Selected Papers

High-speed Switching Operation in a Thermocapillarity
Optical Switch for Application to Photonic Networks
Fusao Shimokawa†, Tomomi Sakata, Mitsuhiro Makihara,and Yoshitada Katagiri
Abstract
We have improved our previously developed waveguide optical switch called Olive (oil-latching interfacial-tension variation effect switch) by developing basic technologies for lowering the viscosity of therefractive-index-matching liquid and shortening the distance the liquid must travel. In combination with
our previously developed highly accurate deep-groove fabrication with a high aspect ratio, batch injection of a very small amount of liquid, and high-performance sealing, these improvements enabled us toachieve a switching time of less than 10 ms while maintaining excellent optical characteristics. This
switching time, which is about one-tenth that of our previous Olive, breaks through an important threshold, allowing Olives to be used for optical crossconnects and add/drop multiplexers.

1. Introduction
Optical switches are key components for thesophisticated optical processing systems required for
optical add/drop multiplexers (OADM), opticalcrossconnects (OXC), and standby optical system ofphotonic networks [1]-[3]. They should be self-latching (i.e., they must retain each state without power
being supplied) and must be have excellent opticalcharacteristics (low insertion loss, high extinction
ratio, low crosstalk, polarization, and wavelength
insensitivity) in addition to fast switching and highreliability. They must also be compact to allow highdensity integration and be inexpensive [4], [5].
We previously developed a silica-based waveguide
optical switch called the oil-latching interfacial-tension variation effect switch (Olive) [6]. It is selflatching and exhibited low insertion loss (below 3 dB
in a 1×8 optical switch), high extinction ratio (>50dB), low crosstalk (<–50 dB), and wavelength
insensitivity (±0.5 µm). The switch chip size is about
10×10 mm [7]. However, its switching time of
50–100 ms is rather slow for OADMs and OXCs,† NTT Microsystem Integration Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: simokawa@aecl.ntt.co.jp

Vol. 1 No. 7 Oct. 2003

though it would be suitable as the optical switch forstandby optical systems and reconfiguration systems[8]. OADMs and OXCs require a switching time on
the order of 1 ms [4]. Achieving such fast switching
would greatly expand the applicability of Olive.
This paper introduces basic technologies that successfully enabled us to reduce the switching time to
1/10 and describes the optical characteristics of a fabricated switch.
2. Structure and operation principle of Olive
Figure 1(a) shows the basic structure of our switch.
It is a waveguide optical switch with a microactuatorfabricated in a planar lightwave circuit (PLC) by
micromachining. It consists of an upper glass substrate and an intersecting waveguide substrate thathas a groove at each crossing point and a pair of
microheaters. The groove is partially filled withrefractive-index-matching liquid and is completely
sealed by the upper glass substrate.
When the liquid is present at the crossing point ofwaveguides, the optical signals pass straight throughthe groove. On the other hand, when the liquid moves
away from the crossing point leaving it filled with air,the optical signals are switched into the crossingwaveguide by total internal reflection [9] on the31

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Microheaters

GrooveTransmission

Upper glass

Waveguide
substrate

Refractiveindex-matchingliquidReflection

3. Basic technologies for improving the switchingtime
3.1 Considerations
The switching time is expressed in terms of thegroove structure (groove width, groove depth, distance traveled by the liquid) and liquid characteristics(absolute viscosity of the liquid and surface tension
difference at the liquid column during heating) (Fig.
2). When the liquid flow is approximated as thesteady-state flow of viscous fluid, the switching time
t is described by [10]

Input
Core

(a) Basic structure
LiquidTransmission

the liquid in the groove to move toward the opposite
side of the groove. The liquid is held there by the capillary pressure, which is determined by the groove
width. These two phenomena in combination provide
the switching operation and self-latching function.

Microheaters
Waveguide
core
Input
Groovet ∝ µ·1/d·w·∆γ ,

(1)

ONON

Liquid movement

where w is the groove width, d is the groove depth,
l is the travel distance of the liquid, µ is the absoluteviscosity of the liquid, and γ is the surface tension difference at the liquid column in heating. To improve
the switching speed, we decided to lower the viscosity of the liquid and shorten the liquid’s travel distance. There is no trade-off between these approaches, so by doing both, we should shorten the switchingtime substantially.

