Selected Papers

Recent Advances in Optical Switches
Using Silica-based PLC Technology
Masayuki Okuno†, Takashi Goh, Shunichi Sohma,
and Tomohiro Shibata
Abstract
We review recent progress in optical switches based on planar lightwave circuit (PLC) technology.
First, we describe the basic configuration and fabrication of silica-based PLC switches. Then, we
describe the configuration and characteristics of N×N strictly nonblocking matrix switches (mainly a
16×16 switch), 1×N switches (mainly a 1×128 switch), a 128×128 switching sub-system consisting of
256 1×128 switch modules, and an optical add/drop multiplexing circuit consisting of arrayed waveguidegratings and 2×2 switches. We also report a low power consumption trial for large-scale PLC switches
and the results of reliability tests.

1. Introduction
To handle the increase in Internet traffic, the bit rateof time division multiplexing (TDM) systems has
increased rapidly and wavelength division multiplexing (WDM) systems have been introduced. To date,
WDM systems have mainly been employed in pointto-point transmission systems. In such cases, the signals are processed in the system nodes after opticalelectrical (O-E) conversion. This O-E conversion
limits the throughput and increases the hardware cost.
Optical nodes such as optical cross-connect (OXC)
and optical add/drop multiplexing (OADM) nodes
handle signals in the optical path layer; they are transparent as hardware and provide a high throughput [1].
Space division optical switches are key devices inthese systems, and many types of switches including
micro-electro-mechanical systems (MEMS) [2], bubble [3]-[4], and planar lightwave circuit (PLC) typeswitches have been developed.
At NTT Photonics Laboratories, we have beendeveloping optical switches using silica-based PLC
technology since the 1980s. A silica-based PLC isessentially a passive device. However, it can be used
† NTT Photonics Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: okuno@aecl.ntt.co.jp

20

to realize an optical switch by combining the thermooptic (TO) effect and an interferometer configuration.
PLC switches have low insertion loss and polarization dependence, are easy to attach to fibers, and aresuitable for mass production. They are also stable andhighly reliable. Many switches have been developed,
and 2×2 and 8×8 matrix switches are already being
marketed by an NTT group company. Currently, PLC
switches capable of large-scale integration and offering high performance levels are attracting considerable attention because of the wavelength increase of
WDM systems, and we have expanded the switch
scale and improved their performance.
This paper reviews recent advances in silica-based
PLC switches developed by NTT Photonics Laboratories. First, we provide a brief description of a PLC
and describe the basic structure of PLC switch units.
We then move on to the configuration and typical performance of N×N matrix switches and 1×N selection
switches. Finally, we review a trial designed to reduce
driving power consumption, and mention the reliability of our PLC switch modules.
2. Silica-based PLC
The silica-based PLC is an optical circuit with thesame structure as an optical fiber that can provide
several functions including filtering and switching.
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Figure 1 shows the process used for fabricating a silica-based PLC switch. This fabrication technique
combines flame hydrolysis deposition (FHD) andreactive ion etching (RIE), as used for other silicabased PLCs such as splitters and arrayed waveguidegratings (AWGs) [5]. First, silica glass particles forthe under-cladding and core layers are deposited on asilicon substrate by FHD. The basic components are
pure silica (SiO2) for the under-cladding and germanium-doped silica (SiO2-GeO2) for the core. Next,
the Si substrate is heated in an electric furnace to consolidate the fine glass particles. Then, unwanted areasof the core layer are removed by photolithography

FHD

SiO2-GeO2
glass particles
SiO2
glass particles

Si substrate
Consolidation
Core
Under-cladding

RIE
Waveguide core

Table 1. Waveguide parameters used in silica-based PLC
switches.
H∆

SH∆

Relative refractive index difference (%)

0.75

1.5

Core size (µm)

6×6

4×4

Propagation loss (dB/cm)< 0.01< 0.05

Minimum bending radius (mm)52Optical fiber coupling loss (dB/point)

