Development of Mini-MT Ferrule
Using Short-Cycle Injection Molding
by Koichi Maeno *, Takahiro Ueno *, Masato Shiino *2, Yutaka Arai *2,
Kazuhiro Takayama *2, Katsuki Suematsu *2, Masao Shinoda *2,
Koichi Takagi *3, Hiroyuki Yamada *3 and Kiyoshi Midorikawa *3

Injection molding is attracting attention as a short-cycle molding method for MiniMT ferrules that comprise optical connectors. The authors investigated the material, molds, molding conditions, and polishing technologies for injection molding Mini-MT ferrules,
and succeeded in developing the ferrules having the same level of precision as those by conventional transfer molding. The 4-fiber Mini-MT connector comprised of the developed 4-fiber MiniMT ferrules gave such excellent optical characteristics as 0.107 dB on average for insertion loss
and 45 dB or better on average for return loss, in addition to stabilized insertion loss changes atreliability tests. Furthermore, the insertion and return losses of the 2-fiber Mini-MPO connector
comprised of the 2-fiber Mini-MT ferrules were 0.18 dB on average and 45 dB or better on average respectively, and the connector showed small insertion loss changes.
Thus, optical characteristics of injection molded Mini-MT ferrules proved to be equivalent tothose of transfer molded ferrules. Since it is possible to make the cycle time of injection molding
shorter than that of transfer molding thereby enabling low-cost production, the use of injection
molded ferrules is expected to broadly spread in future.

ABSTRACT1.

INTRODUCTION

As the optical network business based on the information
highway plan develops in recent years, optical fibers aregradually being employed as backbone lines in on-premises cabling. While conventional metal cables are used infloor-level cabling making only a limited transmission
capacity available, optical fiber cables are capable of highspeed data communication. 1)~3) In the future, FTTD (fiber
to the desk) implementation based on optical fiber cabling
will extend to desktop personal computers thus creating alarge demand for optical connector components. On theother hand, to adapt to the needs of high-capacity datatransmission, optical fiber transmission cables are becoming multi-cored and densely packed, so that a large
demand for multi-fiber optical connectors which permit collective connection of these cables is rapidly developing.
At present, multi-fiber optical connectors in actual use
include the MT (mechanically transferable) connector. 4), 5)
In order to satisfy the required optical characteristics, theMT connector necessitates high precision manufacturing

technologies that assure a positional precision of the submicrons order; and the technologies have already been
established in which MT ferrules are molded by transfer
molding using thermosetting polymer compound.
Recently however, as mentioned before, a large
demand for multi-fiber optical connectors is rapidly developing calling for lower costs. This accordingly requires ashort-cycle molding technology that brings about not only
high precision but also higher productivity than conventional technologies. There has also been a need for connection in 2- and 4-fiber units requiring optical connectors
having a more compact, denser structure than the MPOFiber hole

2.5mm

*

Polymer Processing Sec., Hiratsuka Research Lab., R&D
Div.
*2 Components & Devices Group, Optical Cable Systems
Dept., Opto-Technology Lab., R&D Div.
*3 Precision Machining Technologies Sec., Process
Technologies Dept., Production Engineering Development
Center, Plant & Facilities Div.

2.6mm
Alignment hole
4.4mm
Alignment pin

Figure 1

Alignment hole pitch

Structure of Mini-MT ferrule

143

(multifiber push-on) connector. This led to a design having
no more than four fibers and the development of the MiniMT connector, based on the MT connector. 6)
This report describes the development of injection molded Mini-MT ferrules that provide, at lower molding costs,
optical characteristics which are equivalent to thoseobtained with transfer molded MT ferrules.

2.