Liquid movement

Input
Reflection(b) Basic concept of optical switching
Fig. 1. Structure and switching operation of an Olive.

groove wall, i.e., at the silica-air interface.
Figure 1(b) shows the basic switching concept. Theinterfacial tension of the air-liquid interface isdecreased by heating (thermocapillarity). This allows

3.2 Lowering the viscosity of refractive-indexmatching liquid [10]The refractive-index-matching liquid should havelow viscosity, low volatility, good refractive-index
controllability, and high thermal and chemical stability. To satisfy these requirements, we developed a
new liquid composed of a single substance based on

Surface tension difference atliquid column ∆γ
Microheaters

Groove width w

GrooveGroove depth hLiquid(viscosity µ)
Travel distance of liquid I

Fig. 2. Parameters related to improvement of switching time.

32

NTT Technical Review

CH3H3C

SiCH3CH3OSiCH3PhSiOmPhCH3SiOpCH3

CH350

Absolute viscosity [cP]Fig. 3. Chemical structure ofdimethyldiphenylpolysiloxane.

40

Number of molecules [a.u.]

Selected Papers

1.0

Previous liquid
0.50 210103

104105dimethyldiphenylpolysiloxane (Fig. 3).
Molecular weight
It is well known that the viscosity of a30
liquid can be represented as a functionof its weight-average molecular weight20as shown in Fig. 4. In this figure, the redcircle indicates the previous liquid,
10which had a viscosity of 50 centipoise
New liquid(cP) and weight-average molecularweight of about 3000. To obtain a liquid
00
1000
2000
3000
with viscosity of 10 cP, we should
Weight-average molecular weight
abstract the weight-average molecularFig. 4. Relationship between absolute viscosity and weight-average
weight of about 1000 in
molecular weight.
dimethyldiphenylpolysiloxane. By
using the gel chromatography method
instead of the distilling method, we were able toGroove length L
achieve the target viscosity of 10 cP, which is oneWaveguide core
fifth that of the previous liquid, without any refractive
Input
index mismatch or thermal stability degradation.
A refractive index of 1.449 at 25°C can be obtained(a) Previous structure
by adjusting both the dimethyl and diphenyl contents
in dimethyldiphenylpolysiloxane. We also found that
Liquid column x
Travel distance I
the new liquid has suitable thermal stability (heatdecomposition temperature is about 230°C) and low
Liquid margin length dvolatility (comparable with the previous liquid).
3.3 Shortening the travel distance of the liquid
Figure 5 shows the parameters related to shortening
the travel distance of the liquid. L is the groove
length, l is the travel distance of the liquid, x is thelength of the liquid column, and d is the margin
length of the liquid column. The ideal case, i.e., having no margin, is L=l+x.
We designed the fundamental structure so as toshorten the travel distance of the liquid without
affecting the liquid column length at which effective
TIR is generated and to maintain the liquid margin
length needed for latching.
It should be mentioned here that we achieved highliquid injection accuracy through high uniformity
processing of the sizes of both the groove and liquid
injection slit. The tolerance for the amount of injected liquid decreased from ±10% to ±5%.
As a result of the newly designed fundamental
structure and accurate liquid injection, we were able
Vol. 1 No. 7 Oct. 2003(b) New structure

Fig. 5. Parameters related to shortening the liquid’s
travel distance.

to reduce the travel distance from 120 to 60 µm in aprototype Olive.
4. Fundamental characteristics of the improved
Olive
4.1 Switching speed and optical characteristics
We fabricated a prototype 16-channel 2×2 Olive(Fig. 6) using the above-mentioned technologies andexamined its optical characteristics. Figure 7 shows
the transient optical waveform of the old and newOlives. In the old one, the viscosity of the liquid was
50 cP and the travel distance about 120 µm. In the33

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new one, those values are 10 cP and about 60 µm.
Both switches operate at a driving power of 0.15 W
and heating duration of 4 ms. The switching time ofthe new Olive is only 6 ms, which is about 1/10 thatof our previous Olive. This switching time is comparable to that of MEMS (micro–electro–mechanicalsystems) switches and other waveguide opticalswitches. Table 1 summarizes the characteristics ofFiber ribbon

Ceramic package
16-ch 2 × 2 Olive chip

2 cm

Fig. 6. Appearance of the fabricated 16-channel 2 × 2
Olive.