0.4

>1

and RIE. After that, the core ridge structures are covered with an over-cladding layer by FHD and consolidated again. Finally, thin film heaters for switching
are deposited on the waveguides by the lift-off technique.
PLCs are highly stable and reliable because they
have no moving parts, and the waveguide material,silica glass, is physically and chemically stable. In
addition, they are suitable for large-scale integrationand mass production because they are fabricated by
PLC technology. PLCs offer ease of fiber attachment
and a low propagation loss, which is lower than 0.01
dB/cm at a wavelength of 1.55 µm, because theirmaterial and structure are the same as those of opticalfiber. Typical waveguide characteristics for PLC
switches are listed in Table 1. Waveguides with a relative refractive index difference ∆ of 0.75% between
the core and cladding (High-∆: H∆) are mainly used
for PLC switches because they have a small minimum bending radius of 5 mm and can be densely
integrated. We have been developing optical switches
using 1.5%-∆ waveguides (Super High-∆: SH∆) witha minimum bending radius of 2 mm to achieve aneven higher degree of integration [6].
3. Silica-based PLC switch

FHD+consolidation
Over-cladding

Heater fabrication
Thin-film heater

Fig. 1. Fabrication process of silica-based PLC switch.

Vol. 1 No. 7 Oct. 2003

3.1 Basic configuration
Our basic TO-switch unit is essentially a balanced
bridge Mach-Zehnder interferometer (MZI) composed of two 3-dB directional couplers and two
waveguide arms, as shown in Fig. 2. The waveguidearms are equipped with thin film heaters on theircladding that operate as TO phase shifters. The optical path length difference ∆nL between the two arms
is designed to be zero (symmetric-MZI) or a halfwavelength (asymmetric-MZI). The MZI is in the
cross-state when ∆nL=0: an optical signal is guided
from input port 1 to output port 2 or from input port 2
to output port 1. It is in the bar-state when ∆nL is thehalf-wavelength: an optical signal is guided from
input port 1 to output port 1 or from input port 2 to21

Selected Papers

Waveguide core
Input

Output
2

2

∆L=0

∆L

1

1

Signal light
3-dB directional coupler∆L=Half wavelengthThin-film heater

Fig. 2. Basic configuration of silica-based PLC switch.

output port 2. By varying the refractive index of one
waveguide arm of the MZI using the TO effect, we
can change ∆nL by half a wavelength, which changes
the switch state. The typical temperature increases atthe core and the heater are 15 and 40°C, respectively.
The switching power is 0.35 to 0.45 W. The switching time is 1 to 3 ms, which is sufficient for OXC andOADM systems.
3.2 N×N matrix switches
Large-scale switches are made by integrating thebasic switch units. One such switch is the N×N strictly nonblocking matrix switch*1. We have alreadydeveloped N×N matrix switches with a scale of up to
16×16 [7]-[12]. Figure 3 shows the configuration ofthese switches. We use the double-MZI switchingunits shown in Fig. 3(a) as cross-point switches toobtain a high extinction ratio. The double-MZI
switching unit consists of two asymmetric MZI units
and an intersection. The optical signals are propagated along the cross-path (from input port 1 to output
port 2 or from input port 2 to output port 1) when no
driving power is supplied to the TO heater of either
MZI unit (OFF-state) or along the bar-path (frominput port 1 to output port 1 or from input port 2 to thedummy port) when driving power is supplied simultaneously to the TO heaters of both MZI units (ONstate). In matrix switches, the amount of light leaking
from input port 1 to output port 1 in the OFF-stateshould be kept sufficiently small because it travels toan output port of the matrix switch and increases the
crosstalk. The double-MZI switching unit blocks thelight leaking at each MZI unit, thus achieving a highextinction ratio: twice that of the basic MZI unit. In*1 nonblocking matrix switch: matrix switch that can guide opticalsignals through arbitrary optical paths without obstructing otheroptical paths.

