Development of Mini-MPO Connector
by Naoko Shimoji * , Jun Yamakawa * and Masato Shiino *In recent years there has been a need for connectors that will provide high-density

ABSTRACT packaging in the connection of optical fibers for communications equipment. The

existing MPO connector enables high-density batch connection of up to 12 fibers, but an even morecompact structure is needed to obtain high-density connection of fewer fibers.
The Mini-MPO connector has therefore been developed based on the MPO structure. It is adapted
to the connection of 1, 2 or 4 fibers, and is smaller and simpler in structure than the MPO.
In terms of optical characteristics, an angled-interface single-mode 2-fiber Mini-MPO connector
has an insertion loss averaging 0.2 dB or less and a return loss of 53 dB or less--excellent performance which is equivalent to that of conventional MPO connectors.

1.

INTRODUCTION

In working to achieve fiber-to-the-home implementation,
attention has focused on increasing the number of opticalfibers per cable and raising density, creating the need forconnectors for these cables1)~2). MT connector (mechanically transferable connector) now in use can handle 2- to
12-fiber ribbons3)~4). These MT connectors are generally
used in outdoor applications.
There is a need, however, for a connector structure forindoor and device-to-device use that is easily
reconnectable. The MPO connector (multifiber push-on
connector), a push-pull, reconnectable connector that
adopts the MT connector ferrule, meets such needs5). In
terms of fiber density, the MPO connector is more advantageous the more fibers there are, and in recent years,
with progress in high-speed computers, demand for 8- and12-fiber MPO connectors has increased.
There has also been a need in recent years for connection in 2- and 4-fiber units for data communications, requiring optical connectors having a more compact, denser
structure than the MPO connector. This led to focusing ona design having no more than four fibers, and the development of the Mini-MPO connector, based on the MPO
connector6)~7).

2.

DESIGN CONCEPT

2.1 MPO Connector Structure
Connectors for fibers used in indoor equipment for opticaltransmission are subject to a number of requirements, including:. High-density connection. Easy connection and disconnection. No need for refractive index matching material
The MT connector is designed for the high-density connection of multiple optical fibers. Figure 1 shows its structure. The MT connector has two alignment pins to alignthe ferrule, and a clamp spring. When closed, the MT connector is extremely compact and is thus well suited forhigh-density fiber connection within closures or cabinets.
At points where frequent connection and disconnection isnecessary, a design that has easier reconnectability isneeded.

MT ferrule

Alignment pin

Optical fiber ribbon
Clamp spring* Optical Cable Systems Dept., Opto-Technology Lab.,R&D Div.

Figure 1

MT connector structure91

Coupling sleeve

Plug

Adapter

Plug

Adapter

FerruleFigure 2

FerruleGuide sleeve

SC connector structure

Figure 4

Coupling sleeve

Plug
Adapter

MT ferrule

Figure 3

MPO connector structure

A typical single-fiber connector, on the other hand, isthe SC connector (subscriber loop system optical fiberconnector)8). Figure 2 shows its structure. It connects twoplugs by means of an adapter, which can be connected
and disconnected with a single action.
The MPO connector combines high-density connection
with convenient disconnecting action. Figure 3 shows its
structure. The MPO is ideal in satisfying the need for highdensity packaging in equipment.
2.2 Optical Characteristics
If there is a gap between the optical fibers in a connector,
Fresnel reflection occurs at the interface between the fiber endface and air, causing degradation of both insertion
loss and return loss characteristics. It is thus desirable
that physical contact be maintained between the opposing optical fibers, and in single-fiber SC connectors this isaccomplished by forming the end face of the ferrule into aspherical configuration9).
Since multi-fiber MT and MPO connectors have a larger
connection surface than SC connectors, extremely highaccuracy in fiber endface geometry is required to achieve
perfect physical contact. In MT connectors Fresnel reflection is prevented by the use of a refractive index matchingmaterial at the fiber joint.
Where frequent connection and disconnection are required, however, the application and removal of the re-92

