Development of Vertical Spot Size Converter (SSC) with
Low Coupling Loss Using 2.5%Δ Silica-Based Planar Lightwave Circuit
Yasuyoshi Uchida *, Hiroshi Kawashima *, and Kazutaka Nara *Recently, new planar lightwave circuit (PLC) components are under study, in
response to the requirement of downsizing and cost reduction, to increase theirrefractive index difference Δ between cladding and core. Accordingly, we are developing
2.5%Δ silica-based PLCs. However, it is difficult to put the 2.5%Δ silica-based PLCs directly into
practical applications because their coupling loss with a single-mode fiber is as high as 2.9 dB/
facet, raising a need for developing a spot size converter (SSC) to lower the loss. We havedesigned here, therefore, a new vertical SSC that has a broad core expanded in both horizontal
and vertical directions, and fabricated the vertical SSC by combining plasma enhanced chemical
vapor deposition (PECVD) and shadow mask. As a result, it has been confirmed that the use of
the vertical SSC can significantly reduce the coupling loss between the 2.5%Δ waveguide andfiber to 0.06 dB/facet.

ABSTRACT1.

INTRODUCTION

Recently, as the optical networks becomes in widespread
use accompanied by traffic increases, optical components in use are required to be smaller in size, lower incost and lower in power consumption. In response tosuch requirements, studies on the silica-based planar
lightwave circuit (PLC) components are going forward toincrease the refractive index difference Δ between cladding and core to 1.5% or 2.0% 1), 2). It is known that, byincreasing their Δ, PLC components can have smaller
bending radius, and this leads to advantages such that
optical components like arrayed waveguide grating(AWG) can be reduced in size, and simultaneously that
their cost per chip can be reduced due to an increase in
the number of chips per wafer. For example, whereas with
Δ=0.8%, the bending radius is 5000 μm, it decreases to
800 μm when Δ is raised to 2.5%, and this results in a
AWG chip downsized to about 15 mm square 3). Moreover,in the case of differential quadrature phase shift keying(DQPSK) decoder and the like that needs temperature
control, downsizing results in a reduction in the area size
to be temperature controlled, so that the necessary power
consumption can be reduced.
However, an increase in Δ results in an increase in optical confinement effects, thereby decreasing mode field
diameter (MFD), which in turn increases the mode fieldmismatch along with an increase in the coupling loss withfiber. For example, the coupling loss between waveguideand fiber is 0.5 dB/facet when Δ=0.8%, it rises to 2.9 dB/
facet when Δ=2.5%, and this excessive coupling loss
* FITEL Photonics Lab., R&D Div.

makes it difficult to directly use the waveguide as opticalcomponent. Because of this, it becomes necessary tohave a spot size converter (SSC) to lower the couplingloss. Up to now, an SSC shown in Figure 1 has beenreported, which expands the waveguide width using ahorizontal taper 4). Use of this SSC can reduce the coupling loss to 0.25 dB/facet when Δ=0.8%, and to 1.5 dB/
facet when=2.5%, but lower coupling loss is still needed
for practical applications.

Straight waveguide

Taper section

Waveguide section
after mode field conversion
Single-mode fiber core

Silicon substrate

Figure 1

Structure of horizontal SSC.

Examples of SSCs with even reduced coupling loss
include laterally narrow-tapered SSC 5) and double-corelaterally narrow-tapered SSC 6) as shown in Figure 2.
These SSCs have a narrow waveguide to make light leakout of the core in order to efficiently couple the light with afiber, and achieve reduced coupling losses of 0.5 dB/facet
and 0.2 dB/facet, respectively. But these SSCs have disadvantages in terms of fabricating processes in that, thenarrow-tapered waveguide has to be strictly controlled forits distance between the taper and the end, and that the
double-core laterally narrow-tapered waveguide has avery complicated fabricating process, in which cores having different Δ are grown twice and they are each formed
into a waveguide shape.