22

contrast, light leaking from input port 1 to output port
2 in the ON-state is acceptable in terms of crosstalk,because it is ultimately guided to an idle output portof the matrix switch. Figure 3(b) shows the logicalarrangement of the N×N matrix switch. This is called
the path-independent loss (PI-loss) arrangement, andconsists of N switching stages, each with N switchingunits, and is nonblocking. This arrangement can
reduce the total circuit length to half that of the conventional diamond-shaped crossbar arrangement(2N-1) [7]-[9], because it requires half the number ofswitching stages.
The performance of the fabricated matrix switches
is shown in Table 2. Each switch was fabricated using
0.75%-∆ waveguides. The circuit layout of a 16×16
matrix switch is shown in Fig. 3(c). The 100 × 100
mm chip was fabricated on a 6-inch wafer. A total of
256 cross-point switches, corresponding to 512 MZIunits, are integrated on one chip. This is the largest
number of integrated switches yet achieved in a
waveguide-type switch. We have already made 8×8
and 16×16 matrix switch modules. The N×N matrix
switch module requires N2 electrical terminals to feed
switching power to each cross-point individually.
256 terminals will be needed in the 16×16 switch
module. Since such a large number of terminals
makes the module structure complicated, we developed a 16×16 switch module with a driving circuitthat has a serial/parallel control signal conversion
Table 2. Performance of fabricated N×N matrix switches.
4×4

8×8

16×16

25×65

68×68

100×100

Waveguide length (cm)

14

29

66

Average insertion loss (dB)

2.6

5.1

5.6

Average extinction ratio (dB)

55

60

59

Power consumption (W)

3

6

13

Switch scale
Chip size (mm2)

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Intersection

Output 2(OFF-state)

Input 2
Output 1(ON-state)

Input 1Signal light

Asymmetric MZI switching unit
(a)
1

1
:

2

2
:

:

:
:
N

N
#1

.......

#2#N

N switching stages: OFF-state: ON-state(b)

100 mm

#1

#2

#12

#11
#8

#10
#7

#4

100 mm

#3
#13

#14
#9
#6

#5

#15

#16

(c)

Fig. 3. N×N matrix switch: (a) cross-point switching unit (double-gate configuration), (b) logical arrangement (pathindependent loss configuration) and (c) circuit layout of 16×16 matrix switch.

Vol. 1 No. 7 Oct. 2003

23

Selected Papers

Power supply
terminals

#4

#1

#13

#16

PLC

SW1

DATA

2

64

193

DrIC #1

256

DrIC #4

CLK
LATCH
(a)
Ceramic substrate

Bonding wires
Input fibers

PLC
Driving IC

Output fibers

Connector for control and power supply(b)Fig. 4. 16×16 matrix switch module: (a) schematic diagram of the driving circuit and (b) photograph of the module.

function, as shown in Fig. 4(a) [12]. As a result, wewere able to reduce the number of electrical terminals
to 3 control terminals and 16 power supply terminals.
Figure 4(b) is a photograph of a fabricated 16×16
switch module. In this module, the 16×16 switch
chip, which was attached to two 16-arrayed fibers,
was mounted on a multi-layered ceramic substrateintegrated with the driving circuits. The module was
165 × 160 × 23 mm.
As shown in Table 2, each switch has a low insertion loss and a high extinction ratio, which are thesame as those of mechanical switches. Specifically,
the insertion loss was reduced by improving the fabrication process, and an average loss of 5.6 dB was
achieved in the largest-scale 16×16 switch module.
The power consumption was 13 W, which allows air
cooling.

24

3.3 1×N switches and their applications
We have also developed 1×N switches that have 1
input port and N output ports. These switches are useful for selecting optical light sources and a signal
monitor. We have already made a delivery and coupling switch (DC-SW) for OXC systems that is composed of several 1×N switches and couplers [13].
We adopted two types of logical arrangement forthese 1×N switches [14]. One is the tree arrangement,
which connects symmetric-MZI units with a binary
tree structure, as shown in Fig. 5(a). This is useful forachieving low loss and reducing the chip size because
the required number of switching stages is proportional to log N. The other is a tap arrangement, whichconnects asymmetric-MZI units to the main waveguide, as shown in Fig. 5(b). This is useful for achieving low power consumption because only one MZIunit is in the ON-state and all the others are in the
NTT Technical Review

Selected Papers

Output ports
#GSW
1
2

#7

#6
#5
#4
#3
#2
Input port

#1

128

: 1×2 switch (GSW) unit (symmetric): Gate switch (GSW) unit (asymmetric)(a)
Input port

1

2

3

4

5

6

7

8

Output ports: Tap switch unit (asymmetric): Gate switch unit (asymmetric)(b)

Fig. 5. 1×N switch configurations: (a) tree structure (1×128 switch) and (b) tap structure (1×8 switch).

