Selected Papers

Development of a Large-scale 3D MEMS
Optical Switch Module
Tsuyoshi Yamamoto†, Johji Yamaguchi, Renshi Sawada,
and Yuji Uenishi
Abstract
A three-dimensional (3D) micro-electro-mechanical system (MEMS) optical switch, consisting of twoaxis tilt mirror arrays and free-space optics, is a practical solution for constructing large-scale switching
fabrics due to its high component density and low cost. The MEMS-mirror arrays developed by NTT aresingle-crystal silicon mirrors integrated with high-aspect ratio torsion springs and actuated by electrostatic forces. The free-space optics consist of low-cost high-precision polymer components assembled passively. A prototype module with 100-channel optical fiber input/output exhibited a low coupling loss of
4.0 dB and a switching time of 3 ms.

1. Introduction
The rapid growth of broadband network services isa driving force behind the development of the nextgeneration optical network based on dense wavelength division multiplexing. From the viewpoint of
resource management in the network, wavelengthpath-level optical cross-connects (OXCs) are mission
critical technologies due to their multiple restoration
capabilities. These enable features like high robustness against traffic fluctuations and an optimum network configuration in the most cost-effective manner; they are made possible by dynamically allocating network resources [1]. Recent progress in opticalswitches systems based on micro-electro-mechanicalsystem (MEMS) devices should lead to the construction of a large-scale, all-optical switch for practical
OXCs [2]-[3].
NTT Microsystem Integration Laboratories has
been developing large-scale three-dimensional (3D)
MEMS optical switches that are compact and inexpensive. The main components of the switch areMEMS two-axis tilt mirror arrays and free-spaceoptics. The MEMS mirrors are made of single-crystal† NTT Microsystem Integration Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: t-ymmt@aecl.ntt.co.jp

Vol. 1 No. 7 Oct. 2003

silicon with integrated gimbal structures and are tilted two dimensionally by electrostatic force. The freespace optics for internal connections of the switch
consist of low-cost precision-made polymer components and are passively assembled using conventional dowel pins. We also present the results of a prototype having 100 input and 100 output ports, showing
the optical characteristics for internal connection andthe switching operation.
2. Basic switch structure
3D MEMS optical switches are suitable for compact, large-scale switching fabrics because they utilize spatial parallelism, which enables high-density,
3D interconnections. The ability of this architecture
to achieve input- and output-port counts of over one
thousand is the primary driver of the large-scaleOXCs. In particular, these types of switches provide
high application flexibility in network design becauseof low and uniform insertion loss with low wavelength dependency under various operating conditions. Furthermore, this switch exhibits minimal
degradation of the optical signal-to-noise ratio, which
is mainly caused by crosstalk, polarization dependent
loss (PDL), and chromatic and polarization mode dispersions.
Figure 1 shows the basic configuration of the 3D37

Selected Papers

MEMS optical switch. The optical signals passing through the optical fibers
at the input port are switched independently by the gimbal–mounted MEMS
mirrors with two-axis tilt control andthen focused onto the optical fibers at
the output ports. In the switch, any
connection between input and output
fibers can be accomplished by controlling the tilt angle of each mirror. As aresult, the switch can handle several
channels of optical signals directly
without costly optical-electrical orelectrical-optical conversion.

