High-Density Multi-Fiber Connectors for Optical Interconnection
by Naoya Nishimura *, Katsuki Suematsu *, Masao Shinoda * and Masato Shiino *As the performance limits of electrical transmission become apparent in such
equipment as servers and routers, optical interconnection technology isdrawing attention. Furukawa Electric has been developing, in anticipation of application to
optical interconnection, optical high-density multi-fiber connectors. In this paper, compact multifiber ferrule μ-Joint is presented, along with MU-type plug and backplane connector, both basedon μ-Joint. The μ-Joint has a cross section about one quarter that of conventional MT ferrule, sothat it allows for high-density mounting with a low insertion loss of 0.5 dB or lower. Tests inconformity to Telcordia GR-1435-CORE have been carried out to confirm that the ferrule has highlong-term reliability.

ABSTRACT1.

INTRODUCTION

Data traffic on the Internet continues to increase due to thegrowth of moving picture delivery and the like, resulting in arequirement for high-speed, large-capacity signal transmission functionality for servers and routers. Thus, the issue ofinadequate transmission performance of electrical wiring is
gradually emerging due to the increased number in signaltransmission lines. Accordingly, optical interconnection
technology is recently drawing attention as a promisingmeans to solve this problem. Moreover, in terms of space
reduction, optical interconnection is expected to enhanceheat dissipation efficiency in high-end systems such asservers and super computers, so that application of opticalinterconnection technology to signal transmission between
boards and racks is becoming a real possibility.
We have been developing the optical interconnection
technology assumed to be applied to equipment wiring,in which optical fibers, optical connectors, and highspeed optical modules are downsized than conventionalcounterparts to constitute the product family ofOptoUnity 1).
In this paper, small-sized multi-fiber optical connectors
capable of high-density mounting will be reported for theirstructures and evaluation results, including μ-Joint connector plug and adapter, which have, using ultra-compact
multi-fiber ferrule μ-Joint, the same outer dimensions asfor the MU connector 2).

2.

assuming that the connectors are mainly used on board
ends for signal transmission between boards and racks.
1) Should be smaller than the conventional MT ferrule andMPO connector, enabling high-density mounting.
2) Should be compatible with ThreadWave 1), the bendinsensitive fiber for optical interconnection of Furukawa
Electric.
3) Should allow for low insertion-loss connection.
4) Should be provided with high long-term reliability.
With respect to insertion loss, the target was set at 0.5 dB
or lower even in case single-mode (SM) fiber is used, inconsideration of total loss budget for ultra-fast transmission
in the future. In terms of long-term reliability, the development target for loss variation after the tests in conformity toTelcordia GR-1435-CORE was set at 0.3 dB or lower.

3.

ULTRA-COMPACT MULTI-FIBERFERRULE μ-JOINT

An ultra-compact multi-fiber ferrule μ-Joint capable ofhigh-density mounting has been developed. Figure 2
compares μ-Joint with MT ferrule conventionally used.

RackBackplane

ThreadWave ribbon(containing 80-µm diameter ﬁber)

DEVELOPMENT CONCEPTFigure 1 shows a schematic illustration of application of
the connectors developed here. Development wasadvanced based on the concept mentioned below,* FITEL-Photonics Lab., R&D Div.

Optical module

Figure 1

Appliedplace

Schematic illustration of application of the developedconnectors.

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High-Density Multi-Fiber Connectors for Optical Interconnection

The dimensions of μ-Joint is about half that of MT ferrule,and about 25% in cross section. It uses 0.4-mm diameter
stainless steel guide pins, and is adapted to ThreadWave
optical fiber having a cladding diameter of 80 μm, andcore diameters of about 50 μm and about 5 μm for multimode (MM) fiber and SM fiber, respectively. ThreadWave
MM fiber has a relative reflective index difference Δn ofabout 2%, so that it is less susceptible to bending loss
increase, permitting an allowable bending radius R of
5 mm. Moreover, whereas a fiber ribbon generally has afiber pitch of 250 μm, that of ThreadWave fiber ribbon is
125 μm, making it suitable for compact optical wiring on
printed circuit boards, etc.
Improving the accuracy of ferrule is essential to realize
low insertion loss. In order to achieve the target insertion
loss of 0.5 dB, the target fiber-hole position error was
designed to be not more than 0.5 μm. Figure 3 shows themeasurement results of the misalignment of fabricatedμ-Joint ferrules, in which the fiber-hole position error of
0.5 μm or lower is achieved by improving the dimensional

MT ferruleµ-Joint ferrule

Figure 2

Comparison of MT ferrule and μ-Joint ferrule.

accuracy of the mold.

