Development of Optical Wiring Technology
for Optical Interconnects
Mitsuhiro Iwaya*, Katsuki Suematsu*, Harumi Inaba*,
Ryuichi Sugizaki*, Kazuyuki Fuse*, Takuya Nishimoto*2, Kenji Kamoto*3

We had developed elemental technologies for optical interconnects for its
application in next generation high performance data server and others. To
achieve complicated compact wiring, low loss and high reliability, we applied the small diameterand bend insensitive fiber, ThreadWave. We had also developed the optical connector housing
which can be space-reduced and of high-density connection in anticipation of its combination
with electrical wiring, achieving 3.2 times higher of connection density over conventional ratio.
We had also simplified the assembling method of connectors with the goal of cost reducing andwe had confirmed that, with the easy-assembled connectors, the optical characteristics required
for the high-speed transmission could be maintained. We had developed an experimental modelof blade server structure which is assembled of these elemental technologies, and achieved an
installation of 2000-fiber-channels with the same size as the conventional electrical wiring.

ABSTRACT1.

INTRODUCTION

In recent years, along with the higher-performance ofCPU and the increasing of data traffic, the data processing capacity required to the server has been dramaticallyincreasing. Also, on the server construction, virtualization
technology by consolidating multiple processes in one
CPU is progressing, and the volume of data between
CPUs and the memory capacity required are increasing.
Based on this situation, in the future, it is estimated thatthe input-output of data processing will need several
Tbps of the bandwidth and tens of Tbps of the transfer
rate in total will be required in the interconnects which willtransmit these multiple signals.
Meanwhile, the increase of the transmission capacity
on the conventional electric transmission technology is
about to face a limit in terms of the signal quality and theheat dissipation efficiency. As a new technology, the optical interconnects are expected to solve these problems.
Based on these backgrounds, we have been working
through the development of elemental technologies forthe optical interconnects such as optical modules, opticalconnectors and optical fibers 1).
This time, we had considered the optical interconnects
technology which is intended to apply to the next generation high performance data server and others and we had
developed wiring technique which can be implemented in* FITEL Photonics Labs, R&D Div.
*2 Yokohama R&D Labs, R&D Div.
*3 FITEL Products Division, Telecommunications Company

higher-density and lower-cost than ever before. This isreported herein.
CPU blade
Switch blade

LSI

LSI

Optical mid-plane
Figure 1

2.

Optical transceiver

Structure of optical CPU blade server.

THE CONCEPT OF THE DEVELOPMENT

2.1. Development Subjects and Examination Items
For the assumed application, we set the high performance server in a blade server structure which has ahigh-density mounting. The schematic of the set server
structure is shown in Figure 1. A mid-plane (circuit board)is placed in the center and the slots where blades can beinserted into are placed on the both sides of the midplane. In this development, we assumed that the structurein which the CPU blades, where optical transceivers are
installed, can be inserted from the one side, and that

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Development of Optical Wiring Technology for Optical Interconnects