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Development of Vertical Spot Size Converter (SSC) with Low Coupling Loss Using 2.5%Δ Silica-Based Planar Lightwave Circuit

We have fabricated here, using a 2.5%Δ waveguide, avertical SSC that has a facilitated fabrication process
together with a reduced coupling loss with fiber. As aresult, the coupling loss with fiber has been significantly
reduced to 0.06 dB/facet. The details will be presented inthis paper.

re

co

dth

wi

eg

av

W

e
uid

1m

m±
0.5

θ1

Core width
conversion sectionmm

respect to the other, locally forming a core elongated in
the vertical direction, and this can lead to loss of a specified shape. Accordingly, it was decided to expand thecore in the horizontal and vertical directions at different
positions, so as to mitigate the strict conditions of controlling the relative position of the two tapers, thereby making
fabrication easier. The waveguide after core expansion in
the horizontal and vertical directions, i.e., after mode conversion, is square in cross-section, being equal in thewidth and height. The width is indicated as “waveguidesize” in Figure 3.

Over-cladCore widthfine-tuning section

1st
section2nd
sectionWaveguide size（a）Laterally narrow-tapered SSC.
Fiber core
High-∆ core

Taper section
Low-∆ core（i）
O v e r-

（ii）

Unde
bstratec la d（i） Ultra-high-∆ coreOver-cl ad
Under-cl ad（ii）
Low-∆ core

Low-∆ core

O ver-clad
U nder-clad（b）Double-core laterally narrow-tapered SSC.

Figure 22.

Figure 3

Silicon substrate

Connected withsingle-mode fiber
Vertical taper angle θ

Structure of vertical SSC.

r - c la dn su

SilicoConnected with
2.5%∆ waveguide

Under-clad

Typical structures of SSCs studied.

DESIGN OF SSC

2.1 Structural Design of SSC
The SSC fabricated here has adopted a vertical structurein which the core is expanded in both the horizontal andvertical directions. The core size of a 2.5%Δ waveguide isnormally 3.5 μm × 3.5 μm, which is smaller than that of afiber. By expanding the waveguide core size, therefore,
the MFD is expanded making the mode field mismatch
smaller, to reduce the coupling loss with fiber.
Figure 3 shows the structure of SSC fabricated here, inwhich the core width of a 2.5%Δ waveguide is expanded
first by a horizontal taper to form the first section shown in
Figure 3, and then the height is expanded by a verticaltaper to form the second section. Since the taper expands
the core in the upper direction only and not in the lower
direction down to the substrate, the waveguide has an
asymmetric structure. Thus it is necessary to adopt separate processes to fabricate the horizontal and verticaltapers independently. In doing so, if the design is suchthat the core is expanded in the horizontal and verticaldirections simultaneously, it is likely that the position ofeach taper shifts during the fabrication process with

2.2 Design Parameters of SSC
To design the structure of a vertical SSC, it is necessary tohave two parameters: waveguide size after mode field
conversion, and taper angle in the vertical direction θ.
The waveguide size after mode field conversion should
be such that the coupling loss is minimized. And, thetaper angle in the vertical direction θ should be such avalue that is capable of mode field converting adiabatically without generating higher modes.
First, the waveguide size after mode conversion wasdetermined. Simulations were run using the beam propagation method (BPM) to obtain light propagation conditions in the waveguide with changing waveguide widths
for coupling with fiber, and the coupling loss with a fiber
was calculated based on the optical electric field distribution after mode conversion. In this calculation, a square
being equal in height and width was adopted for the
waveguide shape after mode conversion. The calculated
results are shown in Figure 4. While the coupling loss of a
3.5 μm-wide 2.5%Δ waveguide is normally 2.9 dB/facet, it
decreases as the waveguide width is expanded because
the MFD approaches that of fiber thus reducing the mode
field mismatch. From Figure 4 it can be seen that the coupling loss reaches the minimum value of 0.06 dB/facet ata waveguide width of 12.5 μm. With larger waveguide
widths, because the MFD becomes larger than that offiber, the mode field mismatch rises again, increasing thecoupling loss. Therefore, the waveguide width after mode
conversion was determined as 12.5 μm, where the coupling loss with fiber reaches a minimum.

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Coupling loss（dB）

Development of Vertical Spot Size Converter (SSC) with Low Coupling Loss Using 2.5%Δ Silica-Based Planar Lightwave Circuit

3
2.5
2
1.5
1
0.5
0
0

20

Relationship between waveguide width and calculated coupling loss.

