Vertical Optical Waveguide Comprising Square Base Cuboid Cores

with Size Modulation for Multilayer Chip-to-Chip Interconnection

Songpin Ran, Takaaki Kakitsuka and Kiyoto Takahata

Graduate School of Information, Production and Systems, Waseda University, Kitakyushu, Fukuoka 808-0135, Japan

Keywords: Chip-to-Chip Interconnection, Optical Waveguide, BPM.

Abstract: For chip-to-chip interconnection, a Pbit/s-class transmission capacity is expected to be developed in near

future. To achieve such a huge capacity, a multilayer optical waveguide structure for connection between

chips is indispensable. In this paper, a square base cuboid-core structure with size modulation for vertical

light beam propagation in a polymer multilayer waveguide is proposed for improving optical coupling

between the optical devices and waveguide. A 10-layer polymer waveguide with nonuniform cuboid cores

are designed for single-mode transmission at 1.31 μm. Using a simulation based on the beam propagation

method, it is confirmed that the proposed design can provide a coupling efficiency of −4.4 dB, which is 3.8

dB higher than that of a previously reported uniform cube-core design, for the 10-layer structure and a

crosstalk of −20.6 dB. The simulated results show that the proposed nonuniform cuboid-core structure has

the potential to realize a 10-layer optical waveguide structure for future chip-to-chip optical interconnection

with a huge transmission capacity.

1 INTRODUCTION

With rapid progress in artificial intelligence, Internet

of things, and other emerging IT technologies (Calo

et al., 2017, Atlam et al, 2018), a strong demand for

an increase in the transmission capacity of rack-to-

rack, board-to-board, and chip-to-chip inter-

connections has arisen. For chip-to-chip inter-

connection, a Pbit/s-class capacity is expected in the

near future. However, it is difficult for an electrical

interconnection to achieve such a huge transmission

capacity because of the problems of large

transmission loss in the high-frequency region and

significant heat generation. Therefore, optical chip-

to-chip interconnection has attracted considerable

attention owing to its high-speed and power-saving

operation.

A typical optical chip-to-chip interconnection

configuration is shown in Fig. 1 (Matsuoka et al.,

2012). Two large-scale integration (LSI) chips

adapted with optical interface devices are connected

using an optical waveguide formed in the board. The

optical signal from a vertical cavity surface-emitting

laser (VCSEL) travels downward and is reflected by

a 45° mirror for coupling to a horizontal waveguide.

After being transmitted through the waveguide, the

optical signal is reflected by another 45° mirror and

detected by a photodiode (PD).

Figure 1: Schematic of an optical chip-to-chip

interconnection.

Figure 1 shows an optical interconnection with a

single layer waveguide. However, it is impossible to

achieve Pbit/s-class capacity because of the

limitation of the number of waveguides associated

with the single layer. This means that a multilayer

waveguide structure is indispensable for achieving a

huge interconnection capacity.

There have been several studies on multilayer

chip-to-chip interconnection (Shishikura et al., 2007,

Suzuki et al., 2015). Figure 2 shows a cross-

sectional schematic of a connection between the

VCSELs and a multilayer optical waveguide. As

shown, the optical signal travels a longer distance

from the VCSEL to a waveguide in a deep position,

and the light beam broadens before arriving at the

mirror. This causes optical power loss and degrades

174

Ran, S., Kakitsuka, T. and Takahata, K.

Vertical Optical Waveguide Comprising Square Base Cuboid Cores with Size Modulation for Multilayer Chip-to-Chip Interconnection.

DOI: 10.5220/0009173401740179

In Proceedings of the 8th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2020), pages 174-179

ISBN: 978-989-758-401-5; ISSN: 2184-4364

the overall coupling efficiency between the VCSEL

and photodiode (PD).

To solve this problem and increase the coupling

efficiency, a multilayer optical printed wiring board

with a cube-core structure has been proposed and

demonstrated (Shishikura et al., 2007). The board

contains a two-layer multimode waveguide for a

wavelength of 0.85 μm.

For a rack- or board-level optical interconnection,

0.85-μm multimode VCSELs and multimode fiber

(MMF) have been widely used. For the

demonstration of chip-to-chip optical

interconnection, the same type of VCSELs and

multimode polymer waveguide have been mainly

used (Ishii et al., 2003, Doany et al., 2012). However,

in recent years, introduction of single-mode

transmission to such a short-distance application has

been actively discussed (Vyrsokinos et al., 2016) for

meeting the demand of high-speed signal

transmission over 25 Gbit/s without heavy digital

signal processing at the receiver. Moreover, long-

wavelength (1.31 or 1.55 μm) single-mode VCSELs

operating around 50 Gbit/s (Spiga et al., 2017,

Breyne et al., 2019) and single-mode polymer

waveguide have been reported (Xu et al., 2017, Zuo

et al., 2019).