New Olive(viscosity: 10 cP)

Previous Olive(viscosity: 50 cP)

001020304050

Switching time [ms]Fig. 7. Transient optical waveforms of previous and newOlives.
Table 1. Characteristics of previous and new Olives.
PreviousNewTransmission loss (dB)

0.12

Reflection loss (dB)

1.2

Crosstalk (dB)

< –50 (0 to 40°C)

Driving power (W)Switching time (ms)34

4.2 Reliability
The reliability test for the new Olive was performedin accordance with the Telcordia standard (GR-1221core etc.). We tested heat shock, high temperature andhigh humidity, storage at high temperature, storage at
low temperature, heat cycles at high humidity, shock,vibration, and continuous switching.
In the heat shock test (–15°C to 85°C, 20 cycles),we did not observe any significant change in liquidcolumn length before and after the test. This confirmed the perfect liquid seal in the Olive. In the hightemperature and high humidity test (85°C, 85% RH),
we did not observe any significant degradation of the
optical characteristics, such as the insertion loss or
crosstalk even after 2000 hours. The insertion loss
fluctuation was less than ±0.5 dB. From these results,
it is clear that the epoxy resin for the liquid sealing
and connection between optical fibers and an Olive
did not degrade. The continuous switching test indicated that more than 20 million switching operations
can be achieved without any significant degradationof the optical characteristics. So far, we have not discovered any critical problems in the reliability tests,
indicating that the new Olive has sufficient reliability
for practical use.
5. Applications to optical transmission systems

Optical power [%]100

the old and new Olives. The optical characteristics ofthe new Olive are the same as those of the previous
one except for the switching speed.

0.15
50

65.1 Fundamental performance [11]An Olive must offer not only high-speed (<20 ms)
operation but also high transmission performance forpractical implementation of OADMs or OXC modules. Since the improved switching time (<10 ms) satisfies the former requirements, we discuss transmission performance in this section.
We focused on the coherent crosstalk noise generated at every intersection of the waveguides due tothe superposition of transmitted signals, which may
cause more significant signal-to-noise ratio (SNR)
degradation than power fluctuations do. To investigate this possibility, we performed transmission
experiments using two independent optical signals
modulated at 10 Gbit/s in NRZ (non-return to zero)format.
Figure 8 shows the measured transmission performance when one signal collides with another signal atan intersection. The resulting bit error rate versus signal power indicates error-free performance. The temNTT Technical Review

Selected Papers

poral waveform shows clear eye opening. Considering that the intrinsic channel crosstalk due to undesirable diffraction at intersections is around 45 dB, theOlive can be used in practical optical transmissionsystems.
5.2 Implementation in systems [11]Scalability is a key function for next-generation
data communications and telecommunications systems based on photonic networks. Olives can contribute to the construction of such networks due totheir self-latching, thereby providing low-power
operations under remote control via commandinstructions. We fabricated an OADM using a 1×n
Olive with wavelength-tunable optical modules todemonstrate the merits of the Olive. Figure 9 shows aschematic diagram of the OADM, which expands themultiplexed optical signals to be manipulated in theFourier space between the two arrayed waveguidegratings (AWG) modules and performs add/drop
operation for arbitrary channels. By using the Olive,
we have successfully fabricated a compact OADM
frame (Fig. 10). We plan to improve such modules tohandle a larger number of wavelength division multiplexing (WDM) channels as required in future netSwitch module for the OADM

Bit error rate

10–5
10–6

10 Gbit/s

10–7
10–8
10–9
10–10

–15

–10–5
Optical power [dBm]

0Fig. 8. Transmission performance.

Fig. 10. Appearance of the switch module in the OADM
sub-system.

Add module
Wavelength-tunable transmitter

Switch modules for OADM
(1×8 Olive)

EDFA

Optical signal

DEMUX

M
UX

Optical signal((AWG)AWG)

EDFA
Receiver
Drop module

Fig. 9. Schematic diagram of the OADM.

Vol. 1 No. 7 Oct. 2003

35

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works.
6. Conclusions
By developing basic technologies, we have made ahigh-performance oil-latching interfacial-tension
variation effect switch (Olive) with millisecond-orderswitching speed. This is fast enough to be used as theoptical switch in OADM and OXC systems, which
will greatly increase the applicability of the Olive.

Fusao Shimokawa
Senior Research Engineer, Supervisor, NTT
Microsystem Integration Laboratories.
He received the B.E., M.E., and Dr. Eng.
degrees in electronic engineering from Nagaoka
University of Technology, Niigata in 1981, 1983,and 1995, respectively. In 1983, he joined NTT
Musashino Electrical Communication Laboratories, Tokyo, Japan. He is currently engaged inresearch and development of MEMS devices. He
received the Technical Award from the Japan
Society for Precision Engineering in 1992 and1999. He is a member of the Japan Society ofApplied Physics (JSAP), the Japan Society forPrecision Engineering, and the Institute of Electrical Engineers, and Institute of Electronics,
Information and Communication Engineers(IEICE) in Japan.