OFF-state regardless of the switch scale. Botharrangements have additional asymmetric-MZI units
in their final stage as gate switches to reducecrosstalk. The gate switch can also be used as anattenuator to adjust the optical output level at the
selected output path.
We have fabricated switches ranging from 1×8 to
1×128 [15] with the tree arrangement and a 1×8
switch with the tap arrangement [16]. Table 3 shows
the performance of our 1×N switches. The switches
not using superhigh∆ 1×128 switch were fabricatedusing 0.75%-high∆ waveguides. We obtained a low
insertion loss of 2.6 dB even with the 1×128 tree-type
switch and a high average extinction ratio of above 50
dB.
These 1×N switches can also be used to construct alarge-scale strictly nonblocking N×N switching subVol. 1 No. 7 Oct. 2003

system. Figure 6(a) shows the logical arrangement ofour 128×128 switching subsystem. This consists of
2N 1×N switch modules and over 16,000 opticalfibers. We developed the switch board on which we
mounted two 1×128 switch modules, and the subsystem consisted of two 19-inch rack mounts. The fibersthat connected the 1×128 switch modules were compactly stored by using a fiber management technique[17] developed in our laboratories. A photograph ofthe fabricated 128×128 switching subsystem isshown in Fig. 6(b). We fabricated part of this subsystem and measured its performance. The insertion loss
was below 6.5 dB (average: 5.7 dB) and the extinction ratio was over 90 dB, proving that this subsystem
can be used for large-scale OXC systems.

25Selected Papers

Fiber-wiring system

1

1×128

1×128

1

2

1×128

1×128

2

128

1×128

1×128

128

(a)

(b)

Fig. 6. 128×128 switch subsystem: (a) logical arrangement and (b) photograph of switch subsystem.
Table 3. Performance of fabricated 1×N switches.
1×8(tree)

1×8
(tap)

1×32
(tree)

Waveguide ∆

H∆

H∆

H∆

SH∆

Chip size (mm2)60×66

Switch scale

1×128(tree)

18×60

35×74

67×74

Waveguide length (cm)

6

17

13

11

Average insertion loss (dB)

1.2

1.3

1.5

2.6

Average extinction ratio (dB)
Power consumption (W)

52

56

55

63

1.5

0.5*

2.2

3.5

*: Switch unit: Low power consumption type

3.4 Reconfigurable optical add/drop multiplexer
The silica-based PLC switch can be monolithicallyintegrated with other PLC devices including an AWG
multi/demultiplexer because their fabrication
processes are the same. We have demonstrated areconfigurable monolithic 16-channel OADM withathermal operation [18]. The OADM consists of two
AWG multi/demultiplexers and an array of 16 2×2
PLC switches as shown in Figs. 7 and 8. The 2×2
switch unit is composed of 4 MZI units to reduce thecrosstalk, and 64 MZI units may be activated simultaneously. The refractive index of silica-based waveguides changes with chip temperature, which means
such AWG characteristics as center wavelength
would also change. To solve the problem in thisOADM, we made the AWG characteristics temperature independent by filling grooves formed in thearrayed waveguides with a silicon resin having a negative refractive index temperature dependence thatcancels out the positive dependence of the silica26

DEMUX
(AWG)

2×2 switches

MUX
(AWG)

λ1

λn

Fig. 7. Configuration of optical add/drop multiplexer.

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Selected Papers

Main output port

Drop ports

Waveguides
AWG
for DEMUX

Coupler
HeaterGrooves to cancel out
thermal effects
AWG
for MUX

Main input port

Add ports

Double-gate typeoptical 2×2 switch

Fig. 8. Circuit layout of OADM.

based waveguides.
Figure 9 shows the transmission spectra from themain input port to the main output port for the 1080°C temperature range, when the 2nd, 5th, and 10th
channels were dropped. We obtained insertion losses
below 12 dB and crosstalk below 36 dB. The spectrain Fig. 9 almost all coincide with each other; that is,the characteristics of all the channels are almost completely insensitive to temperature change. Thus, thisOADM works stably when there are chip temperature
variations caused by changes in the add/drop channel
number.