Optical beams

Two-dimensional
optical fiber array

MEMS two-axis
tilt mirror array

n=10

m=10

2.1 MEMS two-axis tilt mirror
MEMS two-axis
Microlens arrays
arraytilt mirror array
The attributes of MEMS mirror
Fig. 1. Basic structure of 3D MEMS optical switch.
arrays depend on whether the mirror ismade of polysilicon or single-crystal
silicon. Polysilicon mirrors are fabriBase layer
Torsion springcated using mature-micromachining
technology, but it is difficult to achieve
Silicon oxide
Mirrorreliable connections in the switch due
substrate
Tilt mirror
to mirror deformation caused by inherent stress in the materials [4]. Singlecrystal silicon mirrors provide reliable
Pivot
connections due to their durable
Trenchmechanical structure [5].
ElectrodeFigure 2 shows a MEMS mirror consubstratesisting of two single-crystal silicon
substrates. The substrates are
Terraced electrodeprocessed independently by silicon
AuSn solder(driving electrode)bulk micromachining [6] and bonded
Fig. 2. Cross-sectional schematic of MEMS two-axis tilt mirror.
together. Each mirror is supported byfolded torsion springs in two orthogonal axes and tilted two-dimensionally by electrostatic
2.2 Low-cost, highly accurate optical components
force. The torsion spring has an aspect ratio of > 6,
for free-space opticsand this high-aspect-ratio spring features a strong
The two-dimensional optical fiber and microlens
bending capability relative to torsion and provides
arrays used to establish the internal connections ofstrong support for the mirror. The tilt angle of the mirthe switch determine the density, hardware volume,
ror can be changed by controlling the applied voltage
and cost of the switch. However, high-precision posibetween the two substrates. In this mirror, the driving
tioning is essential for assembling optical compoelectrode has a three-dimensional terrace structure,nents that use two-dimensional array devices consistand this effectively reduces the control voltage coming of hundreds of small elements. Several assembly
pared with a conventional MEMS mirror.
techniques have been reported [7]-[8], but such conThe appearance of a fabricated MEMS mirror isventional assembly processes may be costly because
shown in Fig. 3(a). The mirror has a diameter of 600
they require highly accurate, time-consuming fiberµm and is integrated with the gimbal structures thathandling.
provide freedom of tilt about two axes.
To overcome these problems, a two-dimensional
optical fiber array using single-mode optical fibers
with metal micro–ferrules and a polymer substrate
38

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Selected Papers

10 µm(b)500 µm

100 µm(a)(c)

Fig. 3. (a) MEMS mirror, (b) the high-aspect torsion spring, and (c) 3D terrace electrode with a pivot.

having precisely aligned holes was developed [9].
Figure 4 shows the configuration of the fiber array.
Optical fibers with the metal micro–ferrules and twodowel pins are inserted into holes in the polymer substrate. Each ferrule end is polished and given an antireflection coating in advance. The polymer substratewith guide holes, made of a thermosetting molding
compound, is molded using a transfer-molding
method based on a common technique for fabricating
ferrules in MT (mechanical transfer) -type opticalconnectors [10]. In this structure, the fiber arrays are
passively assembled in a series of simple steps. The
dowel pins are used for alignment between opticalfiber and microlens arrays.
The optical fiber arrays in the switch module havea 10 × 10 arrangement. The fibers in each array arealigned with a 1.3-mm spacing with an average displacement of less than ±2 µm, which demonstrates

that our two-dimensional optical fiber arrays provide
high-precision fiber positioning. The microlens arrayis based on polymer that is transparent to wavelengths
in the 1300–l600-nm range and is suitable for plasticinjection molding.
2.3 Optomechanics
The optomechanics in the switch generally requireprecise alignment with micrometer and sub-milliradian tolerance. The most critical positioning stability
may be required between the fiber and the microlens
arrays because a small misalignment between themwill greatly increase the coupling loss. Therefore, theplacement of each optical component must be kept
stable over its lifetime with allowable errors measured in micrometers.
To overcome these problems, we used a passive
assembly approach for constructing the optomechanics in the switch. Figure 5 shows a photograph of theMetal frame

MEMS-mirror packages

Optical fibers

4cm

Metal ferrules
Polymer substrate
Dowel pins
1.3 mmFig. 4. Structure of 2D optical fiber array.

Vol. 1 No. 7 Oct. 2003

2D optical fiber arrays

Fig. 5. Photograph of prototype optical switch module
having 100 input and 100 output ports.

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Selected Papers

fabricated switch module having 100 input and 100output ports. The optical components in the module
were mounted on metal frames without any positioning mechanism and firmly fixed to each other using
conventional dowel pins, instead of epoxy resin. Theswitch module has an overall hardware volume of
approximately 170 cc (80 × 60 × 35 mm).

