Selected Papers: Optical Switches for Photonic Networks

Recent Progress in Optical Switching
Device Technologies in NTT
Tohru Maruno†
Abstract
There have been several important developments in photonic networks based on wavelength divisionmultiplexing in response to the explosive growth of Internet and broadband network services. Since optical processing systems such as optical cross-connects and optical add-drop multiplexers are essential forphotonic networks, there is a strong need for optical switching devices, which will be key componentsof these systems. This paper gives an overview of NTT’s recent progress in low-loss and highly reliable
optical switches fabricated by technologies based on planar-lightwave circuits, the oil-latching interfacial-tension variation effect switch, and three-dimensional micro-electro-mechanical systems mirrors.

1. Introduction
The rapid and global spread of the Internet andbroadband network services is accelerating the
growth of optical communications networks. Photonic networks based on wavelength division multiplexing (WDM) systems is playing a key role in increasing the capacity and flexibility of these communications networks. The first WDM networks were pointto-point transmission systems and the signals were
processed electrically in the system nodes after optical-electrical (OE) conversion. Today, a very attractive research topic is next-generation photonic networks, where many point-to-point dense wavelength
division multiplexing (DWDM) transmission systems are connected and optical signals are processed
without OE/EO conversion at the nodes [1]. Such networks require optical cross-connect (OXC) and optical add/drop multiplexing (OADM) systems. Key
devices for these systems are large-scale integrated
space division optical switches.
This paper outlines recent trends of optical switching devices and gives an overview of NTT’s activities
concerning low-loss and highly reliable opticalswitches using planar-lightwave circuits (PLCs), theoil-latching interfacial-tension variation effect switch† NTT Photonics Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: maruno@aecl.ntt.co.jp

12(Olive), and three-dimensional micro-electromechanical systems (MEMS) mirrors. The individual
switches are described in more detail in the subsequent three papers.
2. Configuration of OXCs and OADMs using
optical switch devices
NTT has already developed a new extended signaling and traffic engineering method for a multilayer(photonic and electrical) router based on generalized
multiprotocol label switching (GMPLS), as reported
at SUPERCOMM 2001 [1]-[4]. It consists of an IProuter, a wavelength router, and a GMPLS-routermanager and offers both IP (Internet protocol) packet
switching and wavelength-path switching capabilities. In the router, the wavelength paths, known as
optical label switched paths (OLSPs), are set andreleased in a distributed manner based on the functions provided by GMPLS. Therefore the router lets
us create the optimum network configuration by considering the IP and photonic network resources in a
distributed manner. The OXC in the router, which
establishes both optical cross connection and longhaul WDM transmission functionalities, is constructed using the delivery-and-coupling switch (DC-SW)
architecture shown in Fig. 1. The 8×16 (8 input and16 output ports) cross-connect switch (Fig. 1(b)) consists of 128 1×2 PLC-switches and 16 8×1 optical
couplers. The DC-SW architecture enables us to
NTT Technical Review

Opticalamplifier

AWG

Optical coupler Opticalamplifier

1
2

8×16
DC-SW

8

1

1×2 switch

λ1
λ2
λ8

8×1

1
(2)
1

2

16

(b) DC-SW architecture
PLC-SW

(3)
2

2
(4)

(15)
16

16
(16)

(a) 8×16 fiber port system architecture(c) 8×16 DC-SW sub-board

Fig. 1. Delivery-and-coupling optical switch.

reduce the number of switch elements in the router byusing couplers as multiplexers and to fabricate an
inexpensive practical router.
If we can manufacture a low-cost N×N type matrix
optical switch with low loss and high reliability, we
can easily achieve a wavelength-path cross-connect
by allocating a different wavelength to each port ofthe switch. The operation of the unit switches composing a matrix optical switch is outlined in Table 1.
A digital optical switch has a matrix arrangement
consisting of 1×2 or 2×2 unit switches. Each switch
has two states and acts like an on-off shutter. An input
optical signal can be switched to any desired output
port by operating a unit switch at a specific intersec-

Table 1. Basic structure of large-scale matrix optical switch.
Type

Digital optical switch

Analog optical switch

Architecture

Matrix arrangement of 2×2 unit switches

Free-space optical transmission

Number ofswitches

Number of unit switches increases
exponentially as the number of input
and output ports increases.