BASIC STRUCTURE OF MINI-MT FERRULE

As shown in Figure 1, the Mini-MT ferrule is provided with
two alignment-pin holes together with fiber holes in which
fibers are to be inserted. In order to have such excellent
optical characteristics as are obtained with conventionaltransfer molded ferrules, it is essential for an injection
molded ferrule to have a positional accuracy of the submicrons order in the dimension between the alignment-pin
holes as well as in the relative position between the alignment-pin holes and the fiber holes.
Figure 2 shows the structure of an assembled Mini-MT
connector, in which the ferrules with a fiber ribbon eachare optically connected using two alignment pins and aclamp spring.

3.

INJECTION MOLDING TECHNOLOGY

3.1 Development Concept
Optical characteristics of MT ferrules are controlled by thediameter of alignment-pin holes, the pitch of alignment-pin
holes, fiber hole diameter, and the offset of fiber holes.
The challenging tasks of managing these parameters
have been solved by the development of new thermosetting polymers having low molding shrinkage in addition to
the upgrading of transfer molding technologies, resulting
in the production of MT ferrules with a high dimensional
precision of the submicrons order.
Thus, the MT connector is drawing attention in the fieldof data communication. In order to cope with a large
demand anticipated, the MT connector is required to havemass productivity and low cost. Therefore, in an effort toMT ferrule

realize low cost production of ferrules that provide opticalcharacteristics equivalent to those of transfer molded ferrules, new Mini-MT ferrules have been developed basedon the injection molding technology.
3.2 Material Technology
We have been investigating the injection molding technology as well as the transfer molding technology. The investigation has shown that it is important to select a thermoplastic polymer of low molding shrinkage for injection
molding, and that the stabilization of molding shrinkage
depends, needless to say, on molding conditions andmolds in addition to such important factors as polymer
species and filler shape. We have carefully studied thematerial parameters of base polymers together with theshape, size, distribution, and quantity of filler, finally succeeding in developing a compound suitable for injection
molding.
The developed compound is excellent in molding stability and mechanical strength. The strength of alignment-pin
holes on molded ferrules was measured to prove that theferrule had the same level of mechanical strength as thatof conventional ferrules.
3.3 Molding Technology
Precision transferring of the mold is known to be very
important in molding technology. In injection molding, theprocess is such that the dried compound is supplied into
the hopper of the injection machine, heated to melt in thecylinder, and the melted compound is injected into themold. Properly pressurized until gate solidification, theproduct is cooled in the mold, and then it is taken out.
It is essential, accordingly, to establish the optimum
conditions to carry out the process of injection, pressurizing, and cooling of the melted compound without giving
strain onto the molding pins for the fiber holes. Only undersuch conditions, stabilized products with superior positional precision can be obtained.
3.4 Fiber Ribbon Attachment and Polishing
A fiber-optic ribbon is inserted into the ferrule with its coating removed at the end, and is fixed using epoxy adhesive.
The easiness of subsequent flat polishing of the endface depends substantially on the property of the polymer
used. After various studies, a set of polishing conditions
was successfully found to meet the requirements of theFiber ribbon
Fan-out code for transforming
FC into MT

Alignment pin

Clamp
Clamp

Figure 2

144

Structure of Mini-MT connector

LED light
sourceMaster Mini-MT

Figure 3

Fiber ribbon

SensorOpticalMini-MT under test power meter

Measuring system configuration

Furukawa Review, No. 19. 2000

injection molded ferrules, realizing a satisfactory polishedend-face of excellent flatness.

4.

OPTICAL CHARACTERISTICS

The insertion loss was measured by the versus-mastermethod in which the loss with respect to a standard plug
having an extremely small offset in fiber holes was measured. The light source was an LED of 1310-nm wavelength, and index matching material or matching oil wasapplied to the point of connection. Figure 3 shows themeasuring system configuration.
The core fibers are arrayed symmetrically on the centerline of the two alignment-pin holes, so that measurements
were made in both the normal and the 180° reversed connections.
4.1 Single-Fiber Mini-MT Ferrule
Figure 4 shows the insertion loss histogram of the singlefiber Mini-MT connectors using the single-fiber Mini-MT
ferrule, and Table 1 those of single-, 2-, and 4-fiber connectors. The loss was 0.11 dB on average with a maximum of 0.29 dB for normal connection, and 0.159 dB on
5
Normalconnection

