferrule
in a connector, its endface is gradually deformed over
time by the effect of the compressive contact force W.
This creep phenomenon becames more noticeable when
the plastic ferrule is connected with a zirconia ferrule,
rather than another plastic ferrule.
If two deformed plastic ferrules are joined together
1004
IEICE TRANS. ELECTRON., VOL.E84–C, NO.7 JULY 2001
Fig. 5 Insertion loss histograms of plastic ferrules with four
different degrees of endface deformation.
in the connector, both the insertion and the return
losses will fluctuate. In this subsection, we discuss the
allowable endface deformation ∆Lof the plastic fer-
rule with a view to realizing stable single-mode optical-
fiber connections. It is necessary to stabilize the optical
characteristics of the connectors, when the ∆Lvalue
increases.
First, we deformed the endfaces of plastic ferrules
by leaving them connected with zirconia ferrules at
85◦C in an 85%RH atmosphere for a prescribed period.
We obtained four groups of 10 plastic ferrules. Each
group had a common ∆Lvalue that was different for
each group. We examined the insertion loss fluctuation
of the SC-type plugs that incorporated the deformed
plastic ferrules. Figure 5 shows the insertion loss his-
tograms of the deformed plastic ferrules of each group.
We measured the insertion loss against that of a master
plug whose fiber-core eccentricity in relation to the cen-
ter of the zirconia ferrule outer diameter was <0.5 µm.
All the insertion loss values were less than 0.5 dB, the
average values being less than 0.2 dB regardless of the
∆Lvalues.
Figure 6 shows the insertion loss histograms of ran-
domly concatenated plastic ferrules belonging to the
four goups. There were 45 random concatenations for
each group of ferrules. We found that a ∆Lvalue of less
than 4 µm was necessary to keep the insertion loss fluc-
tuation to below 0.5 dB for the random concatenation
of the deformed plastic ferrules.
Fig. 6 Insertion loss histograms of randomly concatenated
plastic ferrules with four different degrees of endface deforma-
tion.
Fig. 7 Return loss histograms of plastic ferrules with four
different degrees of endface deformation.
Next, we examined the return loss fluctuation of
SC plugs incorporating plastic ferrules from the four
above-mentioned groups. Figure 7 shows the return
SHUTO et al.: LONG-TERM RELIABILITY OF PLASTIC FERRULES
1005
Fig. 8 Return loss histograms of randomly concatenated plas-
tic ferrules with four different degrees of endface deformation.
loss histograms of these plastic ferrules. In the return
loss measurement, the deformed plastic ferrule under
test was coupled to a zirconia ferrule in a master plug.
All the return loss values were greater than 46 dB, and
the average values were greater than 49 dB regardless
of the ∆Lvalues.
Figure 8 shows the return loss histograms of
randomly concatenated plastic ferrules from the four
goups. It is clear that we require a ∆Lvalue of less
than 3 µm if we wish to maintain the return loss of
greater than 40 dB for the random concatenation of the
deformed plastic ferrules.
So we define the ∆Lvalue of 3 µm as the allowable
endface deformation ∆Lcof the plastic ferrule, based
on the above results. When the ∆Lvalue falls to ∆Lc,
the plastic ferrule is judged to be a failure.
However, it is noteworthy that we can only de-
termine plastic-ferrule failure for the random concate-
nation of deformed plastic ferrules. Deformed plastic
ferrules with a ∆Lvalue greater than the ∆Lcvalue
cannot degrade the insertion- and return-loss character-
istics, if they are coupled to zirconia ferrules (see Figs. 5
and 7). For this reason, the initial optical characteris-
tics of the plastic ferrules were maintained in various
environmental durability tests (see [8]).
Therefore, we must keep in mind that the esti-
mated lifetimes based on the ∆Lcvalue, whose esti-
mation is described below, are those for the worst case,
in which two deformed plastic ferrules are incidentally
joined together in the connector.
Fig. 9 Fluctuation in endface deformation for plastic ferrules
at: (a) various temperatures and (b) various relative humidities.
3.2 Creep Behavior of Plastic Ferrule
We examined the creep behavior of the plastic ferrule
in detail at high temperature and/or in a high relative
humidity atmosphere.