Mini-MPO connector structure

fractive index matching material becomes tedious. In MPO
connectors, the fiber protrudes slightly from the surrounding ferrule, and physical contact is achieved by polishingits endface configuration.
Further, in MPO connectors the fiber endface is generally polished on an angle, which is chosen so that the lightof Fresnel reflection is not returned to the core but is deflected to the clad. By this means Fresnel reflection is reduced and good reflection characteristics can be obtainedat all times10)~11).
2.3 Development of the Mini-MPO Connector
2.3.1 Basic Structure
The Mini-MPO connector was developed with the objective of high-density connection of up to four optical fibers.
Figure 4 shows the structure of the Mini-MPO connector. The design of the Mini-MPO is based on a configuration similar to the MPO connector, and the mechanism forconnection and disconnection and the method of polishing to achieve physical contact used on the MPO connector ferrule are retained.
2.3.2 Reduction in Connector Size
Table 1 compares the dimensions of the Mini-MPO andconventional MPO connectors. As can be seen, the closer
pitch of the alignment pins in the ferrule makes it possible
Table 1

Dimensions of MPO and Mini-MPO Connectors
MPO

FerruleMini-MPO

Fiber pitch:
0.25 mm

Adapter

Plug

Furukawa Review, No. 18. 1999

to reduce the height dimension and make the plug andadapter more compact.
In addition, by simplifying the structure of the Mini-MPO
adapter, an even more compact configuration has beenachieved. By this means it has been possible to reduce
both the height and width dimensions. Instead of a reinforcing member, the Mini-MPO connector has a guidesleeve that is inserted into the coupling sleeve. The dimensions of the Mini-MPO adapter are 10 x 8 x 24 mm,
which is the same as or even less than the dimensions of
the SC adapter (12 x 9 x 24 mm).
Based on the design concept described above, prototype Mini-MPO connectors were manufactured and their
optical characteristics evaluated.

3.

OPTICAL CHARACTERISTICS

3.1 Initial Characteristics
The first prototype Mini-MPO built was for two single-mode
fibers. The end face of the ferrule was polished to an angleof 8° and no index matching material was used in the evaluations.
Figure 5 shows the results of measurements of insertion loss. The light sources used for the measurements
were LEDs of 1310- and 1550-nm wavelength, and the

40

30

λ
mean
max
min

20

n

Frequency (times)

1310nm1550nm1310nm0.190dB
0.46dB
0.01dB
0.102dB
801550nm0.190dB
0.40dB
0.00dB
0.087dB
80

data is the insertion loss value with respect to a standard
plug. For both wavelengths the average loss was satisfactory at 0.2 dB or less with a maximum of 0.4 dB--values
just as good as those of the conventional MPO connector.
Figure 6 shows return loss characteristics. Measurements were made using LDs of 1310- and 1550-nm wavelength, and the return loss between a reference plug andthe connection point was measured. The results werehighly satisfactory, with an average of approximately 60
dB and a minimum of 53 dB.
3.2 Reliability Tests
Next the Mini-MPO connectors were tested for connection-disconnection performance and for temperature cycling. Table 2 shows the results.
Figure 7 is representative of the results of connectiondisconnection tests. A total of 500 reconnections were performed, and the insertion loss was measured. The change
in insertion loss due to connection-disconnection was extremely stable within ±0.2 dB. Furthermore no particulardamage to the connectors due to repeated connectiondisconnection was observed. This confirmed that the structure was capable of withstanding frequent connection anddisconnections.
Figures 8 and 9 show insertion loss change and returnloss in an environment of -25 to 70°C. The insertion losschange was extremely stable--within +0.2 dB--over the fulltemperature range. Return loss also was 50 dB or more, asatisfactory result that was no different from room temperature performance.

Table 2
10

0

Item

0

0.2

0.4

0.6

0.8

Figure 5

Insertion loss of single-mode 2-fiber Mini-MPO
connector

40

λ
mean
max
min

30

n

1.3 m
59.9dB
64.0dB
55.5dB
1.7dB
80

1.5 m
61.0dB
65.8dB
53.8dB
1.9dB
80

Connectiondisconnection test

500 connectiondisconnections
λ = 1310 nm

Insertion loss change< 0.2 dB: (n=6)

Temperature
cycling test

10 cycles -25/+70˚C
λ = 1310 nm

Insertion loss change< 0.2 dB: (n=6)
Return loss change< 50 dB: (n=4)

0.8

1.3 m
1.5 m

Insertion loss (dB)Frequency (times)

Results

1.0

60
50

Conditions

1.0

Insertion loss (dB)

Results of Reliability Tests

20

Fiber 2

0.4
0.2

10
0
50

Fiber 1

0.6

60

70

0.0

0

Return loss (dB)Figure 6

Return loss of single-mode 2-fiber Mini-MPO
connector

100

200

300

400

500

Reconnection cycle

Figure 7

Insertion loss of two fibers in the same 2-fiber MiniMPO connector during 500 reconnections