Number of unit switches increases inproportion to the number of input andoutput ports.

IssuesPath dependence of insertion loss

Stable operation and reliability

ExamplesPLC-switch, bubble reflection switch,
electronic switch

3D-MEMS mirror

Characteristics

Vol. 1 No. 7 Oct. 2003

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Selected Papers

Table 2. Configuration of large-scale matrix switches using small-scale digital optical switches.
Type

Architecture

Crossbar

2N 1×N

3-stage Clos
1
2

1
2
3
4

1
2

1

1×N

N×1

1

2

1×N

N×1

2

N

1×N

N×1

N

N
1 2 3 4
Number ofswitchesN

N2

tion. Highly reliable digital switches with a small
matrix-scale such as 1×16 and 16×16 PLC switches
are commercially available [5], [6]. However, they
are disadvantageous in terms of both optical loss andthe cost of large-scale switch fabrication, because therequired number of unit switches increases geometrically. If we consider N input fibers and N output
fibers, the total number of mirrors required is equal to
N2. An analog optical switch can be created using afree-space transmission structure, such as a threedimensional (3D) MEMS mirror optical switch (seesection 3.6). A 3D MEMS array operates by steering
beams of light in an analog fashion in 3-dimensional
space. For N fibers in and N fibers out the total number of mirrors needed is 2N and the distance that thelight travels does not increase as quickly with the port
count as it does for two-dimensional (2D) configurations. Therefore, a 3D-MEMS mirror optical switch
is much more suitable for fabricating large-scale
devices. The characteristics of a large-scale prototype
optical switch (about 1000×1000) have already beenreported [7]. Further work is required on the design ofsystems for mirror control and free-space opticaltransmission to ensure that analog optical switches
operate stably.
A method for designing large-scale matrix switches using small-scale digital switches is shown in
Table 2. When an N×N crossbar matrix optical switch
is fabricated with a digital optical switch, the value of
N in one switch chip is often limited to a few tens.
Multi-chip arrangements of the three-stage (Clos*1)
switch-fabric architecture and/or the 2N 1×N typeswitch enable us to overcome this limit and fabricate
large-scale (100×100 or more) optical switches easily. The number of unit switches for a 3-stage Clos isless than that for the crossbar type above 32×32,*1 Named after C. Clos of AT&T Bell Labs.

14

N

N
approx. 6N1.5–3N

N (N–1)

2×2 optical switch

AWG

AWG

λ1

λn

AWG: Arrayed waveguide grating

Fig. 2. Basic structure of OADM system.

although the crossbar type has fewer unit switches
when the scale is smaller.
Figure 2 shows the basic arrangement of an OADM
system, which consists of an optical multiplexer/
demultiplexer (MUX/DEMUX) and 2×2 switches. In
an OADM system, the number of 2×2 switches
should match the number of WDM transmission
wavelengths.
3. Optical switch devices
There have already been reports of the design andfabrication of several kinds of optical switch device[8]. Table 3 summarizes the characteristics of thesedevices. They can be roughly categorized into two
types depending on whether they use optical waveguides or free-space transmission. The optical waveguide type can be subdivided according to the structure of the switch unit: Mach-Zehnder interferometer(MZI), Y-branch waveguide, bubble reflection, moving optical fiber, and semiconductor optical amplifier(SOA) types. The free-space transmission type uses a
2D or 3D MEMS mirror.
NTT is proceeding with the research and development of the MZI, Y-branch waveguide, and bubble
NTT Technical Review

Selected Papers

Table 3. Characteristics of optical switch devices.
Type

Switching principle

Free-spacetransmission

Characteristics

Silica waveguide,
Heater as phase shifter

Reliable,
Simple driving circuit

Electric current
injection

Semiconductor opticalwaveguide

Short switching time of less than 1 µs

Polymer optical waveguide,Thin film heater

Small wavelength dependence,
Simple driving circuit

Bubblegeneration

Bubble generating liquid,
Silica optical waveguide

Short switching time of less than 1 ms,
Small wavelength dependence

Bubblemovement

Oil,
Silica optical waveguideSelf-latching, Low wavelength dependence,
Small power consumption

