Optical Coupling between Optical Devices and Opto-Electronic
Printed Wiring Boards
Naoyuki Kojima *, Shunichiro Sato *, Osamu Mikami *2,
Tomonori Ogawa *2, and Masahiro Kanda *2

Recently, several interconnection technologies such as optical interconnection,
three-dimensional integration of semiconductor chips and wirelessinterconnection are under study, in order to solve the so-called pin bottleneck problem thatcauses inadequate inter-chip transmission capacity thus limiting the computer processing
speed. We have been paying attention, among these, to the optical interconnection technology
based on optical wiring, and are working on the structures of optical interconnector that enableefficient connection between, and mounting of, optical devices and optical waveguides. To be
more precise, the structure consists of an optical interconnector made of ultraviolet ray curable
resins having different refractive indexes, being mounted on the connecting part between opticaldevices and waveguides. In this work, the optical interconnector was formed on a VCSEL
mounted on an FR-4 substrate, and its optical characteristics were studied. As a result, it has
been confirmed that this optical interconnector is provided with an optical confinement capability,and enables high-efficiency, large positional tolerance connection between optical devices andwaveguides.

ABSTRACT1.

INTRODUCTION

In recent years, as digital home appliances become widespread and computerization accelerates, data volume
handled continues to grow thereby increasing the transmission bandwidth of network equipment such as servers
and routers. This trend creates a need for performance
improvement of data processing systems. However,
although the integration degree and processing ability of
LSI chips themselves improves year by year in accordance with the ITRS (International Technology Roadmap
for Semiconductors) roadmap, the pin bottleneck problem arises causing an inadequate inter-chip transmission
capacity and at the same time limiting computer processing speed. In order to solve this problem, several interconnection technologies including wireless interconnection as well as optical interconnection and three-dimensional integration of semiconductor chips are under study.
We have been paying attention, among these, to the optical interconnection technology based on optical wiring,
and are working on structures that enable efficient connection between, and mounting of, optical devices andoptical waveguides. To be more precise, the structure* Components & Advanced Interconnect Technologies Dept.,
Ecology & Energy Lab., R&D Div.
*2 Graduate School of Engineering, Tokai University

consists of an optical interconnector made of ultraviolet
ray curable resins (UV resin) having different refractive
indexes, being mounted on the connecting part between
optical devices and waveguides. In this work, the opticalinterconnector was formed on a VCSEL (Vertical Cavity
Surface Emitting Laser) mounted on an FR-4 substrate,and its optical characteristics were studied. In this paper,
the effectiveness confirmed of this optical interconnector
will be presented.

2.

OPTICAL INTERCONNECTION TECHNOLOGY

2.1 Construction of Optical Interconnections
Current data processing systems are basically structured
by electrical interconnections between electrical components centered on LSI chips. Accordingly, to solve the pinbottleneck problem by using optical interconnection, it isessential that optical wiring be as easy and simple toachieve interconnection as electrical wiring, allowing us toperform "optics-unconscious" interconnection. Figure 1
shows the schematics of interconnections that have been
implemented so far.
In transmission equipment, interconnections between
electronic components and equipment have a hierarchicalstructure such as, for example, "chip~package~module~board/card~backplane~rack~cabinet." Among these,

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Optical Coupling between Optical Devices and Opto-Electronic Printed Wiring Boardsin these days, optical fibers are used for interconnection
between equipment such as the rack~cabinet part, andthe use of optical fibers and waveguides is expanding in1)
the backplanes connecting the board/card part .
On the other hand, in the interconnection between electrical components centered on LSI chips, although thestudy on the use of optical fibers for the board/card part
between the module and backplane has recently beenlaunched aimed at its realization in near future, the interconnection still relies on electrical means. Even now,
interconnection techniques for the chip~package~module
part remain yet in the research phase.

enables optical interconnection that is as easy and convenient as electrical wiring, without being conscious of optical wiring. It is thus said that this technology will be introduced in future, if its reliability problems are solved.