To clarify the temperature and humidity depen-
dences of the ∆Ldegradation, we examined fluctua-
tions in the ∆Lvalues of the plastic ferrules under var-
ious environmental conditions. Figure 9(a) shows the
fluctuation in the Frvalues at various temperatures un-
der a constant relative humidity of 90%RH. There was
considerable fluctuation in the ∆Lvalue with increas-
ing temperature.
Furthermore, Fig. 9(b) shows the fluctuation in the
∆Lvalues at a constant temperature of 65◦C and var-
ious relative humidities. The fluctuation in the ∆L
value increased with increasing relative humidity. This
indicates that the moisture uptake of the plastic ferrule
plays an important role in the degradation of the ∆L
value. From these fluctuation data, shown in Fig. 9, we
were able to estimate the time-to-failure tfat which
the ∆Lvalue reached the allowable endface deforma-
tion ∆Lc(3 µm). We then estimated the acceleration
parameters by using the tfvalues.
1006
IEICE TRANS. ELECTRON., VOL.E84–C, NO.7 JULY 2001
3.3 Estimation of Acceleration Coefficients for Plastic
Ferrule
Based on the theories of Weibull statistics [14], the cu-
mulative failure probability F of the plastic ferrule un-
der the compressive contact force Wis related to the
time-to-failure tfas follows:
F(tf)=1−exp −(tf
η)
m(1)
where mis the shape parameter and ηis the scale pa-
rameter [15].
From Eq. (1) we obtain
ln ln 1
1−F(tf)=mln tf−mln η(2)
The median lifetime ξfor the Weibull distribution
is given by
ξ=η(ln 2)
1
m(3)
We estimated the time-to-failure tfof all the plas-
tic ferrules (20 samples) tested under each environmen-
tal condition. The tfvalues for the plastic ferrules are
shown in Fig. 10, where the cumulative failure proba-
bility Fis plotted against the tfvalues on a Weibull
scale. Figure 10(a) shows Weibull plots for the tfval-
ues at various temperatures under a constant relative
humidity of 90%RH, and Fig. 10(b) shows the plots at
a constant temperature of 65◦C and various relative
humidities. We obtained a linear relationship between
the left-hand term of Eq. (2) and the logarithms of the
tfvalues. This means that the tfvalues are in accor-
dance with the Weibull distribution. We were able to
estimate the Weibull parameters (m,η) from the data
in Fig. 10 by using Eq. (2). That is, we estimated the
mand ηvalues, respectively, from the slopes and the
intercepts of the lines shown in Fig. 10.
We then calculated the median lifetimes ξby us-
ing the estimated Weibull parameters and Eq. (3). The
estimated mand ξvalues under various environmental
conditions are listed in Table 1.
The change in the mvalue is relatively small when
the test temperature and relative humidity are changed.
So we assumed that the mvalues are constant (1.540)
under various environmental conditions.
By contrast, the ξvalues decrease rapidly with in-
creasing temperature and relative humidity. The tem-
perature Tand relative humidity RH dependences of
the ξvalues are expressed by using the following equa-
tion [16]:
ξ=Cexp E
kT exp A(RH)2(4)
where Cis a constant, Ais the humidity acceleration
Fig. 10 Weibull plots for time-to-failure of plastic ferrules at:
(a) various temperatures and (b) various relative humidities.
Table 1 Shape parameters and median lifetimes of plastic
ferrule.
coefficient, and Eis the temperature acceleration co-
efficient or the activation energy for the ξdegradation
process. From Eq. (4), the logarithm of ξis obtained
as follows:
ln ξ=lnC+E
kT +A(RH)2(5)
The logarithm of ξis proportional to both 1/kT and
(RH)2as shown in Eq. (5).
An Arrhenius plot for the ξdata at 65◦C/90%RH,
SHUTO et al.: LONG-TERM RELIABILITY OF PLASTIC FERRULES
1007
Fig. 11 Plots of median lifetime versus temperature and rela-
tive humidity for plastic ferrule: (a) temperature and (b) relative
humidity.
Table 2 Acceleration coefficients of plastic ferrule.
55◦C/90%RH, and 45◦C/90%RH is shown in Fig. 11(a).