Moving optical fiber

Silica optical fiber,
MagnetSmall wavelength dependence,
Small polarization dependence, Self-latching

Optical adsorption of SOA

SOA

Short switching time of less than 1 ns,
Works as an amplifier

2D-MEMSSi process

Small wavelength dependence,
Small polarization dependence,

3D-MEMSSi process

Good for mass production,
Small wavelength dependence,
Small polarization dependence

Y-branch waveguide and TO effect
Opticalwaveguide

Base technology

TO effect

Mach-ZehnderInterferometer

Bubble reflection

Micro-mirror reflection

NTT’s activities

reflection type optical waveguide switch technologies. A maximum switch scale of about 32×32 ports
can be fabricated in a single chip. Figure 3 shows thebasic structure of each switch unit. NTT is also examining the 3D-MEMS mirror optical switch as a technology that enables the fabrication of large-scale
optical switch chips with over 100×100 ports.
3.1 MZI type optical switch
Figure 3(a) shows the basic structure of an MZItype 2×2 optical switch element. Two 3-dB directional couplers are connected by a pair of arms. Each arm
includes a phase shifter that can introduce a difference in optical path length ∆L between the two arms
by changing the refractive index of the waveguide.
There are two main choices for implementing thephase shifters: a thin film heater that utilizes the thermo-optic (TO) effect or an electrode that injects electric current. At NTT, we have been developing TO
switch devices using silica-based PLC technology
since the 1980s [9]. Although silica-based PLC isessentially a passive device, it can be used to make an
optical switch by combining the TO effect and aninterferometer configuration. In a PLC switch, therefractive index of one arm waveguide is increasedby the thin film heater. The optical path length difference ∆L between the two arms is designed to beeither zero (symmetric-MZI) or a half-wavelength(asymmetric-MZI). The MZI can be in the crossstate, in which an optical signal is guided from input
Vol. 1 No. 7 Oct. 2003

port 1 to output port 3 when ∆L is zero, or in the barstate, in which an optical signal is guided from input
port 1 to output port 4 when ∆L is a half-wavelength.
By varying the refractive index of one waveguide armof the MZI using the TO effect, we can change ∆L byhalf a wavelength, and thus change the switch state.
Typical temperature increases at the core and theheater are 15 and 40°C, respectively. The switching
power is 0.35 to 0.45 W for an MZI. The switchingtime is 1 to 3 ms, which is sufficient for OXC andOADM systems. Figure 4 shows the progress that has
been made in optical circuit integration and power
consumption in PLC switches. We have already fabricated a practical 16×16 optical switch module andprototype switch chips up to 1×128 and 32×32. Moreover, we successfully performed a fundamental teston the operation of a 128×128 switch subsystem, consisting of 2N 1×N switch modules and over 16,000
optical fibers [10]. The results show that the multichip arrangement is effective for constructing largescale N×N switching subsystems. The high power
consumption of the previous PLC switch, which wasits greatest disadvantage, has been significantly
reduced to about 1/10 of its former value of 45 mW
by using heat insulating grooves [11]. A total power
consumption of only 1.4 W is expected for a 16×16
matrix switch with this low power technology.
One of the most important advantages of PLC
switches is their high reliability. Using eight arrayed
2×2 switch modules for OADM systems, we carried
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Selected Papers

Waveguide core

OpticalsignalThin film heaters

2

3

1

4

∆L=0

∆L=λ/23-dB directional couplers(a) Mach-Zehnder interferometer typeY-branch optical waveguides

Thin film heaters
Substrate(b) Y-branch waveguide typeTransmission

Reflection
OpticalsignalWaveguide core(c) Bubble reflection typeFig. 3. Unit structure of optical waveguide type switch
devices developed in NTT.

out a long-term reliability test that included six typesof storage test: heat-damp, high-temperature, lowtemperature, heat-cycle, vibration, and impact tests,
in accordance with Telcordia*2 standards [12]. There
were no problems such as drift of the optimum operating point, change in heater resistance, or deterioration in loss or extinction ratio during the 5000-hour
test. We also encountered no problems when operating the heater at 65°C or in mechanical tests. These
results show that silica-based PLC switch modules
are highly reliable. The next paper, “Recent Advances*2 http://www.telcordia.com/