(a) Jumper type (next-generation)
Opticalinterface

LSI

Electricalconnector Electricalconnector
moduleInterposer LSI

PWB
Backplane(b) OE-PWB type

Electricalcable(a) Electric typeFigure 2
Opticalconnector
module LSIInterposer

Opticalconnector
moduleLSI

Opticalconnector

PWB
Backplane(b) Optical connector type
Opticalconnector

LSI

Interposer

LSI

Optical cable

Opticalconnector
moduleBackplane(c) Jumper type (present)
Figure 1

Schematic view of interconnection (I)2.2 Opto-Electronic Printed Wiring Board
Study on the use of optical means for the chip~package~module part has lead to two possible candidates asshown in Figure 2 (a) and (b): the extension of the jumper
type and the opto-electronic printed wiring board(OE-PWB) type, in which optical waveguides are incorporated in the substrate.
In the case of the extension of jumper type shown in
Figure 2 (a), it is advantageous from the standpoint oftechnology development in that, it is an extension of theconventional technology that uses fiber and connector.
But it can be disadvantageous from the standpoint ofusers involved in design and assembly in that, this technology means coexistence of numerous fibers and opticalwaveguides in a limited space on the substrate, raisingpossible problems of handling and heat dissipation.
On the other hand, in the case of the OE-PWB typeshown in Figure 2 (b), in which optical modules as well as
optical devices and packages are mounted for opticalconnection on a substrate with built-in optical waveguides, it is advantageous from the standpoint of users in
that, since the handling and heat dissipation issues are
similar to those in conventional electrical wiring boards, it

Opticalconnector
moduleSchematic view of interconnection (II)However, more detailed study on this OE-PWB technology has shown that this technology actually involves,
beside the reliability problem, several problems such thatthe optical devices and optical waveguides must be, due
to their optical characteristics, optically connected withhigh efficiency, that the mounting apparatus must have alarge tolerance in mounting precision, and that sealing
must be performed, to cope with the use environment,
assuredly after mounting. In other words, the optical connection can not be achieved by simply surface mounting
optical devices on an OE-PWB, but it is not until these
problems are solved that this technology becomes aseasy as electrical wiring. Thus, probably due to the developmental tasks mentioned above, not much study has
been conducted in fact on the mounting structures of
optical devices on the OE-PWB.
Accordingly, to achieve high coupling efficiency andlarge positional tolerance, we have proposed an opticalstructure, in which two UV resins with different refractive
indexes and photomasks are used to form, on the opticaldevices and the optical port of an OE-PWB, an opticalinterconnector having a light-guiding function. In thiswork, the optical interconnector was fabricated by photomask pattern transfer on optical devices in an opticalmodule, which comprises the optical devices mounted on
an FR-4 substrate. The optical characteristics of the fabricated optical interconnector were measured, and theeffectiveness of this optical interconnector structure wasconfirmed.

3.

OPTICAL INTERCONNECTOR

3.1

Mounting Structure of Optical Devices on
OE-PWB
Figure 3 shows a simple model of optical interconnector
studied here, together with the photos of a prototype. The
structure consists of an optical module accommodating
optical devices and its driver IC, which is mounted on an

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Optical Coupling between Optical Devices and Opto-Electronic Printed Wiring Boards

Driver IC

Bonding WireCore

Photonic Device

Clad
Dam

Solder
FR-4

Clad
Core
Clad

Light
Photonic Device

Figure 4

MirrorOptical interconnector.

（a）Model3.2 Manufacturing Method
Below will be described in detail the method of forming
the optical interconnector proposed here, onto the module substrate on which optical devices are mounted.
Figure 5 illustrates the process steps. For this process,both the photomask and the UV resin were prepared in
two types, for the core and the clad.

（b）Photos of prototype

Figure 3

Simple mounting structure of optical module on
OE-PWB.

OE-PWB. Since optical devices generally have electrodes
formed on the light-emitting and -receiving surface, wirebonding is needed for their mounting on the module. The
loop of wire bonding has to be several tens to one hundred microns in height at least, and accordingly, the distance from the light-emitting and -receiving surface to theoptical port on the OE-PWB becomes considerably large.
In cases where a driver IC thicker than optical devices isused, the distance may become larger even a cavity
structure is adopted for the module. In addition, sinceelectrical circuits for module mounting are needed on the
OE-PWB, circuit patterns with minimized functions
become necessary on the optical waveguide, together
with a resist layer to protect the surface circuitry. After
mounting, need arises for some sealing to protect thebonding wires and optical devices, giving rise to thenecessity for further increase in the device-to-OE-PWB
clearance. In total, therefore, the distance from the lightemitting and -receiving surface of the optical device to themirror surface of optical waveguide, as shown in Figure 3,
would be several hundreds microns, and this is expected
to result in a significantly large coupling loss.
Accordingly, as shown in Figure 4, an optical interconnector with a light-guiding function was formed on the
optical port of the OE-PWB. The optical interconnector
allows for confining the diffusive light beam within the
coupler, thereby achieving beam injection into the opticaldevice or optical waveguide with high efficiency and large
positional tolerances.