each CPU blade can be connected to switch blades on
the other side of the mid-plane. We also targeted that theserver chassis size is the same as the conventional electric wiring server.
Since many blades are densely installed in a blade
server, the connecting space allowed for one blade is limited. Also, considering power supply or low-speed electric
signal transfer to the blades, the space allowed for opticalsignal connection is further decreased. To achieve multiple channel connections under such conditions, the drastic space-reduction and the high-concentration of the
optical connectors and transmission media are required.
Additionally, the optical signal transmission media isrequired to allow the flexibility of the wiring design andthe ease of installation. Also, from a repair and replacement viewpoint, it is desirable not to have an excessive
integration.
Lastly, reducing costs of components and mounting
process are important evaluation points for the realizationof the optical interconnects. We tried to examine, thistime, the cost reduction of the optical connection fromboth viewpoints of the component cost and the mounting
process cost.
2.2. The Concept of the Development
In our development of the product and its evaluation, we
proceeded following the concept below.
As mentioned above, we set a mid-plane type blade
server as the assumed server structure where several
CPU blades and switch blades can be interconnected.
For the connection diagram of CPU blades and switch
blades, we assumed two types, namely, one where CPU
blade connects to several switch blades and one where
one CPU blade connects to one switch blade, and wedesign so that these are placed together on the optical
mid-plane.
An optical transceiver is installed on the CPU blade andoptically connected to the optical mid-plane at the opticalconnector on the edge of the blade. Meanwhile, on theswitch blade, in order to extract an arbitrary optical signal
and to confirm the connection characteristics or the wiring of the mid-plane by loopback, the connectors areattached to the wirings of the optical connector on theedge of the blade and terminated. The number of the
optical signal channels is set as 96 channels per blade.
We selected optical fiber as the optical transmission
media. The advantages of the optical fiber applied to theblade server are:1.
Low crosstalk between channels,
Flexibility and ease of installation,
2.
3.
Production cost,
4.
Ease of length standardization in the unit ofmodule, by ribbon fiber such as 12-fiber ribbon
and others.
Also, in this prototyping, we assumed that the structure
where the optical fiber installation is only on one side of
the mid-plane circuit board, this is the side where CPU
blades are inserted in Figure 1.
The high-density connection of the optical signals is

considered on the basis of a conventional mechanicallytransferable (MT) connector structure. Also, since opticalcomponents and electric components need to be connected simultaneously when the blade is inserted, we
took a backplane (BP) optical connector housing.
For cost reduction, we are essentially trying to reducethe components cost and the assembling cost.

3.

ELEMENTAL TECHNOLOGIES

3.1. Optical Fiber
The strictest limitation of the high-density installation with
optical fiber is the minimum bending radius. It is limited
by long term reliability (failure rate) of the optical fiber andthe optical signal loss when bending the optical fiber. The
minimum bending radius R is around 30 mm for standard
optical fiber. And even for so-called “bend insensitive
fiber”, which inhibits bending loss by controlling refractive
index profile of core and cladding, it is considered to bearound 7.5 mm in terms of long term failure rate for thefiber.
On the other hand, to achieve complex optical fiber wiring on the mid- plane and maintaining minimum bending
radius, it is desirable to have the space which is, at least,
twice as high as the minimum bending radius on the midplane. However, it is difficult to have such space because
the insertion length of the blades is specified if conventional electric components are used.
Also, to install as many as 2000 fibers efficiently, it isdesirable that the optical fiber is as small and flexible aspossible to avoid interference and generation of tensile
force between optical fibers.
To meet such requirements, we used ThreadWave, thesmall diameter fiber of 50 μm core, 80 μm cladding and125 μm coating in diameters 2). The comparison between
cross-section shapes of ThreadWave and standard fiber
is shown in Figure 2. Figure 3 shows total failure in time(FIT) number of 2000 fibers with one turn loop of 5 mm
radius plotted against the cladding diameter. The FIT at
80 μm cladding diameter is less than 1.0, the value shownin black solid line, and it is understandable that theThreadWave has sufficient reliability to the small-radiusbending. For the optical characteristics, as gaining therelative refractive-index difference Δ from 1% to 2% of the
GI profile core, good characteristics as 0.1 dB or less ofbending loss with one turn loop of 5 mm radius and
200 MHz-km or more of transmission band are achieved.
In this development, we assumed the total channel
numbers of transmitter and receiver modules as up to
12 fibers and examine with 12-ribbon fibers of
ThreadWave. The 12-ribbon fibers of ThreadWave has
each fiber in 125 μm-pitch alignment and can reduce thecross-section area down to about one-quarter of
12-ribbon of standard fiber with 125 μm cladding diameter. In addition to that, thanks to its smaller diameters of
cladding and coating, it is more flexible than standard ribbon fiber. With these characteristics, high-density andcompact wiring can be achieved easily.