16

in Optical Switches using Silica-based PLC Technology,” describes the PLC switch in more detail.
The electron-injection type MZI optical switch isfabricated on a substrate made of a semiconductor
such as Si. Switching is achieved by injecting an electric current into the electrode to control the refractive
index of the waveguide arm. A high switching speedof less than 1 µs can be expected, although there arestability problems due to changes in the optical transmittance caused by the injected electric current.
Moreover, the high refractive index of core materials
such as Si makes it necessary to improve the opticalfiber connection method to reduce the connection andreturn losses.
3.2 Y-branch waveguide type optical switch
Polymers have large TO coefficients (more than ten
times that of SiO2). This enables us to develop a Ybranch waveguide type optical switch. Figure 3(b)shows the basic structure of a 1×2 switch. When one
arm is heated to reduce its refractive index, the optical output appears only in the other arm. This switch
is independent of wavelength in principle, which isone of its advantages over the MZI type.
NTT has developed 2×2 and 1×8 optical switches
using newly synthesized silicone resin and polyimide
resin as the optical waveguide materials [13]. In the
1×8 switch, we achieved low power consumption of360-450 mW, on/off ratio of 40 dB, insertion loss of
3 dB, and polarization-dependent loss of 0.3 dB. The
switching times obtained from the pulse response
characteristics for a 90%-rise and 90%-fall were 6.0
and 3.4 ms, respectively, and there was no deterioration in the switch characteristics after 107 switching
operations. Moreover, we confirmed that simultaneous switching of wavelengths in the 1.3 and 1.55 µm
bands was possible.
3.3 Bubble reflection type optical switch
This type of switch consists of intersecting opticalwaveguides that have partially fluid-filled grooves at
each waveguide intersection. Optical switching isachieved by controlling two optical states: the transmission state and the reflection state. An optical signal passes straight through the groove when it is filledwith fluid that has the same refractive index as the
waveguides (the transmission state). In contrast, thesignal is reflected into an intersecting waveguide at
the groove wall by total internal reflection when there
is a bubble in the groove (the reflection state). There
are two main methods based on using bubbles. In one,air bubbles are generated in liquid by using technoloNTT Technical Review

Selected Papers

Forecast
1×128
32×32
Heat insulating
groove0.5SH∆* waveguideCircuit integration (a.u.)

0.4
R&D

6

1×128
128
0.3

Practical use
16×16

16×16

4

1×16

Device design
1×8

1×8

0.2
1×16

2

Power consumption (W per unit switch)

8

0.1
8×8
1990

8×8
1992

1994

1996

1998

2000

2002

2004

*SH∆ (super high delta): over 1% refractive index difference between core and cladding

Fig. 4. Progress in optical circuit integration and power consumption in PLC switch devices.

gy originally developed for inkjet printers [14]. In theother, heat is used to move a bubble in a partially fluidfilled groove [15].
NTT has developed an optical switch called the oillatching interfacial-tension variation effect switch(Olive). Figure 3(c) shows its basic structure. Theinterfacial tension of the air-liquid interface isreduced by heating. This allows the liquid in thegroove to move toward the opposite side of thegroove. The liquid is held there by capillary pressure,which is determined by the width of the groove. Thismeans that the switch only consumes power tochange state: each state is maintained stably without
power, a useful property called self-latching. Olive
has exhibited a low insertion loss (below 3 dB in a
1×8 optical switch), a high extinction ratio (>50 dB),
low crosstalk (<–50 dB), and wavelength insensitivity (±0.5 µm). The switch chip size is about 10
mm×10 mm. Recently, the switching time was
reduced from about 50–100 ms to less than 10 ms byusing new basic technologies designed to lower theviscosity of the liquid and shorten its traveling distance. The third paper in this issue, “High-speed
Switching Operation in a Thermocapillarity Optical
Switch for Application to Photonic Networks,” provides further details.