In this work, the cube-core structure is applied to

a multilayer single-mode polymer waveguide for a

wavelength of 1.31 μm, and a new nonuniform

cuboid-core structure design is proposed to provide

high coupling efficiency for an optical

interconnection board comprising 10 waveguide

layers.

Figure 2: Cross-sectional schematic of a connection

between VCSELs and a multilayer optical waveguide.

2 UNIFORM CUBE CORES

A cube-core structure was proposed and developed

for a multilayer multimode optical waveguide

(Shishikura et al., 2007). Figure 3(a) shows a cross-

sectional schematic of the connection between the

optical devices and the multilayer waveguide with

cube cores. The cube-core structure acts like a lens

or a waveguide core for vertical light beam

propagation. The cube cores are composed of

waveguide-core material and can be fabricated

simultaneously in a waveguide-core fabrication

process. They require no additional fabrication

process as in conventional processes for fabrication

of multilayer optical waveguides. As shown in Fig. 3

(b), this structure confines the light beam and

suppresses the spread of the beam from the VCSEL.

This leads to a high coupling efficiency between the

optical devices and horizontal waveguides even in

the lower layer.

Figure 3: (a) Cross-sectional schematic of a connection

between VCSELs and a multilayer optical waveguide with

cube cores. (b) Optical confinement via cube cores.

By introducing the cube core, a vertical

waveguide structure for a 10-layer single-mode

waveguide was designed (Fig. 4). The design

parameters are listed in Table 1. The operating

wavelength was 1.31 μm. For a waveguide design in

this study, ultraviolet (UV) curable acrylate

polymers were set as the core and clad materials of

the waveguide (ChemOptics Inc., 2019). The

refractive indexes were set at 1.48 and 1.45 for the

core and clad, respectively. A single-mode

horizontal waveguide having a width of 3.0 μm and

height of 3.0 μm was designed. In this work, the

height of the cube core was fixed to 3.0 μm because

the cube cores should be formed simultaneously

with the waveguide core, as previously described.

The size of the base area of all cube cores was set at

3.0 × 3.0 μm

. The pitch between the cube cores,

which is equal to the pitch of the layers, was fixed at

18 μm with consideration of a balance between the

waveguide density and crosstalk.

The designed model was analyzed using the

beam propagation method (BPM). Figures. 5(a) and

(b) show simulated beam propagation diagrams for

the 10-layer models without and with cube cores,

respectively. It is obvious that the cube cores confine

the light beam and guide it up to the 10

layer.

Vertical Optical Waveguide Comprising Square Base Cuboid Cores with Size Modulation for Multilayer Chip-to-Chip Interconnection

175

Figure 4: Model of the vertical waveguide comprising

cube cores and a mirror.

Table 1: Design parameters.

Item Value

Wavelength 1.31 μm

Waveguide mode Single-mode

Core height 3.0 μm

Pitch 18 μm

10 μm

7 μm

Refractive index

Clad 1.45

Core 1.48

*1 d

: Distance between VCSEL and the 1

core

*2 d

: Distance between VCSEL and the layer surface

Figure 5: Simulated light beam propagation: (a) Without a

cube core and (b) With a cube core.

To evaluate the effect of the cube-core structure,

the coupling efficiency was calculated using models

without and with cube cores. The coupling

efficiency is the ratio of the monitored light power to

the output light power of the VCSEL. At a

propagation distance z, the light power in a monitor

area was calculated and the coupling efficiency was

obtained. In this work, the size of the monitor area

was set at 5.2 × 5.2 μm

, which is almost equal to

the mode field size of the single-mode waveguide

with a 3.0 × 3.0 μm

cross section. The z

dependence of the coupling efficiency is shown in

Fig. 6; the position of the input face of each cube

core is shown with a marker on the curve. The cube-

core structure effectively increases the coupling

efficiency. In the 10

layer, the improvement is 7.6

dB; however, the coupling efficiency remains only

−8.2 dB, which corresponds to a power loss of 85%.

Even in the 5

layer, the coupling efficiency is less

than −5 dB.

Figure 6: Coupling efficiency for the vertical waveguide

with and without cube cores.

3 NONUNIFORM CUBOID

CORES

To improve the coupling efficiency of the uniform

cube-core structure, we propose square base cuboid

cores with base-size modulation, as shown in Fig.

7(a). The cross-sectional size of the cuboid core

gradually changes from the 1

to the 10

layer while

its height is fixed to 3 μm. It is called a nonuniform

cuboid-core structure in this paper. The simulated

light beam propagation through the vertical

waveguide consisting of 10 size-modulated cuboids

is shown in Fig. 7 (b). Compared with the beam

propagation of the uniform cube (Fig. 5(b)), the

confinement of the light power is strengthened. The

improvement is particularly obvious in the 8

. 9

and 10

layers.