4. Low power consumption technology
The increase in traffic volume means that largescale and more compact switches are required. We
have made larger-scale PLC switches by using 1.5%∆ waveguides. Finding a way to reduce the switching
power of the MZI unit is very important for largerscale PLC switches. We have already achieved
switching power as low as 90 mW using a Si trench
structure and heat-insulating grooves with 0.75%-∆
waveguides [19]. Moreover, we have already successfully reduced the switching power to 45 mW by

010°C

Main-main

Transmittance (dB)

–10

20°C
30°C

–20

40°C
–30
50°C
–40

60°C
70°C

–50
80°C
–60
1545

1550

1555

1560

Wavelength (nm)Fig. 9. Transmission spectra of OADM.

Vol. 1 No. 7 Oct. 2003

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Selected Papers

Switching power (mW)using heat-insulating grooves
A
Thin-film heaterwith 1.5%-∆ waveguides andWaveguide
optimized structural parameters [20].
Figure 10 shows the strucSignal light
Bar-state (φ=π/2)ture of an MZI switch unit
with low power consumption.
It has heat-insulating grooves
Cross-state (φ=0)
arranged symmetrically withrespect to the TO heater. The
switching power depends on
the thickness of the undercladding used to suppress heatradiation into the Si substrate
MZI
and the width of the cladding
Heat insulating grooveA’
ridge used to suppress lateral(a)heat diffusion. With a thicker
Over-cladding
under-cladding, the response
Thin-film heater
time is slower because the heat
Cladding ridge widthCoreflow into the Si substrate
becomes slower. Therefore, aAA’narrow cladding ridge is much
more effective than a thickunder-cladding for reducing
the switching power while
maintaining a reasonable
response time. Figure 11
Heat insulating groove
Under-cladding thickness
shows the dependence of the
Si substrateexperimental switching pow(b)ers on the cladding ridge widthFig. 10. Structure of low power consumption switch: (a) top view and (b) crossat under-cladding thicknesses
sectional view.
of 20 and 50 µm. The solid
plots are obtained experimentally, open plots show data for the conventional
400
switches, and the solid lines are theoretical curves.
UC 50 µm
The switching power decreases as the cladding ridge
UC 20 µm
becomes narrower and the under-cladding becomes300
thicker. We achieved very low switching power of 45
mW with an under-cladding thickness of 50 µm and200
a cladding ridge width of 20 µm. This power is about
one-tenth that of a conventional switch and half thatof a conventional low-power-consumption switch
100[19]. The switching response time was about 3 ms,
which is the same as that of the conventional switch.
With this low-power MZI, the total power consump0
tion of a 16×16 matrix switch should be only 1.4 W.
20
30
40
50
60
70
80 No groove
Cladding ridge width (µm)

Fig. 11. Dependence of switching power on cladding
ridge width.

28

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Selected Papers

Table 4. Reliability test results: changes in mean performance value.
Test items

Loss
(dB)

Extinction
ratio (dB)

Heaterresistance (%)

Long periods in ON-state (65°C, 5003 h)

–0.2

–1.5

0.0

Driving
power (%)
0.7

Damp heat (75°C, 90%, 5007 h)

0.2

0.9

0.0

–0.2

High temperature (85°C, dry, 5003 h)

–0.1

0.0

0.0

–0.2

0.3

–0.2

0.0

Heat cycle (–40 to 75°C, 500 cycles)

0.0

–0.4

0.0

0.1

Vibration (20 to 2 kHz, 4 min, 4 cycles)

–0.2

–0.2

0.0

–0.5

Impact ( 500 G, 6 directions, 5 cycles)

0.0

–0.3

0.0

–0.3

Low temperature (–40°C, 2014 h)