3.3 External View and Configuration
Figure 6 (a) and (b) show photos of the prototype, after
the core and dam were fabricated. Acrylic UV resins having refractive index of 1.569 and 1.542 were used for thecore and dam material and for the cladding material,
respectively. The core diameter is 30 μm. Figure 6 (c) is aphoto of the optical interconnector proposed here, with its
clad being filled. The size of the clad is 1000 μm x 4000μm x 500 μm (height). From Figure 6 (c), it can be confirmed that the VCSEL and the bonding wires are successfully embedded in the clad, together with the core,
which is accurately located at the light-emitting spot.
Since this prototyping was intended for confirmation of
the effectiveness of the optical interconnector, measurements were made only for evaluation of optical power,
without using a driver IC. For this reason, a special module substrate provided with a wiring pattern for power
supply only was used in the prototyping.

4.

OPTICAL CHARACTERISTICS

4.1 Measuring System
Figure 7 shows the measuring system used for evaluationof optical characteristics. A laser driver is used to power
supply the module substrate provided with VCSEL and
optical interconnector, to directly activate the VCSEL; andthe emitted light passing through the optical interconnector is observed for its optical intensity distribution usingnear field pattern (NFP) measuring equipment; then theequipment is replaced with an optical power meter combined with graded-index multi-mode fiber (GI-MMF) tomeasure the I-L characteristics and the positional tolerance in optical coupling.
4.2 Optical Characteristics
The measurement results of the NFP and optical intensity
distribution are shown in Figure 8, in which the NFPs in
the cases of core only and clad filled are shown in (a),and the optical intensity distribution on the X axis of the

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Optical Coupling between Optical Devices and Opto-Electronic Printed Wiring Boards

Core
Dam
Clad

（a）Photo Mask

The ﬁrst process : Manufacturing of core and dam

1) UV resin for core is ﬁlled
over the optical device.

2) Photomask for core anddam is set.

3) UV ray is irradiated throughthe photomask.

4) The photomask is removed,
and uncured UV resin iscleansed.

7) UV ray is irradiated throughthe photomask.

8) The photomask is removed,
and uncured UV resin iscleansed.

The second process : Manufacturing of clad

5) UV resin for clad is ﬁlled
into the space between
the core and dam.

6) Photomask for clad is set.

（b）Manufacturing steps
Figure 5

Manufacturing method of optical interconnector.

NFPs shown in (a) is presented in (b). It can be seen thatin the case where the clad is filled, although a slight leakof light from the core to the clad can be observed, astrong intensity of light is propagated through the core. In
the both cases, since the optical output has an intensity
peak within the core diameter of 30 μm, it is consideredthat the light beam propagates being confined in the core.
The leak from the core has been confirmed to be very
weak.
Core

Dam

100 µm VCSEL（a）Core and dam

NFP

Core

Dam

500 µmThe fact that the optical intensity, for the cases of core
only and clad filled, has different peak positions may be
attributable to several causes including: that the core surface is coated during cladding with a thin film of cladding
material due to the small clearance caused by the thickness of the photomask, bringing about light scattering;that light leakage occurs due to the changes in opticalpropagation modes caused by the difference in the refractive index difference (Δn) of UV resin; and that the core

VCSEL
Bonding
Wire
Monitor（b）Close-up side view of core and dam

PC

Clad
Coreφ30 µm
GI-MMF（ core/clad=50 µm/125 µm ）500 µmLaser DriverPower Meter（c）Core and dam after clad being ﬁlled

Figure 6External views of prototyped optical interconnector.

Figure 7

Measuring system.

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Optical Coupling between Optical Devices and Opto-Electronic Printed Wiring Boards

possibly becomes inclined during the process of clad formation. After all, however, considering that the proposed
optical interconnector has a peak optical output within thecore diameter of 30 μm, the optical interconnector is functioning as an optical waveguide.
MAX

the waveguide formed on the light emitting surface of the
VCSEL supports radiation-less light propagation that isless sensitive to the environment, and that the waveguide
reduces the effects of light reflection at the interface
between the light emitting surface and air, thereby
increasing the light intensity. As stated above, the effectiveness of the optical interconnector proposed here has
been confirmed.

10 μ
10 µm

10 µm
Core only（a）NFP

900

Core only
Optical inter connector

1.5

Air gap10.5

光結合器interconnector
Optical800

0Core only

700

1

2

3

4

6

5

7

8

9

10

Driving current（mA）

600
500

Figure 9400

I-L curves.