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Table 3
Insertion loss

Temperature
0

Figure 8

2

4

6

8

10

Change in insertion loss in Mini-MPO connector
during temperature cycling test

70
Return loss

Return loss (dB)Multi-mode (62.5 m)

Insertion loss (dB)mean=0.33
max=0.48

mean=0.02
max=0.10

Return loss (dB)mean=47.9
max=41.0

mean=37.8
max=34.8

-0.2

Cycle

60

50

Temperature
40

0

2

4

6

8

10

Cycle

Figure 9

Return loss in Mini-MPO connector during
temperature cycling test

STUDIES OF FLAT INTERFACES

4.1 Problems with Flat Interfaces
With recent progress in optical array device technology
thought is being given to laser diode modules that can bejoined by MT connectors in the hope of using optical fiberconnections for high-speed computers and large-capacity exchanges. It is further expected that development willproceed not only in MT (MPO) connectors but in MiniMPO connectors as well.
Connecting MPO and Mini-MPO connectors and optical modules will require both angled interfaces and flat
interfaces. With a flat interface and no physical contact,
there will be a marked degradation in return loss due toFresnel reflection; to obtain satisfactory return loss characteristics it is essential that positive physical contact between the fibers be maintained. Accordingly attention wasgiven to techniques of achieving physical contact with aflat interface in Mini-MPO connectors.
4.2 Evaluation of Optical Characteristics
A prototype flat-interface 2-fiber Mini-MPO connector was
manufactured for single-mode and multimode fibers (corediameter 62.5µm). In evaluating optical characteristics, norefractive index matching material was used.

94

Single Mode

0.0

-0.4

4.

Optical Properties of Flat-Interface Mini-MPO
Connectors (n=16)

0.2

Table 3 shows data for insertion loss and return loss
characteristics. Both are the values when connected to areference plug. The results show that there is no practicalproblem in terms of either insertion loss or return loss.
Return loss in particular is 47 dB average and 41 dB minimum for single mode and 37 dB average and 34 minimum
for multimode --excellent results that are just as good as
those for ordinary MT connectors using refractive index
matching material.
4.3 Confirming Physical Contact
Next tests were carried out to confirm physical contact. Ina flat interface, a gap between the fibers will result in aconsiderable degradation of both insertion loss and return loss. In connectors in which physical contact is notachieved, a refractive index matching material is appliedin the gap between the fibers to improve optical characteristics. Thus to investigate the situation with physicalcontact, it is appropriate to compare the optical characteristics with and without refractive index matching material.
Figure 10 shows the relationship between insertion loss
characteristics before and after application of refractive
index matching material. In connectors in which physicalcontact is not achieved, application of refractive index
matching material reduces insertion loss. This result, byshowing that the difference in characteristics with and without the matching material is very small, suggests that physical contact was substantially achieved.
1.0

0.8
With matching material (dB)Change in insertion loss (dB)

0.4

0.6

0.4

0.2

0.0
0.0

0.2

0.4

0.6

0.8

1.0

Without matching material (dB)Figure 10 Relationship between insertion loss with andwithout index matching material for flat- interface
Mini-MPO connectors

Furukawa Review, No. 18. 1999

5.

CONCLUSIONS

A new connector known as the Mini-MPO has been developed based on the existing MPO connector but connecting small-core fibers at higher density. It is smaller andsimpler in structure, with an adapter cross-section measuring approximately one-half that of the MPO connectors.
In terms of optical characteristics it gives an average of
0.2 dB or less for insertion loss and 53 dB or more for
return loss in single-mode angled-interface connectors-satisfactory characteristics that are equivalent to thoseobtained with conventional MPO connectors. The characteristics obtained in temperature cycling tests and connection-disconnection tests were stable, and constituted
no practical problem. Good return loss characteristics werealso obtained for flat-interface connectors.
Both the Mini-MPO and MPO connectors are expected
to be used in an increasing range of applications as timegoes on.

REFERENCES

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Multifiber Connectors for Future Large Scale Subscriber Networks", ECOC'93, MoP1.5, (1993), 29.

3) Nagasawa, Furukawa, Makita and Murata: "MechanicallyTransferable Single-Mode Multifiber Connectors", IOOC'89,
21C2-1, (1989), 48.
4) Ota, Kihara and Shigematsu: "High-Performance Multiple-Fiber Connectors", Furukawa Review, No.11, (1992), 9.
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a Plastic Ferrule", IEEE Photon. Technol. Lett., Vol. 3, No. 10,(1991) , 937.
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Manuscript received on June 29, 1998.

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