5. Reliability
The reliability of PLC devices including fiber connections has been confirmed with splitters and
AWGs. Since PLC switches use additional components such as thin-film heaters and power feed electrodes, their reliability must be verified. We carriedout a long-term reliability test for switch modules
where we also took into account the active conditions[21]. Our test samples were eight arrayed 2×2 switch
modules for OADM systems. We performed two categories of test, namely operational and storage tests.
Two of the eight arrays of switches were kept in the
ON-state in the operational test since the ratio of the
add/drop state in a conventional OADM system was
20-30% of all channels. We set the ambient temperature at 65°C to take account of the temperature
increase in the cabinet. We carried out six types ofstorage test: heat-damp, high-temperature, low-temperature, heat-cycle, vibration, and impact tests, inaccordance with Telcordia standards. The test results
are shown in Table 4. There were no problems such as
optimum point drift, heater resistance change, or
deterioration in loss and extinction ratio over the
5000-hour duration of the operational test. We also
encountered no problems in the ambient temperatureand mechanical tests. These results show that silicabased PLC switch modules are highly reliable.
6. Conclusion
This paper reported recent progress in silica-based
PLC switches. First, we outlined the PLC, N×N
matrix switches (up to 16×16), 1×N switches (up to
1×128) and a 128×128 switching subsystem, which
combines 1×128 switches. Then, we introduced amonolithic OADM, in which 2×2 switches and
AWGs are integrated, and low power consumption
technology. Finally, we reported on the reliability of
PLC switch modules.
Vol. 1 No. 7 Oct. 2003

0.2

Optical switches are key devices for constructing
flexible and low-cost photonic networks, and many
types of switches have been developed by many companies. Of these switches, one of the most useful isthe silica-based PLC switch, which is highly stable
and reliable. PLC switches with many configurations
and scales have been made and some integrated
switches are already being marketed by an NTT
group company. Further advances in PLC switch
technology will contribute greatly to the constructionof high-performance photonic networks.
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30

Masayuki Okuno
Senior research engineer, supervisor, Hyperphotonic Component Laboratory, NTT Photonics Laboratories.
He received the B.S., M.S., and Ph.D. degrees
in electronic engineering from Hokkaido University, Hokkaido in 1982, 1984, and 2000, respectively. In 1984, he joined NTT Electrical Communications Laboratories, Tokyo, Japan, where
he has been engaged in research on optical components and guided-wave optical devices. He is
now with NTT Photonics Laboratories, Kanagawa, Japan. He is a member of the IEEE and theInstitute of Electronics, Information and Communication Engineers (IEICE) of Japan.

Takashi Goh
Research Engineer, Photonic Transport Network Laboratory, NTT Network Innovation Laboratories.
He received the B.S. and M.S. degrees in electronic and communication engineering fromWaseda University, Tokyo in 1991 and 1993,
respectively. In 1993, he joined the NTT Optoelectronics (now Photonics) Laboratories, Ibaraki, Japan, where he has been engaged in researchon silica-based planar lightwave circuits. In
2002, he moved to NTT Network Innovation
Laboratories, Kanagawa, Japan and is now
engaged in the research and development of optical communication systems. He received theYoung Engineer Award from IEICE in 2000. He
is a member of the IEICE and the Japan Societyof Applied Physics (JSAP).

Shunichi Sohma
Researcher, Hyper-photonic Component Laboratory, NTT Photonics Laboratories.
He received the B.S. and M.S. degrees in electronic engineering from Tohoku University,
Sendai, Miyagi in 1997 and 1999, respectively.
In 1999, he joined NTT Photonics Laboratories,
Ibaraki, Japan. He is a member of IEICE. In
2002, he received the Young Researcher Award
from the International Conference on Solid State
Devices and Materials.

Tomohiro Shibata
Senior Research Engineer, Hyper-photonic
Component Laboratory, NTT Photonics Laboratories.
He received the B.S. degree in physics, theM.S. degree in applied physics, and the Ph.D.
degree in crystalline materials science from
Nagoya University, Nagoya in 1983, 1985, and1995, respectively. In 1985, he joined NTT Laboratories, where he engaged in research on theepitaxial growth of thin film semiconductors utilizing ECR (electron cyclotron resonance) plasma technology. He is now studying the fabrication of silica-based planar lightwave circuits. He
is a member of JSAP and the Surface Science
Society of Japan.

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