Vol. 1 No. 7 Oct. 2003

3.4 Moving optical fiber type optical switch
This type of optical switch works by physicallymoving optical fibers, for example by using magnets[16]. These switches are self-latching but have a low
operating speed of about 10 ms, so they might be suitable for situations where switching is infrequent orneed not be fast, such as setting up optical transmission paths in a telephone exchange for new service
subscribers. However, this approach is not suitablefor high-density mounting because each switch must
be magnetically shielded.
3.5 SOA type optical switch
Optical switching can also be achieved by changing
between two states of a semiconductor optical amplifier (SOA): SOA-on with high optical transparency
and SOA-off with high optical absorbance. The SOA
response time is very fast: on the order of nanoseconds. Moreover, SOAs can compensate for loss byamplifying the signals [17]. However, the switch iscurrently impractical because the SOA still has manytechnical problems and the fabrication technology isimmature.
3.6 MEMS mirror reflection type optical switch
A 2D-MEMS mirror works as a digital opticalswitch. Some manufacturers have already marketed a17

Selected Papers

16×16 switch and reported its optical properties andreliability [18]. The matrix switch arrangement is acrossbar; the optical loss varies with the selected
input and output ports because of the difference in
optical path length. Eliminating the path length
dependence of the insertion loss is an important issue
in the fabrication of a large-scale switch system.
A 3D-MEMS mirror operates as an analog opticalswitch. Because the number of unit switches increases only linearly in proportion to the number of input
and output ports, the 3D-MEMS mirror should be
more suitable for use in fabricating a large-scale
switching system than a digital optical switch. Moreover, since the path length dependence of the insertion loss is lower in the 3D-MEMS than in the 2DMEMS, there is no need to apply techniques to cancel out the loss differences between the selected input
and output ports.
For a free-space transmission type switch such as
3D-MEMS, it is important to stabilize the mirror
against optical and mechanical stress to reduce any
loss change caused by vibration and temperaturechanges in the system. Stability is also improved byshortening the free-space transmission path length,
but this requires a high tilt angle, high processing
accuracy, and appropriate electrostatic drive voltages
for the mirrors. NTT’s MEMS-mirror arrays are sin-gle crystal silicon integrated with high-aspect ratio
torsion springs, and are tilted two-dimensionally byelectrostatic force. The prototype switching modulewith 100-channel optical fiber input/output has a low
coupling loss of 4.0 dB and a switching time of 3 ms.
The fourth paper in this issue, “Development of a
Large-scale 3D MEMS Optical Switch Module,” provides more details.
4. Future of optical switch devices
Figure 5 summarizes the integration scale at thechip level and the switching time of various opticalswitch devices. The integration scale is limited by thesubstrate size because chips are fabricated on, forexample, silicon substrates. Moreover, waveguideloss, electrode density, and heat dissipation areimportant parameters in chip design. In general, themaximum integration scale in a single chip is about
32×32 for optical waveguide type switches. The
switching time ranges from the millisecond order (forthe TO effect and bubble reflection) to the nanosecond order (for the SOA type). In contrast, a largescale matrix switch of over 100×100 with a millisecond-order switching time can be fabricated in one
chip using the 3D-MEMS free-space transmission
type switch.

3D-MEMS

Switch scale (N×N)

1,000

100
Opticalc waveguide type

Electric
current
injection

Thermooptic effectBubblereflection

0
Mo
oving
optical
p
fiber

SOA

1

10–9

10–6

10–3

1

Switching time (s)Fig. 5. Switch scale and switching time of various optical switch devices.

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NTT Technical Review

Selected Papers

The optical switch devices described in this paper
have both merits and demerits, so one should selectthe best switch according to the situation. After theannouncement of a large-scale free-space transmission optical switch using 3D-MEMS mirrors by
Lucent Technologies in 1999, the research goal has
been to develop a 1000×1000 switch. However, fromthe practical point of view, an important target is tomake a smaller OXC (e.g., with 32 wavelengths on 8
or 16 lines) that provides high reliability at a low cost.
In addition, high-speed switching devices will benecessary if we are to develop next-generation
switching technology such as optical burst switchingand optical packet switching.
5. Conclusion
This paper outlined recent trends related to opticalswitching devices and introduced the principles ofand experimental results for devices fabricated at
NTT. Some devices have excellent characteristics andwill enable us to design large-scale optical switchingsystems. Of course, further research and developmentthat considers not only device performance but alsoreliability and total cost, including both fabricationand maintenance, will be necessary when applying
these devices to optical cross-connects and optical
add/drop multiplexers. Moreover, it is important todevelop all-optical systems that eliminate the opticalelectrical-optical conversion required for electricalswitches.