300
200

Core diameter
30 µm

100
0
‑30

‑20

‑10

0

10

20

30

X-axis（µm）（b）Light intensity distribution

Figure 8

Next, the positional tolerance for 1-dB down optical
coupling efficiency was measured while shifting the
GI-MMF in the X-axis direction. The results are shown in
Figure 10. The positional tolerance for 1-dB down optical
coupling efficiency of -15/+14 μm was obtained, confirming that the optical interconnector proposed has a broad
positional tolerance.

NFP and light intensity distribution.

Next, the I-L characteristics were evaluated using a
GI-MMF (core/clad=50/125 μm). Measurements were carried out after adjusting the position of the fiber to obtain amaximum coupling efficiency. The measured objects werean optical interconnector with core only for purposes ofcomparison, an optical interconnector with cladding, andan elemental VCSEL. Figure 9 shows the results of I-Lcharacteristics measurement. The results show that the
optical interconnector with cladding results in an optical
output that is comparable to, but slightly lower than, thatof the optical interconnector with core only. This decreasein optical output may be attributable to, as mentioned
before, several causes including: that the core surface iscoated with a thin film of cladding material, bringing aboutlight scattering; that light leakage occurs due to thechanges in optical propagation modes caused by the difference in the refractive index difference (Δn) of UV resin,
resulting in an increase in coupling loss with the GI-MMF;
and that the core possibly becomes inclined during theprocess of clad formation. Comparison of optical output
between the optical interconnector and elemental VCSEL
shows that the output of the former has been significantly
improved. This may be attributable to such causes that

Optical coupling eﬃciency（dB）

Optical power (a.u.)

MIN

With clad

Optical power (mW)2‑30 ‑25 ‑20 ‑15 ‑10 ‑5
0

0

5

10

15

20

25

30

‑1
‑2
‑3
‑4
‑5

‑15 〜 +14 µm
1 dB down tolerance width

‑6
‑7
‑8
‑9
‑10

X-axis（µm）

Figure 10 Positional tolerance.

5.

ANALYTICAL STUDY OF OPTICAL
COUPLING EFFICIENCYIn order to validate the coupling structure between the
optical module and optical wiring board as proposed in
Figure 4, positional tolerance in optical module-to-module
coupling was analyzed using the ray tracing technique.
The analytical model is shown in Figure 11. The analysis

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Optical Coupling between Optical Devices and Opto-Electronic Printed Wiring Boards

was carried out under the assumption of: refractive index
of resin for core: 1.569; that for clad: 1.542; and radiation
angle of light source: 20°. Figure 12 shows the result ofanalysis. The result shows a broad positional tolerance
for 1-dB down coupling efficiency of -19/+19 μm, confirming the superior potential of this optical module.

Light source

REFERENCES
1) "Proposals for Widespread Use of Optical Interconnection," editedby Optical Circuitry Installation Roadmap Working Group, Japan
Institute of Electronics Packaging, Oct. 9, 2008 (in Japanese)2) Ogawa et al.; "Study on Coupling Structures for Connecting Optical
Module with a Self-Forming Rod and Optical Wiring Board,"
Proceedings of 23 rd General Assembly, Japan Institute ofElectronics Packaging, Mar., 2009, pp. 97~98. (in Japanese)

Refractive index
Core : 1.569
Clad : 1.542
500 µmCore :φ30 µm
Clad : 1 mm

Core :φ50 µm
500 µmClad : 1 mm
PD
Substrate

X

0

Optical coupling eﬃciency（dB）

Figure 11 Analysis model.

0‑2‑19 〜 +19 µm‑4‑6

1 dB down tolerance width

‑8
‑10
‑12
‑40

‑30

‑20

‑10

0

10

20

30

40

X-axis（µm）

Figure 12 Analysis result.

6.

CONCLUSION

We have proposed an optical interconnection structure
capable of high-efficiency connection between, andmounting of, optical devices and optical waveguides, and,using ultraviolet curable resins, demonstrated this technology by fabricating an optical interconnector on an optical module that is provided with optical devices. Moreover,
the optical characteristics of the structure have been measured and analyzed, thereby validating the effectivenessof this technology, in which an optical interconnector ismounted on the light-emitting and -receiving surface ofthe optical device in the module and on the optical port of
OE-PWB. Furthermore, the connecting structure for optical wiring board has been studied using this optical interconnector, and it has been confirmed that the structure
has a broad positional tolerance in alignment.

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