Frequency

4

Reversedconnection

3

average with a maximum of 0.46 dB for reversed connection --satisfactory connection performance. Moreover,since no much difference is seen between the normal andreversed connection losses, it may be concluded that
there is no specific tendency in the direction and quantityof the offset of fiber holes, and that the targeted positional
precision has been almost realized.
Table 2 shows the return loss of Mini-MT connectors,
satisfactory results of 45 dB or better on average.
4.2 2-Fiber Mini-MT Ferrule
Figure 5 shows the insertion loss histogram of the 2-fiber
Mini-MT: 0.21 dB on average with a maximum of 0.43 dB
for normal connection, and 0.262 dB on average with amaximum of 0.54 dB for reversed connection --satisfactory connection performance. The fact that the insertion
losses are nearly the same for the normal and reversed
connections shows that the 2-fiber holes are arranged
almost symmetrically with respect to the ferrule center.
4.3 4-Fiber Min-MT Ferrule
Figure 6 shows the insertion loss histogram of the 4-fiber
Mini-MT: 0.107 dB on average with a maximum of 0.24 dB
for normal connection, and 0.195 dB on average with amaximum of 0.44 dB for reversed connection --satisfactory connection performance. Although the positional deviation of each fiber hole with respect to the alignment pins
tends to increase with the number of fibers, the insertion
loss is seen to maintain its small values thanks to theselections of material and mold as mentioned before.
35

2
n =7
λ =1310 nm

Normalconnection30

1Frequency

25
0
0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Insertion loss (dB)Figure 4

Reversedconnection

20
15

n =60
λ =1310 nm

10Insertion loss histogram of single-fiber Mini-MT connector

50Table 10Insertion loss of Mini-MT connector

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Insertion loss (dB)
Average insertion loss (dB)Single-fiber Mini-MT

2-fiber Mini-MT

4-fiber Mini-MT

Figure 5

Connection Normal Reversed Normal Reversed Normal Reversed
7

7

60

60

40

40

Average

0.16

0.159

0.21

0.262

0.107

0.195

Maximum

0.29

0.46

0.43

0.54

0.24

0.44

λ =1310 nm

Table 2Return loss of Mini-MT connector

25

FrequencynInsertion loss histogram of 2-fiber Mini-MT connector

20

Normalconnection

15

Reversedconnection

10
n =40
λ =1310 nm
5Return loss (dB)Single-fiber Mini-MT

2-fiber Mini-MT

4-fiber Mini-MT

10

20

24

0

n
Average

45 dB or better

45 dB or better

0

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Insertion loss (dB)

45 dB or better

λ =1310 nm

0.1

Figure 6

Insertion loss histogram of 4-fiber Mini-MT connector

145

Table 3

Results of mechanical tests
Insertion loss change (Average)
Conditions

Test item

Injection

Transfer

0.20 dB or less0.20 dB or less0.20 dB or less0.20 dB or less0.20 dB or less0.20 dB or less0.20 dB or less0.20 dB or less0.20 dB or less0.20 dB or less0.20 dB or less0.20 dB or less

100 connection-disconnections
Connection-disconnection

Loss measurement every 10 repetitions
With refractive index matching material

Pull

5.9 N in the optical axis direction
Frequency: 10~55 Hz

Vibration

Amplitude: 1.5 mm
Direction: 3 directions
Duration: 2 hr/direction

Shock

Intensity: 100 G x 6 ms
Direction: 3 directions
Repetition: 3 times/direction
Load: 6.8 N/fiber at fiber-end 1 m away

Bend

Direction & Repetition:±90 deg. x 100 times bending
Loss measurement every 10 repetitions
Height: 1.5 m

Drop

Fiber length: 2 m
Repetition: 3 times on concrete surface

λ =1310 nm
In accordance with JIS C 5981

Table 4Thus, it has been confirmed that the fiber holes are located almost as exactly as designed.

