Plastic Split Alignment Sleeves for Single-Mode Fiber Connection of MU-and SC-Type Ferrules

Abstract

Split alignment sleeves for the single-mode optical fiber connection of MU-and SC-type ferrules were successfully fabricated for the first time with a precise injection-molding technique using a thermosetting epoxy resin. The optimum inner radii of the plastic sleeves were determined with a view to realizing both suitable gauge retention forces for the ferrules and a small fiber offset of 1 um between them. The adapters incorporating the plastic sleeves showed excellent performance with an average connection loss of 0.09 dB and a return loss of > 50 dB.

Full text

218 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 3, MARCH 2001
Plastic Split Alignment Sleeves for Single-Mode
Fiber Connection of MU- and SC-Type Ferrules
Yoshito Shuto, Shuichi Yanagi, Masayoshi Ohno, Hirotsugu Sato, Michiyuki Amano, Shinichi Iwano, Shin Sumida,
and Shunichi Tohno
Abstract—Split alignment sleeves for the single-mode op-
tical-fiber connection of MU- and SC-type ferrules were success-
fully fabricated for the first time with a precise injection-molding
technique using a thermosetting epoxy resin. The optimum inner
radii of the plastic sleeves were determined with a view to realizing
both suitable gauge retention forces for the ferrules and a small
fiber offset of 1 m between them. The adapters incorporating
the plastic sleeves showed excellent performance with an average
connection loss of 0.09 dB and a return loss of 50 dB.
Index Terms—Epoxy resin, injection molding, optical connec-
tors, optical fiber connecting, plastics, split alignment sleeve.
I. INTRODUCTION
HIGH-PERFORMANCE and low-cost optical connectors
are essential for realizing optical subscriber systems such
asthefiber-to-the-home(FTTH)project.Severalopticalconnec-
tors have already been developed that are capable of connecting
single-mode(SM)opticalfiberswithlowlossandlowreflection.
Among them, MU- and SC-type optical connectors were devel-
oped for use as all-purpose connectors with which to couple to-
gether simplex or duplex SM optical fiber cables [1], [2].
MU- and SC-type optical connectors are designed based on a
push–pull coupling mechanism consisting of a split alignment
sleeve and precise cylindrical ferrules. An MU ferrule has a
outer diameter of 1.249 mm [1], compared with 2.499 mm [2]
for an SC ferrule. The combination of a zirconia or copper-alloy
split alignment sleeve and precise cylindrical zirconia ferrules
has been widely used for these connectors. However, no suit-
able split alignment sleeve (MU/SC-conversion sleeve) has yet
been developed for the SM optical fiber connection of MU- and
SC-type ferrules.
We have recently adopted a novel fabrication process using
a thermosetting epoxy resin, and developed plastic split sleeves
for SC connectors [3], [4]. In the work reported in this letter, we
studied the feasibility of the technique by developing and fab-
ricating plastic MU/SC-conversion sleeves and examining their
optical characteristics.
II. DESIGN OF PLASTIC SPLIT SLEEVE
The structure of an SM optical fiber connection made with
MU- and SC-type ferrules and a plastic MU/SC-conversion
Manuscript received July 24, 2000; revised November 28, 2000.
Y. Shuto, S. Yanagi, M. Ohno, M. Amano, S. Iwano, S. Sumida, and S. Tohno
are with NTT Photonics Laboratories, Nippon Telegraph and Telephone Corpo-
ration, Tokai, Ibaraki 319-1193, Japan.
H. Sato is with NTT Electronics Corporation, Naka, Ibaraki 311-0122, Japan.
Publisher Item Identifier S 1041-1135(01)01975-9.
Fig. 1. Connection structure made with MU- and SC-type ferrules and a plastic
MU/SC-conversion sleeve.
sleeve is shown in Fig. 1. The outer diameters of the plastic
sleeve were set at about 4.02 mm for the SC-side and about
2.25 mm for the MU-side, respectively, by considering the
inner dimensions of conventional SC and MU adapters. The
optimum inner radii of the plastic sleeve were determined with
a view to realizing both suitable gauge retention forces for
the MU- and SC-sides of the sleeve, and a small fiber offset
between the MU- and SC-type ferrules inserted into the sleeve.
When the MU- and SC-type ferrules are inserted into the
plastic sleeve, the slit in the sleeve is expanded and the sleeve
centers of the MU- and SC-sides are shifted to the initial slit po-
sitions, as shown in Fig. 2. The variation in the sleeve center
for the MU- or SC-side can be approximately estimated by the
difference between the sleeve inner radius and the ferrule
outer radius .
At the same time, radial gripping forces and for
the ferrule alignment are generated in the MU- and SC-sides of
the sleeve. The gripping forces are related to the experimen-
tally determined gauge retention forces as follows [5]:
(1)
where is the friction coefficient between the zirconia
ferrule and the plastic sleeve. The estimation of the values
has already been described in detail elsewhere [4].
Fig. 3 shows the relationships between the values and the
variations in the MU- and SC-sides of the plastic sleeve. The
arrows in this figure show the allowable ranges (1.0–2.5 N