The ξdata formed a straight line. We calculated the
temperature acceleration coefficient Efrom the slope
of the line by the least-squares method (see Eq. (5)).
Figure 11(b) shows the relationship between the
second power of the relative humidity and the loga-
rithms of the ξdata at 65◦C/90%RH, 65◦C/75%RH,
and 65◦C/60%RH. The ξdata also formed a straight
line. We estimated the humidity acceleration coeffi-
cient Afrom the slope of the line. The parameter C
was calculated by using the estimated coefficients (E,
A) and Eq. (5). The estimated acceleration coefficients
are listed in Table 2.
We estimated the ξvalue of the plastic ferrule un-
der arbitrary environmental conditions of temperature
T and relative humidity RH by using the coefficients
(C,E,A) and Eq. (4).
Fig. 12 Plots of hazard rate versus time for plastic ferrules at:
(a) 50%RH and (b) 90%RH.
3.4 Hazard Rates of Plastic Ferrule
The hazard rate λ(tf) for the Weibull distribution is
given by
λ(tf)=mt
m−1
f
ηm
=mt
m−1
fln 2
ξm(6)
As shown in Eq. (6), the hazard rate λcan be calcu-
lated by using the shape parameter mand the me-
dian lifetime ξ. We estimated the λvalues of the
plastic ferrule under various environmental conditions
by using m= 1.540 and Eq. (6). The calculated λ
values are plotted in Fig. 12. The λvalues increase
with increasing time and temperature. In a normal
atmosphere (25◦C/50%RH), a small λvalue of <0.1
FIT (FIT=10−9/hour) is expected for 20 years of use.
This λvalue satisfies the requirement (<0.1 FIT
for 20 years of use) for switchboard and transmission
equipment components [12]. In a more severe case of
25◦C/90%RH, a relatively small λvalue of 1 FIT is
expected for 20 years (see Fig. 12(b)). This λvalue
is larger than the requirement (<0.1 FIT) for switch-
board and transmission equipment components, but is
smaller than the reported λvalues for silica-based pla-
nar lightwave circuit devices (<40 FIT for 30 years of
1008
IEICE TRANS. ELECTRON., VOL.E84–C, NO.7 JULY 2001
use [17]) and for InP planar PIN photodiodes (<10
FIT for 20 years of use [16]).
In contrast, relatively large λvalues of 1.6 and
45 FIT are expected for 20 years at 35◦C/50%RH and
35◦C/90%RH, respectively (see Fig. 12). The λvalue
of 1.6 FIT is smaller than the reported λvalues for
silica-based planar lightwave circuit devices (<40 FIT
for 30 years of use) and for InP planar PIN photodi-
odes (<10 FIT for 20 years of use). However, the λ
value of 45 FIT is larger than the reported λvalues for
both silica-based planar lightwave circuit devices and
InP planar PIN photodiodes.
Therefore, we can conclude that the plastic ferrule
has sufficient reliability when used in a normal atmo-
sphere and at 25◦C/90%RH, but its reliability becomes
poor when it is used at temperatures above 25◦Cina
very humid atmosphere.
Here it is worth noting that the estimated FIT val-
ues based on the ∆Lcvalue are those for the worst case,
in which two deformed plastic ferrules are incidentally
joined together in the connector. If we are careful to
avoid the worst case, the plastic ferrules can be used for
a period of 20 years even at temperatures above 25◦C
in a very humid atmosphere.
4. Conclusions
We examined the creep properties and hazard rates of
plastic ferrules to ensure the long-term reliablity of op-
tical fiber connections. We found that the endface de-
formation ∆Lmust be less than 3 µm if we wish to
maintain the insertion loss of the plastic ferrule to less
than 0.5 dB while retaining the return loss of greater
than 40 dB for the worst case of random concatenation
of similarly deformed ferrules. We examined the fluctu-
ations in the ∆Lvalues of the plastic ferrules in detail
at high temperature and/or at high relative humidity.