Results of temperature tests
Insertion loss change (Average)

Item

Conditions
Injection

5.

RELIABILITY TESTSResults of typical reliability tests on selected 4-fiber MiniMT connectors are given below.
The reliability tests consist of mechanical characteristics
evaluation under mechanical strains and temperature
characteristics evaluation done from an environmental
standpoint. The measurement results with conventionaltransfer molded connectors are shown for comparison.
5.1 Evaluation of Mechanical Characteristics
Table 3 shows the test item, test conditions, and results ofmechanical characteristics evaluation. The test conditions
are in accordance with JIS C 5981.
Connection-disconnection tests were performed reconnecting two connectors 100 times and the insertion losschange was measured every 10 repetitions. The result
was 0.20 dB or less, as good as transfer molded connectors.
Pull tests were done by measuring the insertion losschanges before and after pulling the connector at 5.9 N in
the axial direction. The result was satisfactory, with 0.20
dB or less.
Furthermore, vibration, shock, bend, and drop tests
were performed; the insertion loss change was 0.20 dB orless in every test, equivalent to the performance of transfer molded MT connectors obtained in similar tests. Thisconfirmed that there was no practical problem in terms of
mechanical characteristics.

146

Heat-cycle
Hightemperature
Lowtemperature
Humid-40~60°C, 10 cycles,
6 hr/cycle

Transfer

0.20 dB or less 0.20 dB or less

80°C x 96 hr

0.20 dB or less 0.20 dB or less

-40°C x 96 hr

0.20 dB or less 0.20 dB or less

60°C x 95% RH x 96 hr 0.20 dB or less 0.20 dB or less

λ =1310 nm
In accordance with JIS C 5981

Injection-moldedFiberMini-MT Ferrule
Alignment pin

Adapter

Plug

Figure 7

Structure of Mini-MPO connector

5.2 Temperature Characteristics Evaluation
Table 4 gives representative of the test item, conditions,and results of temperature characteristics evaluation.
Heat-cycle tests were carried out by measuring theinsertion loss change under the conditions of -40~60°C,
10 cycles, and 6 hr/cycle. The result was satisfactory, with
0.2 dB or less. Furthermore, high-temperature, low-tem-

Furukawa Review, No. 19. 2000

perature, and humid tests were performed; the insertion
loss change was 0.20 dB or less in every test, equivalent
to the performance of transfer molded connectors.

6.

MAKING MINI-MT FERRULE INTO
MPO 7), 8)

6.1 StructureMini-MPO connector, a push-on/pull-off type connector as
shown in Figure 7, has been developed using the Mini-MT
ferrule.
6.2 Optical Characteristics
As representative of Mini-MPO connectors, the 2-fiberconnector using injection molded 2-fiber Mini-MT ferruleand 2-fiber ribbon has been developed. The end-face ofthe ferrule was polished to achieve flat PC (physical contact) and the connector was evaluated without refractive
index matching material.
Figures 8 and 9 show insertion loss and return loss
respectively. While the insertion loss was 0.18 dB on average with a maximum of 0.36 dB --low enough values--, thereturn loss was 46.2 dB with a minimum of 40 dB --highenough values--.

6.3 Mechanical and Temperature Characteristics
Table 5 gives representative of the test item, conditions,and results of mechanical and temperature characteristics
evaluation. The test conditions are in accordance with JIS
C 5982.
Mechanical characteristics were evaluated for connection-disconnection, straight pull, side bend, and side pull.
In connection-disconnection tests, a total of 500 reconnections were performed, and the insertion loss was measured every 50 repetitions. The change in insertion loss,
within 0.2 dB, was satisfactory and no particular damage
to the connectors causing insertion loss increase wasobserved.
The test conditions for the straight pull, side bend, andside pull tests are shown in Figures 10 to 12. The insertion
loss change due to every test was 0.20 dB or lower.
Heat-cycle tests were performed, to evaluate the temperature characteristics, under the conditions of -25~70°C,
10 cycles, and 2.5 hr/cycle. The change in insertion loss
due to the test was 0.20 dB or less --a satisfactory value.
Table 5 shows the results.