References[1]S. Nagaoka, “Design of Latching Type Single-Mode Optical Fiber
Switch, “IEICE, C-II Vol. J79-C-II, No. 11, pp. 649-656, 1996.
[2] J. E. Fouquet, “Compact Optical Cross-Connect Switch based on
Total Internal Reflection in a Fluid-Containing Planar Lightwave Circuit,” OFC 2000, Baltimore, Maryland, U.S.A., TuM 1, pp. 204-206,
March 2000.
[3] L. M. Lunardi, L. Y. Lin, and E. L. Golsdtein, “Micromachined Optical Cross Connects with Integrated Signal-Monitoring Functionality,”
OFC’99, San Diego, California, U.S.A., FH 3-1, pp. 128-130, Feb.
1999.
[4] N. A. Jackman, S. H. Patel, B. P. Mikkelsen, and S. K. Korotky “Optical Cross Connects for Optical Networking,” Bell Labs TechnicalJournal, Jan.-Mar., pp. 262-281, 1999.
[5] K. Hane, “Micromachine for optical communications,” T. IEE Japan,Vol. 120, No. 11, pp. 683-686, 2000.
[6] 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.
[7] M. Sato, F. Shimokawa, S. Inagaki, and Y. Nishida, “Micromechanical Intersecting waveguide Optical Switch based on Thermo-capillarity,” NTT R&D, Vol. 48. No. 1, pp. 9-14, 1999 (in Japanese).
[8] M. Sato, F. Shimokawa, S. Inagaki, and Y. Nishida, “Waveguide optical switch for 8:1 standby system of optical line terminals,” Technical
Digest of OFC’98, San Jose, California, U.S.A., pp. 194-195, Feb.
1998.
[9] H. Ishikawa and K. Ukimatsu, “Photonic Switching Technology,”
The Telecommunications Association, p. 58, 1993.
[10] T. Sakata, H. Togo, M. Makihara, F. Shimokawa, and K. Kaneko,“Improvement of switching time in a thermocapillarity opticalswitch,” J. of Lightwave Technol., Vol. 19, pp. 1023-1027, 2001.
[11] Y. Katagiri and E. Hashimoto, “Automatic Control of Photonic Networks based on Absolute Wavelength Management,” Technical
Digest of SPIE’s International Conference on Optomechatronic systems III, Stuttgart, Germany, Session II, Nov. 2002.

36

Tomomi Sakata
Senior Research Engineer, NTT Microsystem
Integration Laboratories.
He received the B.S. and M.S. degrees inchemistry from Tohoku University, Sendai,
Miyagi in 1989 and 1991, respectively. In 1991,
he joined NTT Musashino Electrical Communication Laboratories, Tokyo, Japan. He is currently engaged in research and development ofMEMS devices. He is a member of JSAP.

Mitsuhiro Makihara
Senior Research Engineer, NTT Microsystem
Integration Laboratories.
He received the B.E. and M.E. degrees inmechanical engineering from Kyushu Institute ofTechnology, Fukuoka in 1987, and the University of Tokyo, Tokyo in 1989. In 1989, he joined
NTT Musashino Electrical Communication Laboratories, Tokyo, Japan. He is currently engaged
in research and development of optical switchingequipment for optical communications. He is amember of the Japan Society of MechanicalEngineers and IEICE.

Yoshitada Katagiri
Senior Research Engineer, Supervisor, NTT
Microsystem Integration Laboratories.
He received the B.S. and M.S. degrees in energy science from Tokyo Institute of Technology,
Ookayama, Tokyo in 1983 and 1985, respectively, and the Dr.Eng. degree in microoptomechatronics from the University of Tokyo,
Hongo, Tokyo in 1999. He joined NTT ElectricalCommunications Laboratories, Musashino,Tokyo in 1985. He has worked on optical devices
for information systems and telecommunications
systems. He received the Award of Optical
Memory Symposium in 1990. He published
Optomechatronics (Tokyo: Kyoritsu, 2000),with Professor Kiyoshi Itao and Professor
Hiroshi Hosaka of the University of Tokyo. He is
also a co-author of Optomechatronic Systems(CRC, 2002). He has served on a topical committee on optomechatronics of the Optoelectronic Industry and Technology Development Association from 2000. He is a member of the OpticalSociety of America and the Laser & ElectroOptics Society of IEEE.

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