[6] for the MU-type adapter and 2.0–3.9 N [5] for the SC-type
1041–1135/01$10.00 © 2001 IEEE
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SHUTO et al.: PLASTIC SPLIT ALIGNMENT SLEEVES FOR SINGLE-MODE FIBER 219
Fig. 2. Model of split alignment sleeve for analyses.
Fig. 3. Calculated gauge retention force versus variations in inner radius of
MU- and SC-sides of plastic sleeve.
adapter). It is clear from this figure that the values increase
with increasing values, and that the increment of the
MU-side is larger than that of the SC-side.
In Fig. 3, the hatched areas show the allowable ranges for
the MU- and SC-sides. The allowable values for the MU-side
are smaller than those for the SC-side. The fiber offset be-
tween the MU and SC ferrules inserted in the MU- and SC-sides
can be estimated by using the allowable values of these sides
as follows:
(2)
where and are the allowable values of the SC- and
MU-sides of the plastic sleeve, respectively. The theoretical loss
increase (dB) due to the fiber offset ( m) is given by [7]
(3)
where is the spot size (4.5 m) of the SM fiber. According
to (3), a value of 1 m results in a loss increase of about
0.2 dB. Therefore, we employed suitable inner radii for the MU-
and SC-sides, whose values satisfy both the allowable gauge
retention forces and a small fiber offset of about 1 m.
III. FABRICATION OF PLASTIC SPLIT SLEEVES
A thermoset injection-molding technique was used to fabri-
cate the plastic sleeves. First we formed a pipe by utilizing a
Fig. 4. Connection loss histogram of adapters incorporating plastic sleeves.
35-ton horizontal injection-molding machine specially designed
for thermosetting epoxy resin. A slit was then formed by cutting
the pipe along its axis. The lengths of the MU- and SC-sides
were designed to be about 3.4 and 5.7 mm, respectively.
We measured the gauge retention forces of the MU- and
SC-sides of the plastic sleeves. The average values for the
MU- and SC-sides were 1.26 and 2.34 N, respectively, and we
obtained suitable values (1.0–2.0 N for the MU-side and 2.0–
3.0 N for the SC-side) as a result of the dimensional design
described above.
IV. OPTICAL CHARACTERISTICS OF PLASTIC SLEEVES
We prepared adapters incorporating the plastic sleeves, and
used two SM fiber cables and a 1.31- m LD light source to
measure the connection loss of the adapters. The connection loss
was measured against that of a master SC-plug whose fiber-core
eccentricity to the center of zirconia-ferrule outer diameter was
0.5 m. The endfaces of the zirconia ferrules used in the mea-
surement were polished using the Advanced PC (AdPC) pol-
ishing technique [8]. The experimental details of the optical
measurements have been described elsewhere [4].
Fig. 4 shows a connection loss histogram for the measured
adapters. All the connection loss values we obtained, which
included the loss due to imperfections in the adapter and the
normal MU-plug of the second SM fiber cable, were less than
0.5 dB, the average value being 0.09 dB. The standard deviation
of the connection loss was 0.07 dB, which was comparable to
the average value (0.09 dB). The measured loss values indicate
that there are small fiber offsets of 1.4 m in the adapters in-
corporating plastic sleeves.
We also measured the return loss of the adapters at 1.31 m.
The average adapter return loss was 52 dB, and all the return
loss values we obtained were greater than 50 dB. Here, it is
noteworthy that the return loss for the MU- and SC-type con-
nectors is mainly determined by the characteristics of the fiber
endfaces, and the split sleeve can only degrade these charac-
teristics by hindering endface contact due to too much friction
or by causing gross ferrule misalignment. The measured results
indicate that there was no increase in friction and no ferrule mis-
alignment in the adapters incorporating the plastic sleeves.
Furthermore, the initial optical characteristics of the plastic
sleeves were maintained in both temperature and humidity tests
under conditions of 10 C–65 C and 93% RH, and heat-cycle
tests, which were carried out in the temperature range between
25 C and 75 C.
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220 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 3, MARCH 2001
V. CONCLUSION
We employed a precise injection-molding technique using
thermosetting epoxy resin to fabricate split alignment sleeves
for the SM optical fiber connection of MU- and SC-type fer-
rules. The optimum inner radii of the plastic sleeves were de-
termined with a view to realizing both suitable gauge retention
forces ( ) for these ferrules and a small fiber offset of 1 m
between them. The plastic sleeves showed average values of
1.26 and 2.34 N for the MU- and SC-sides of the sleeve, respec-
tively.
The adapters incorporating the plastic sleeves had an average
connection loss of 0.09 dB and a return loss of 50 dB. The ini-
tial optical characteristics of the plastic sleeves were maintained
in temperature and humidity tests and heat-cycle tests. These re-
sults indicate satisfactory levels of performance for SM optical
fiber connection.
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