4.

Whereas MPO connector is commonly used for multi-fiber
optical connection, MU connector housing is adopted
here aimed at higher mounting density. The MU connector is standardized, and is practically used in transmission
equipment installed in telephone stations.
Figure 4 shows μ-Joint-MU connector plugs and a duplex
adapter, where the μ-Joint ferrule is applied to the MU
housing. The plugs come in two types --male with guidepins, and female without guide pins-- and they are used in
opposition to each other. A push-pull coupling mechanism
is provided for ease of connection and disconnection. The
adapter has a pitch of 4.5 mm between connectors, making possible higher mounting density than the MPO connector. In order to reduce costs, we have adopted thedesign such that the plug shares several off-the-shelf parts
with the MU connector that is commercially available, andthat two identically shaped parts are assembled back-toback to constitute the adapter.
The ferrule compression force at the time of connector
engagement is an important parameter to achieve physicalcontact (PC). In consideration of its application to backplane, the μ-Joint-MU connector plug was designed to provide a compression force equivalent to that of MU connectors. But, since this raised some fears of insufficient compression force when the ferrule was applied to a multi-fiberconnector, the method of polishing the ferrule endface wasoptimized to ensure PC connection. Figure 5 shows theresults of PC connection verification tests on fabricatedconnectors, in which the insertion loss was measured
before using an index matching material on the mating
portion and after applying the matching material in drops.
It can be seen that the difference between the two measurements are extremely small, and hence PC connection
has been achieved with respect to all the fibers studied.

5.
Y-axisR（R2＝X2＋Y2）

MU-TYPE HOUSINGBACKPLANE HOUSING

In anticipation of application to optical backplanes,
μ-Joint-BP connector has been developed, in which theX-axis
140
Avg. 0.19
Max. 0.49
Min. 0.01
Std. 0.11
n = 360

120

Frequency

100
80
60

Duplexadapter

40
20
0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Fiber-hole position error R（µm）

Figure 3

Fiber-hole position error of μ-Joint ferrules.

µ-Joint-MU
connector plug（female）

Figure 4µ-Joint-MU
connector plug（male）

μ-Joint-MU connector plug and duplex adapter.

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High-Density Multi-Fiber Connectors for Optical Interconnection

μ-Joint-MU connector plug is adapted to a backplaneconnector. Figure 6 shows the connector developed here.
The μ-Joint-BP connector has a structure such that theprinted circuit board housing engages with the backplanehousing, allowing for connection and disconnection en
bloc of multiple connectors together with the board.
Moreover, simultaneous use of electrical connectors onthe board is possible. Since the μ-Joint-BP connector isdesigned to have a compression force equivalent to thatof the MU connector --a single-fiber connector, excessive
loads on the latch portion made of plastics can be avoided while the mounting density is significantly improved.
Inheriting the mating mechanism of the single-fiber MU
connector, the accuracy in the positioning mechanism of
the μ-Joint-BP connector has been improved along withthe mounting density enhancement.

EVALUATION RESULTS

Figures 7 and 8 show the insertion loss of μ-Joint connector plugs developed here engaged with an adapter. Theinsertion loss for MM fiber is 0.14 dB maximum at theMeasurement wavelength: 850 nm
Max. 0.14
Min. 0.01
Ave. 0.03
Std. 0.03
n = 96

1400.5120

Avg. 0.16
Max. 0.41

0.4Frequency

Min. 0.05
Std. 0.10
n = 96

0.3

100
80
60
40

0.2

201

0.
90.
8

0.
7

0.
6

0.
5

0.
4

0.
3

0.
2

00.
1

00.1

Insertion loss（dB）
000.1

0.2

0.3

0.4

0.5

Figure 7Insertion loss without index matching material（dB）

Figure 5

Insertion loss of μ-Joint-MU connector for multi-mode
fiber.

Results of physical contact verification tests ofμ-Joint-MU.

Measurement wavelength: 1310 nm

160Max. 0.41
Min. 0.01
Ave. 0.12
Std. 0.09
n = 96140

Frequency

120
10080
60
40
20

μ-Joint-BP connector.