[7]

D. Bishop, C. R. Giles, and G. Austin: “The Lucent LamdaRouter:
MEMS technology of the future here today,” IEEE Commun. Mag.,
Vol. 40, pp. 75-79, 2002.
Y. Yamabayashi, “Optical switching devices and systems,” IEICE
Technical Report, 102, PS2002-14, 2002 (in Japanese).
[9] M. Okuno, A. Sugita, T. Matsunaga, M. Kawachi, Y. Ohmori, and K.
Katoh: “8×8 optical matrix switch using silica-based planar lightwave
circuit,” IEICE Transactions on Electronics, Vol. E76-C, No. 7, pp.
1215-1223, 1993.
[10] T. Goh, H. Takahashi, T. Watanabe, M. Kobayashi, T. Shibata, M.
Ishii, S. Sohma, Y. Abe, M. Okuno, and Y. Hibino, “Scalable
128×128 optical switch system composed of planar lightwave circuitand fiber sheet for optical crossconnect,” ECOC 2002, Copenhagen,Denmark, Sep. 2002.
[11] S. Sohma, T. Goh, H. Okazaki, M. Okuno, and A. Sugita: “Lowswitching power silica-based super high delta thermo-optic switch
with heat insulating grooves,” Electron. Lett., Vol. 38, No. 3, pp. 127128, 2002.
[12] M. Okuno, “Highly integrated PLC-type optical switches for OADM
and OXC systems,” Tech. Dig. OFC’2003, Tu1, Atlanta, Georgia,
U.S.A., Mar. 2003.
[13] S. Toyoda, N. Ooba, T. Kitoh, T. Kurihara, and T. Maruno, “Thermooptic switch and wavelength tunable filter using polymer waveguides,” Proc. SPIE, Vol. 4532, pp. 62-72, 2001.
[14] J. E. Fouquet, S. Venkatesh, M. Troll, D. Chen, P. W. Barth, and H.
F. Wong, “Compact scalable fiber optic cross-connect switches,”
LEOS Summer Topical Meeting, Vol. 59, 1999.
[15] M. Makihara, M. Sato, F. Shimokawa, and Y. Nishida, “Micromechanical optical switches based on thermo-capillarity integrated inwaveguide substrate,” J. Lightwave Technol., Vol. 17, No. 1, pp. 1418, 1999.
[16] S. Nagaoka, “Compact latching-type single-mode-fiber switches fabricated by a fiber-micromachining technique and their practical applications,” IEEE Journal of Selected Topics in Quantum Electronics,
Vol. 5, No. 1, pp. 36-45, 1999.
[17] K. Kato, “10-Gb/s Photonic cell switching with hybrid 4×4 opticalmatrix switch module on silica based planar waveguide platform,”
OFC’98, PD3-1, pp. 437-439, 1998.
[18] P. De Dobbelaere, K. Falta, L. Fan, S. Gloeckner, and S. Patra, “Digital MEMS for optical switching,” IEEE Commun. Mag., Vol. 40, pp.
88-95, 2002.
[8]

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Vol. 1 No. 7 Oct. 2003

Tohru Maruno
Executive Manager, Hyper-photonic Component Laboratory, NTT Photonics Laboratories.
He received the B.S., M.S., and Ph.D. degrees
in engineering from Kyoto University, Kyoto in
1977, 1980, and 1997, respectively. In 1980, hejoined NTT Electrical Communications Laboratories, Tokyo, Japan, where he engaged inresearch on optical materials, organic thin films,
and guided-wave optical devices. He is now withNTT Photonics Laboratories, Kanagawa, Japan.
He is a member of the Institute of Electronics,
Information, and Communication Engineers ofJapan, the Japan Society of Applied Physics, andthe Society of Polymer Science.

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