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Development of Optical Wiring Technology for Optical Interconnects

Conventional Fiber

ThreadWave

Core
Cladding
Coating

Center pitch 250 μm

50Core [μm]50125

Cladding [μm]

80

250

Coating [μm]12512-φ80 μm（Pitch 125 μm）
Figure 4

Figure 2

The comparison between cross-section size of
ThreadWave and standard fiber.
Figure 2 The comparison between cross-section
size of ThreadWave and standard fiber

1.0E+06

FIT [1turn * 2000fiber]

1.0E+04

R =5 mm

1.0E+02
1.0E+00
1.0E-02
1.0E-04
1.0E-06
1.0E-08
1.0E-101.0E-121.0E-14

0

20

40

60

80

100

120

140

Cladding diameter (μm)
Figure 3

Cladding diameter of fiber and total FIT number.

3.2. Assembling Method of Optical Connectors
We used optical connector of MT ferrule which has thesame size as the conventional one. The appropriate number of fibers for one MT is assumed as 24 fibers from the
aspects of the balance with wiring density, ease of connection diagram designing between blades, and manufacturability.
For the alignment positioning of 24 fibers, “2 rows of
12 fibers” or “single row of 24 fibers” are available. We
selected “single row of 24 fibers” to avoid increase in therigidity with bending of vertically stacked ribbon fibers.
This fiber alignment is shown in Figure 4.
We reviewed the manufacturing process of the MT connector to reduce the optical components cost and adopted no- polish quick assembly technique in this development. This is the technique where ribbon fiber is cut off by
a cleaver and then fixed by adhesive after adjusting theposition of the end faces of fiber and MT connector. It is
possible to reduce manufacturing cost drastically by omitting the polishing process which exists in standard
assembly process of MT connector.

Fiber position of 24 MT ferrule.

3.3. Optical Connector Housing
As described before, for the blade server application, inorder to connect the electric connectors and the opticalconnectors simultaneously when the blade is inserted,
the backplane connector structure for optical connector
housing is required. There is a multi-fiber push on backplane (MPO-BP) connector as a currently standard backplane optical connector. In case of MPO-BP connector,
first a MPO connector is assembled with a MT ferrule
inserted inside, and then this MPO connector is mounted
on the BP housing. This process is shown in Figure 5.
Although it has the advantage for such construction that
the MPO connector is detachable from the BP housing
and is easy to exchange or to replace, it is difficult toincrease the mounting density of the MT connector
because components are many and the size becomeslarge.
To solve such problems, we developed a new high-density backplane connector housing. The outline of its structure is shown in Figure 6. This connector has the structure where 4 pcs of the MT ferrules are aligned in
2x2-array and mounted directly to the backplane connector housing. With this connector structure, the number ofnecessary BP connector housing parts is drasticallyreduced as one set of housing is for 4-MT ferrules, ascompared with the conventional requirement of one set
for each. Also, we made the structure where each MT ferrule is mounted and fixed with minimum and simplified
components. With this structure, the number of component parts for 4-MT ferrules is reduced in half and drastic
reduction of the part’s cost is expected.
The front views (to scale) of the conventional MPO-BP
connector housing and the new housing are shown in
Figure 7 compared with the same scale. As shown in
Figure 7, for the new housing, even though the entire size
is slightly increased from the conventional MPO-BP connector housing, the installation density of the optical fiber
has increased by 3.2 times because 4 pcs of MT ferrules
are mounted. In addition, since 1 pc of MT ferrule has
24 fibers, 96 fibers can be connected at a time per one
set of backplane connector.

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Development of Optical Wiring Technology for Optical Interconnectsnique without index matching. Also, it is understandable
that insertion loss will be further reduced with properindex matching.
Also, equivalent insertion losses are obtained for bothclipped mating of MT ferrules only and for 4 pcs of MT
ferrules mounted in the new backplane housing, andhence, it is confirmed that the new developed housing
can connect appropriately 4 pcs of MT ferrules at once.