10.
90.
80.
70.
6

Insertion loss（dB）

Figure 8
Figure 60.
4Printed circuitboard housing

0.
3

Backplane
housing

0.
2

000.
1

Insertion loss with index matching material（dB）

1600.
56.

measurement wavelength of 850 nm, and is 0.41 dB maximum for SM fiber at 1310 nm --satisfactory insertion loss
characteristics in each case.
Figure 9 shows the insertion loss of μ-Joint-BP connector. The loss is 0.30 dB maximum at the measurement
wavelength of 1310 nm.
Reliability evaluation tests in conformity to Telcordia
GR-1435-CORE were carried out to confirm that the connectors have good environmental and mechanical characteristics. The results are shown in Table 1. Specifically,Figures 10 and 11 show the results of thermal cycling
tests and durability tests, respectively, of μ-Joint-MU plugconnected with an adapter. Test conditions for thermal
cycling tests are: -40℃~75℃, 8 hours/cycle, 336 hours;
and those for durability tests are: 200 times in total, either
endface of connector is cleaned every 25 times, both
endfaces of connector are cleaned every 50 times. The
insertion loss variations for thermal cycling tests anddurability tests are 0.1 dB or lower and less than 0.2 dB,
respectively --satisfactory results.

Insertion loss of μ-Joint-MU connector for singlemode fiber.

Furukawa Review, No. 34200815High-Density Multi-Fiber Connectors for Optical Interconnection

REFERENCES

CONCLUSION

Ultra-compact multi-fiber optical connectors comprising
μ-Joint-MU connector plug, adapter, and backplane connector have been developed, in anticipation of application
to optical interconnection. The evaluation results haveshown that the connectors have good initial performanceas well as long-term reliability.

Measurement wavelength: 1310 nm

160Max. 0.30
Min. 0.01
Ave. 0.09
Std. 0.06
n= 48140120Frequency

1) M. Iwase et al.: Furukawa Review, No. 32, pp. 26 (2007)
2) R. Nagase et al.: NTT Gijutsu Journal, Vol. 15, No. 10, pp. 8 (2003)(in Japanese)

100
80
60
4020ch20.4

ch30.3ch4

0.2

ch50.1ch60ch7-0.1

ch8-0.2

ch9-0.3

ch10-0.4

ch11-0.5ch12050

Insertion loss of μ-Joint-BP connector for singlemode fiber.

Measurement wavelength: 1310 nm
290
240

0.4
0.3

190

0.20.1

140

090

-0.1
-0.2

40-0.3-0.4-10-0.5-0.6100200Time（hour）

300

-60

Temp.（℃）

Insertion loss variation

0.5ch1ch2ch3ch4ch5ch6ch7ch8ch9ch10ch11ch12Temp.（℃）

Figure 10 Thermal cycling test results of μ-Joint-MU connector
for single-mode fiber.

150200Figure 11 Durability test results of μ-Joint-MU connector forsingle-mode fiber.

Table 10.6

100

Repetition times

10.
90.
80.
7

0.
60.
50.
4

0.
30.
2

00.
1

Insertion loss（dB）

Figure 9ch1

0.5-0.60

Measurement wavelength: 1310 nm

0.6

Insertion loss variation（dB）

7.

Test item

Results of reliability test.

Test results
Test conditions (GR-1435-CORE) µ-Joint-MU µ-Joint-BP(SM)(SM)

Thermalaging test

85℃, 336 hr

0.1 dB orlower

0.1 dB or
lower

Humidity
test

65℃, RH 95%, 336 hr

0.15 dB orlower

0.1 dB or
lower

Thermal-40℃~75℃, 8 hr/cycle, 42 cycle
cycling test

0.1 dB orlower

0.1 dB or
lower

Vibration
test

1.5 mm in amplitude,
3 directions, each 2 hrs

0.1 dB orlower

0.1 dB or
lower

Tensiontest

Media 1 (fiber ribbon), Level 1,
Load: 2.2 N

0.15 dB orlower

0.1 dB or
lower

Impact test

Impacting on concrete block
9 times from 1.5-m height0.1 dB orlower

0.1 dB or
lower

Durability
test

Repeated connection anddisconnection, 200 times in total

0.2 dB orlower

0.2 dB or
lower

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