WIDTH DIRECTION（µｍ）

Next, the taper angle θ for mode field conversion in thevertical direction was determined. Changes in light propagation conditions in the waveguide with changing taper
angles were simulated using the BPM. The shape ofwaveguide structure was regarded to expand to the upperdirection only, as shown in Figure 3. Figure 5 shows thesimulation results of propagating conditions. It can beseen that, whereas adiabatical mode field conversion isachieved in the case of θ=0.4° or θ=1.0°, higher modes
are generated in the case of θ=2.0°.

25
20
15
10
5
0
-5
-10
-15
-20
-25

0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

0

500
1000 1500 2000 2500
PROPAGATION DIRECTION（µｍ）

3000

WIDTH DIRECTION（µｍ）（a）θ＝0.4°
25
20
15
10
5
0
-5
-10
-15
-20
-25
0

0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

0.6
Excess loss（dB）

Figure 45
1015
Waveguide width（µｍ）

Subsequently, excess loss was calculated based on thesimulation results, and the calculated results are shown in
Figure 6. It is seen that, while the excess loss show virtually no change for θ<1.0°, it increases forθ>1.0°.
Consequently, the requirement for the taper angle in thevertical direction is θ>1.0°. On the other hand, the smaller the tapering angle, the longer the taper length.
Accordingly, it was decided to adoptθ=0.6°, taking boththe chip size and process variations into consideration.

0.4

0.2

0

Figure 6

3.

01

WIDTH DIRECTION（µｍ）

4

DEVELOPMENT OF FABRICATION PROCESS FOR VERTICAL TAPER

A vertical SSC was fabricated on a silicon substrate, using
PECVD and reactive ion etching (RIE). On this occasion,
the taper in the vertical direction was fabricated, as illustrated in the process flow chart shown in Figure 7, in two
steps using the PECVD and a shadow mask. First, thefirst core was deposited to have a height equivalent tothat of normal waveguide. Next, the second core wasdeposited, using a combination of PECVD and shadow
mask having a gap D from the substrate, to the height ofwaveguide after mode field conversion. If the gap D（a）Deposition of 1st coreusing PECVD.
500 1000 1500 2000 2500
PROPAGATION DIRECTION（µｍ）

500
1000 1500 2000 2500
PROPAGATION DIRECTION（µｍ）

1st core

3000
Silicon substrate
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

0

3

Relationship between taper angle and calculated
excess loss.

（b）θ＝1.0°
25
20
15
10
5
0
-5
-10
-15
-20
-25

2
Taper angle（°）3000（b）Deposition of
2nd core using
PECVD andshadow mask.

Under-clad

Shadow mask
2nd core

Over-clad（c）Processing including
photolithography,
RIE and FHD.

（c）θ＝2.0°(5.3) θ = 2.0°

Figure 5

Relationship between taper angle and simulated
results of propagation.

Figure 7

Fabrication process of vertical SSC.

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Development of Vertical Spot Size Converter (SSC) with Low Coupling Loss Using 2.5%Δ Silica-Based Planar Lightwave Circuit

between the shadow mask and the substrate is eliminated, deposition will not take place on the portion beneaththe shadow mask so that the layer has a thickness of thefirst core, whereas the other portion has an increased
thickness that includes the second core. But here, since a
gap D was provided between the substrate and shadow
mask as illustrated in Figure 8, the raw material gas flows
into the gap during the PECVD process, thereby forminga gentle slope of layer at the end of the shadow mask 7).
This slope was utilized as the taper in the vertical direction. Subsequently, in the same manner as in the normal
PLC fabrication process, a planar circuit pattern including
the horizontal taper was formed, using photolithography
and RIE, and finally, the core was embedded by depositing the over-clad using the flame hydrolysis deposition(FHD) method.

Shadow maskCore
Under-clad

Gap D

4.

FABRICATION OF SSC

A vertical SSC was fabricated under the conditions determined so far. First, the vertical structure of a 2.5%Δ core
was fabricated on a silicon substrate, using PECVD andshadow mask. Next, a vertical SSC was fabricated using
photolithography and RIE, based on the positional relationship between the horizontal and vertical tapers asshown in Figure 3. Finally, the SSC core was embedded
using FHD.

5.