45

35

40

30

35
n=70
λ =1310 nm

20Frequency

Frequency

25

15
10

n=76
λ =1310 nm

30
25
20
15
10

5

5
0

0
0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

35

0.8

40

Insertion loss histogram of 2-fiber Mini-MPO connector

Table 5

disconnection

55

60

65

70

Figure 9

Return loss histogram of 2-fiber Mini-MPO connector

Results of mechanical and temperature tests on 2-fiber Mini-MPO connector

Item

Connection-

50

Return loss (dB)

Insertion loss (dB)Figure 845Insertion loss

Conditions

change (Average)

Return loss (Average)

500 connection-disconnections
Loss measurement every 50 repetitions

0.20 dB or lessWithout refractive index matching material

Pull

68.6 N x 1 min

0.20 dB or lessBend

4.9 N x 100 times

0.20 dB or lessSide pull

20 N

0.20 dB or lessHeat cycle-25~70°C, 10 cycles, 2.5 hr/cycle

35 dB or better

0.20 dB or less

λ =1310 nm
In accordance with JIS C 5981
BELLCORE specification was referred to in side pull test

147

Adapter

Plug

Adapter(fixed)20 N
Adapter(fixed)Seen from A

Plug

A

68.6 N
1 min.

Fiber cord

( λ = 1310 nm)

Applied in 8 directions(at 45-deg. intervals)( λ =1310 nm)

Figure 12 Side pull test

Figure 10 Straight pull testwith PC contact and without refractive index matchingmaterial, gave an average insertion loss of 0.18 dB and an
average return loss of 46.2 dB.
Since injection molded ferrules have shorter cycle timethan transfer molded ones, it is possible to bring the costlower than that of conventional transfer molded ferrules.
FixturePlug

Adapter
1 cycleFiber cord
4.9 N

( λ = 1310 nm)

Figure 11 Side bend test

7.

IN CONCLUSION

We have developed Mini-MT ferrules and Mini-MPO connectors comprised of the ferrules using injection-molding
technology. The developed products gave good performance, which are practically satisfactory, in terms of initial
optical and mechanical characteristics as well as longterm reliability.
To summarize their performance, the 4-fiber Mini-MT
ferrule, representing the single-, 2-, and 4-fiber Mini-MT
ferrules, gave an average insertion loss of 0.107 dB andan average return loss of 45 dB or better; while the 2-fiber
Mini-MPO connector, comprised of the Mini-MT ferrule148

REFERENCES

x 100 cycles

1) Yamamoto et al.: "Configuring Methodology of Optical
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2) Tanifuji et al.: "Next Generation Optical Fiber Cable
Technology", NTT Technology Journal, (1993.7), pp.13-16. (inJapanese)
3) Ohta et al.: "High-Performance Multi-Fiber Connector",
Furukawa Electric Review, Special Issue on Optical
Components, No.89, (1991.12), pp.8-14. (in Japanese)4) Nagasawa et al.: Proc.IOOC'89,paper 21C2-1(1989), pp.48-49
5) Takaya et al.: "Basic Design and Development of Fast-Molding
and Fast-Assembling MT Connector", Proceedings IEICE, Fall
1998, B10-13. (in Japanese)6) Shimoji et al.: "Development of Mini-MPO Connector",
Furukawa Review, No.18, May 1999, pp.91.
7) Arai et al.: "Development of Single-Mode 2-fiber Mini-MPO
Connector Using Injection-Molded Ferrule", IWCS Proceedings(1998),pp.574-578.
8) Takaya et al.: "Design and performance of Miniaturized MPO
Type Connector for High-density Optical Duplex Connection,"
IEEE Photon. Technol. Lett., Vol. 10, No. 1, pp. 102-104, 1998.
Manuscript received on June 28, 1999.

Furukawa Review, No. 19. 2000