MPO housing

350
Avg. 0.29 dB
Max. 0.79 dB
Std. 0.13 dB
N=768

Backplane connector
Figure 5

Conventional MPO type backplane connector.

Frequency

300
250
200
150
100
Board-side connectors(4 sets)

50
3

2

1.

1

1.

0

1.

9

1.

8

0.

7

0.

6

0.

5

0.

4

0.

3

0.

0.

1

2

0.

Insertion loss [dB]

Blade-side connectors(4 sets)

New backplane connector.

Loss with index matching [dB]

Figure 8

Backplane connector
Figure 6

0.

0.

0

0

Insertion loss distribution of non-polish quick
assembly MT connectors.

0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Loss without index matching [dB]
Figure 9
Conventional (MPO-BO)
24 fibers / 13 mm x 19 mm(24-fiber MT x 1)
Figure 7

4.

New housing
96 fibers / 14 mm x 22 mm(24-fiber MT x 4)

Comparison of fiber density.

CHARACTERISTICS

4.1. Insertion Loss
The insertion loss distribution of quick assembly opticalconnector without index matching material is shown in
Figure 8. On the data for connected sample number of N
= 768, 0.29 dB of the average insertion loss and 0.79 dBof maximum insertion loss at 850 nm are obtained. Also,
the insertion loss difference of matings with index matching material between end faces of the MT ferrules isshown in Figure 9.
From these results, we had confirmed that sufficientconnecting characteristics can be obtained with the MT
ferrules assembled with no-polish quick assembly tech-

Insertion loss difference between matings withand without index matching.

4.2. Transmission Characteristics
We evaluated the 10 Gbps transmission characteristics ofthe quick assembly connector. Using 850 nm wavelengthof VCSEL (vertical cavity surface emitting LASER), thesignal of PRBS-7 (pseudo-random binary sequence 7) is
generated and eye-pattern is observed. Observed eyepattern is shown in Figure 10. As measurements are conducted on all 24 fibers, sufficient eye opening is obtainedat all channels and it is considered that the quick assembly connector has sufficient characteristics for 10 Gbps
transmission.

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Development of Optical Wiring Technology for Optical Interconnects

REFERENCES
1) Masayuki Iwase et al.: “Optical Components for High-Density
Optical Inter-Connect System: OptoUnity®”, Furukawa Electric
Review No. 32 (2007), 26-33.
2) Ryuichi Sugizaki et al.: “Small Diameter Fibers for Optical
Interconnection and Their Reliability”, Proceedings of the 57 th
IWCS, 2008.

Figure 10 Eye-pattern diagram at 10 Gbps transmission.

5.

EXPERIMENTAL MODEL OF BLADESERVER

We had made an experimental model of blade server withthe techniques described above. We achieved the size of
500 mm width x 500 mm height, which is almost equivalent to the conventional electric wiring blade server. For
the depth, as we cannot make an easy comparison
because a cooling fan is attached externally and theblade slots are simplified. However, since we designed
the optical mid-plane so that its effective thickness
remains the same as that of the one which only has electric connectors mounted on, it is expected that the server
size will be equivalent to the conventional electric wiring
type at end products.
On the mid-plane, there are 2000 fibers installed in total
with ThreadWave 12-ribbon fibers, throughput performance of 20 Tbps is achieved when it is operated at
10 Gbps per channel.

6.

CONCLUSION

We had developed a new optical interconnecting technology in expectation of its application to the next generationhigh performance data server. With the use of the small
diameter fiber, ThreadWave, and the development of the
new type backplane housing, we realized much higherdensity connection over conventional optical interconnects technique, and compact and high capacity opticalwiring. Also, by reviewing the components and the manufacturing processes, prospect for high reduction of massproduction cost is obtained.
In the future, consideration of further cost reduction,
reliability evaluation of optical connection, and moredetailed transmission characteristics evaluation arescheduled to proceed.

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