From the fluctuation data, we estimated the time-to-
failure tfat which the ∆Lvalue reached 3 µm. We
then estimated the acceleration parameters and the me-
dian lifetimes ξby using the tfvalues based on Weibull
statistics. The ξvalues decreased rapidly with increas-
ing temperature and relative humidity. We calculated
the hazard rate λusing the ξvalue. The λvalues in-
creased with increasing time and temperature. In a
normal atmosphere (25◦C/50%RH), we can expect a
small λvalue of <0.1 FIT for 20 years. Even in a more
severe case of 25◦C/90%RH, we can expect a relatively
small λvalue of 1 FIT for 20 years. This indicates
that the plastic sleeve has sufficient reliability when it
is used in a normal atmosphere and at 25◦C/90%RH.
Acknowledgements
The authors thank Ryo Nagase, Kazunori Kanayama,
Shinichi Iwano, Masaru Kobayashi, and Ryuji Honda
for helpful discussions and encouragement.
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SHUTO et al.: LONG-TERM RELIABILITY OF PLASTIC FERRULES
1009
Yoshito Shuto received the B.S.,
M.S. and Ph.D. degrees from Kyushu Uni-
versity, in 1977, 1979, and 1990, respec-
tively. In 1979 he joined the NTT Elec-
trical Communications Laboratories, Ni-
ppon Telegraph and Telephone Corpora-
tion, Ibaraki, Japan. He was engaged
in research on oriented crystalline and
liquid-crystalline polymer materials for
optical-fiber jackets. Since 1990, he has
been engaged in research on diazo-dye-
substituted polymer materials for second-order nonlinear optics.
He is presently engaged in research on injection molding poly-
mer materials for single-mode optical connection technology with
NTT Photonics Laboratories. Dr. Shuto is a member of the
Japan Society of Applied Physics, the Chemical Society of Japan,
the Optical Society of Japan, and the Optical Society of America.
Shuichi Yanagi received the B.S.,
M.S., and Ph.D. degrees from Keio Uni-
versity, in 1992, 1994, and 1997, respec-
tively. In 1997 he joined the NTT Opto-
electronics Laboratories, Ibaraki, Japan.
He is presently engaged in research on
injection molding polymer materials for
optical connection technology with NTT
Photonics Laboratories. Dr. Yanagi is a
member of the Japan Society of Applied
Physics.
Masayoshi Ohno received the B.S.
and M.S. degrees in chemistry from Tokyo
Institute of Technology, in 1979 and 1981,
respectively. In 1981 he joined the NTT
Electrical Communications Laboratories,
Ibaraki, Japan. He is presently engaged
in research on injection molding polymer
materials for optical connection technol-
ogy with NTT Photonics Laboratories.
Mr. Ohno is a member of the Japan Soci-
ety of Applied Physics.
Hirotsugu Sato received the B.S. and
M.S. degrees from Hokkaido University, in
1974 and 1976, and Ph.D. degree from To-
kyo Institute of Technology in 1987, re-
spectively. In 1976 he joined the NTT
Electrical Communications Laboratories,
Ibaraki, Japan. He was engaged in re-
search on injection molding polymer ma-
terials for optical connection technology
with NTT Photonics Laboratories. He is
presently a manager with NTT Electron-
ics Corporation, Ibaraki, Japan. Dr. Sato is a member of the
Chemical Society of Japan, the Japan Society of Polymer Pro-
cessing, and the Polymer Processing Society.
Shin Sumida received the B.S., M.S.,
and Ph.D. degrees from Keio University,
in 1976, 1978, and 1981, respectively. In
1981 he joined the NTT Laboratories,
Ibaraki, Japan. Since joining NTT, he has
been engaged in the research and develop-
ment of optical passive components such
as optical fiber cables and planar light-
wave circuits. He was with Photonic Inte-
gration Research Inc. based in Columbus,
OH, as a production manager from 1987
to 1996. He is presently a manager in the research and devel-
opment of optical fiber connection technology. Dr. Sumida is a
member of MRS.
Shunichi Tohno received the B.S.,
M.S. and Eng. D. degrees from Hokkaido
University, in 1976, 1978, and 1988, re-
spectively. In 1978 he joined the NTT
Electrical Communications Laboratories,
Ibaraki, Japan. He has been engaged in
research on crystal growth and character-
istics analysis for III-V and II-VI semi-
conductor compounds. He is presently a
project manager in the research and de-
velopment of photonics integration tech-
nology. Dr. Tohno is a member of the Japan Society of Applied
Physics.