Figure 7: Nonuniform cuboid-core structure: (a)

Schematic and (b) Simulated light beam propagation.

PHOTOPTICS 2020 - 8th International Conference on Photonics, Optics and Laser Technology

176

For the design of the 10-layer single-mode

waveguide with a nonuniform cuboid-core structure

(Fig. 8), the side lengths of the square base of the

cuboid cores were optimized within a range of 3.0 –

8.0 μm with a 0.5-μm step for each layer. During the

optimization process, the cuboid-core size in the

lowest layer was fixed at 3.0 × 3.0 μm

because the

45° mirror was placed in this layer. Figure 9 shows

the optimized square-base size of the cuboid cores

for each vertical waveguide. As shown, the side

length of the square base of the cuboid was

gradually modulated.

For the optimized nonuniform cuboid-core

design, the coupling efficiency was calculated (Fig.

10). The coupling efficiency of the uniform cube-

core structure is also plotted for a comparison. The

coupling efficiency of the nonuniform cuboid-core

structure is significantly better than that of the

uniform cube core for layer numbers greater than 4;

it is greater than −5 dB for all layers. In the 10

layer, the coupling efficiency reaches −4.4 dB,

which is 3.8 dB better than that of the uniform cube-

core structure.

Figure 8: Schematic of the 10-layer single-mode

waveguide with the nonuniform cuboid-core structure.

Figure 9: Optimized square-base size of cuboid core.

Figure 10: Coupling efficiency for the vertical waveguide

with uniform cube cores and nonuniform cuboid cores.

For layer numbers greater than 6, the coupling

efficiency of the nonuniform cuboid-core structure

gently increases. It might be caused by a crude

setting of the monitor area in the simulation.

In the multichannel interconnection, crosstalk

between channels is an important issue. The

crosstalk between adjacent channels was analyzed

for both the uniform cube-core and nonuniform

cuboid-core structures using the models shown in

Fig. 11. In each model, two adjacent vertical

waveguides with cuboid cores were designed with

the channel pitch of 18 μm for the 10-layer

waveguide.

The calculated crosstalk is shown in Fig. 12.

With beam propagation in the vertical direction, the

crosstalk increases. For both the uniform cube-core

and nonuniform cuboid-core structures, the crosstalk

is less than −30 dB up to the 3

layer. It reaches

approximately −20 dB in the 5

layer and remains at

approximately −20 dB up to the 10

layer. Though

the nonuniform cuboid-core structure can provide

stronger optical confinement, it does not provide a

significant improvement in the crosstalk; it has a

crosstalk similar to that of the uniform cube-core

structure. This result is probably because the

crosstalk light from the adjacent channel can more

readily couple with the larger cuboid cores. In the

layer, the crosstalks of the uniform cube-core

and nonuniform cuboid-core structures are −18.1 dB

and −20.6 dB, respectively; thus, an improvement of

2.5 dB was obtained.

Vertical Optical Waveguide Comprising Square Base Cuboid Cores with Size Modulation for Multilayer Chip-to-Chip Interconnection

177

Figure 11: Crosstalk analysis models for the (a) uniform

cube-core and (b) nonuniform cuboid-core structures.

Figure 12: Crosstalk dependence on propagation distance.

4 CONCLUSIONS

For a Pbit/s-class chip-to-chip interconnection with a

multilayer optical waveguide, cuboid-core structures

for vertical light beam propagation were studied.

To solve the problem of poor optical coupling

between the optical devices and waveguide in the

lower layer, a cube-core structure was previously

reported for improving the optical confinement of

the vertical light beam propagation in a multilayer

multimode waveguide. In this study, it was applied

to the design of a multilayer single-mode waveguide

for 1.31-μm light. In the simulation, the structure

drastically improved the coupling efficiency;

however, it could only provide a coupling efficiency

of −8.2 dB for a 10-layer structure.

To further improve the coupling efficiency, a

nonuniform square-base cuboid-core structure was

proposed. The base size of the cuboid gradually

changed along with the light beam propagation. The

new design showed a coupling efficiency of −4.4 dB

for a 10-layer structure and provided a crosstalk of

−20.6 dB, which is 2.5 dB less than that of the

uniform structure.

These simulated results show that using the

proposed nonuniform cuboid-core structure, a 10-

layer single-mode polymer waveguide structure for

chip-to-chip optical interconnection with a huge

transmission capacity can be realized.

ACKNOWLEDGEMENTS

The authors would like to thank Prof. Takahiro

Watanabe for his valuable suggestions.

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