FABRICATION RESULTS

The vertical SSC fabricated here has, as shown in
Figure 3, an endface of normal size to be coupled with anormal 2.5%Δ waveguide and another endface aftermode conversion to be coupled with a fiber. These endface shapes were observed using an optical microscope,and the photos are shown in Figure 10. The sizes of thesecores are 3.8 μm×3.5 μm and 12.8 μm×13.0 μm, respectively, substantially the same size as the design.

Siliconsubstrate

Figure 8

PECVD process using shadow mask.

Figure 9 shows the taper shapes changing with different
gap D between the substrate and shadow mask during thedeposition of the second core. The taper angle θ in
Figure 9 is defined as the maximum slope of the taper. With
D=0.6 mm, taper length and θ are 6 mm and 0.6°, respectively; and with D =1.2 mm, they are 9 mm and 0.3°,
respectively. In other words, the wider the gap D, the smaller the taper angle, but the longer the taper length. On thisoccasion, since θ=0.6°, gap D was selected as 0.6 mm.

Thickness (µm)8（a）Normal-sized core.

Taper length

6
4

8

10µm

Taper angle θ200

Thickness (µm)10µm（b）Core after mode field conversion.
2

4
6
Length（mm）

810Figure 10 Photos of vertical SSC cores on both ends.

（a）
D =0.6 mm
Taper length

64Taper angle θ

2
0
0

2

4
6
Length（mm）

8

10

（b）
D =1.2 mm

Figure 9

Taper shape in the vertical cross-section.

While inputting light from the normal-sized endface of avertical SSC, the near field pattern of light emitted fromanother endface after mode field conversion was measured in order to confirm the spot size. In addition, for thesake of comparison, measurement was carried out using
a normal 2.5%Δ waveguide to determine the light emitted
from a normal-sized core. Figure 11 shows the measurement results of near field patterns. The photo on the top
shows the measurement result of the light emitted fromthe normal-sized core, with an MFD of about 5 μm. And
the bottom photo shows the measurement result of, while

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Development of Vertical Spot Size Converter (SSC) with Low Coupling Loss Using 2.5%Δ Silica-Based Planar Lightwave Circuit

normal-sized core, and 1.5 dB/facet in the case of using ahorizontal SSC. And, in the present case where a vertical
SSC fabricated here is used, the coupling loss has beensignificantly reduced to 0.06 dB/facet.
Coupling Loss @1550 nm（dB/facet）

inputting the light from a normal-sized core, the light emitted from the core after mode field conversion by a vertical
SSC, and the MFD is about 11 μm. From these measurements it has been demonstrated that the MFD has been
converted from 5 μm to 11 μm, using the vertical SSC fabricated here. Moreover, from the fact that the circularshape is maintained unchanged, it is considered thatmode field has been converted adiabatically in single
mode.

（a）Normal-sized core

Figure 11 Measurement results of near field pattern of vertical
SSC.

Figure 12 shows the measurement result of the nearfield pattern of light emitted from a single-mode fiber. The
MFD is about 11 μm also, and the size and shape is thesame as for the one converted by the SSC. From theseresults, it has been confirmed that the vertical SSC fabricated here has converted the MFD of normal-sized coreto that of single-mode fiber.

3
2.5
2
1.5
1
0.5
0

withoutSSC

withhorizontal
SSC

with
vertical
SSC

Figure 13 Coupling loss with SMF when SSC is used.

6.

（b）Core after mode field conversion.

3.5

CONCLUSIONS

A novel structure of SSC was designed in which the core
size was expanded to both horizontal and vertical directions.
By means of depositing using a combination of PECVD
and shadow mask, a vertical SSC was fabricated using a
2.5%Δ waveguide.
It has been confirmed from the measured near field pattern of the fabricated SSC that the MFD of a 2.5%Δ waveguide has been converted to the MFD of a single-mode
fiber. And by using the vertical SSC, the coupling loss
between 2.5%Δ waveguide and fiber has been reduced
to 0.06 dB/facet.

REFERENCES

Figure 12 Near field pattern of SMF.

Subsequently, the coupling loss between the fabricated
SSC and a fiber was determined. The insertion loss at
1550 nm of a waveguide chip having a vertical taper wasmeasured first, then, the propagation loss of 2.5%Δ
waveguide was subtracted to obtain the coupling loss
with fiber. Figure 13 compares the measurement resultwith those of with a horizontal SSC and without SSCs. It isseen that the coupling loss is 2.9 dB/facet in the case of

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