DrivingsignalWaveform
shaping

D/A
converter

MEMS
mirror

2.4 Mirror-motion control
MEMS mirrors generally work as high-Q resonators with relatively ineffective damping mechanisms. This severely limits the settling time of theMEMS-mirror tilt motion, which is mainly determined by the resonant frequencies and the damping
factors of the mirrors. Therefore, another damping
scheme is needed for high-speed switching operation.
To solve this problem, a high-speed mirror motion
controller with an open-loop control was developed[11].
Figure 6 shows a block diagram of the mirrormotion controller. The controller uses open-loop control and has three functional blocks: a waveformshaping block, a digital-to-analog (D/A) converter
block, and a high-voltage amplifier (Amp) block. The
waveform-shaping block consists of a calculation
unit for polynomial expressions to process the driving-signal stream applied to the MEMS mirror. Eachcoefficient in the expressions is obtained by aninverse-function calculation using mirror dynamics.
This waveform-shaping operation eliminates vibration around the resonant frequency in mirror motion,
thereby reducing the settling time. Figures 7(a) and(b) show the step responses of the mirror motion
without and with the mirror-motion control. They
confirm that the settling time for mirror motion iseffectively reduced from 20 to 3 ms by using this mirror-motion control. We also developed a real-time
servo control system for the MEMS mirrors, which
maintains the stability of internal connections in theswitch.

Fig. 7. Step response of MEMS-mirror motion (a) withoutand (b) with mirror-motion control.

3. Performance of a prototype switch module

Table 1. Summary of optical characteristics in the module.

AmpFig. 6. Block diagram of mirror-motion controller.
175
Mirror-tilt motion [%]1501251007550250–25051015Time [ms](a)202530051015Time [ms](b)202530175
Mirror-tilt motion [%]150
1251007550250–25Insertion loss

First, we experimentally evaluated the optical characteristics of the switch module. The typical insertion
loss of a port was 4.0 dB, the return losses weregreater than 30 dB, the PDLs were less than 0.5 dB,
and the crosstalk into adjacent ports was less than –60dB, as shown in Table 1. These results experimentally confirm that our switch module has good opticalcharacteristics for practical applications.
We also evaluated the switching operation of the40

4.0 [dB]

Return loss

> 30 [dB]PDL< 0.5 [dB]

Crosstalk< –60 [dB]

module. Figure 8 shows an example of the 1 × 2
switching operation when driving voltages for theMEMS mirrors were under 50 V. The switching time
was about 3 ms with the newly developed high-speed
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Selected Papers

input/output has a low coupling loss of 4.0 dB andswitching time of 3 ms.
References

Port 1T[1]1Port 2

T23 ms(2 ms/div.)

Fig. 8. Example of 1 × 2 switching operation.

mirror-motion controller, which demonstrates thefeasibility of the compact low-cost 3D MEMS opticalswitch module.
4. Conclusion
We have developed a compact, low-cost switch
module based on MEMS two-axis tilt mirror arrays
and free-space optics using polymer components.
The MEMS-mirror arrays consist of single-crystal
silicon mirrors supported by high-aspect-ratio torsion
springs that provide the highly durable mechanicalstructure needed for reliable connections in theswitch module. The free-space optics, which comprise low-cost and highly accurate two-dimensional
optical fiber and microlens arrays based on polymer
components, are fabricated by passive assembly forlow cost. Experimental results showed that a prototype switch module with 100-channel optical fiberVol. 1 No. 7 Oct. 2003

K. Sato, N. Yamanaka, Y. Takigawa, M. Koga, S. Okamoto, K.
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Mag., Vol. 40, No.3, pp. 75-89, 2002.
[3] V. Kaman, R. Anderson, R. Helkey, O. Jerphagnon, A. Keating, B.
Liu, H. Poulsen, C. Pasarla, D. Xu, S. Yuan, and X. Zheng, “Opticalperformance of a 288 × 288 photonic cross-connect system,” in Proc.
of PS2002, paper-PS.TuA4, pp. 59-61, 2002.
[4] V. A. Aksyuk, F. Pardo, D. Carr, D. Greywall, H. B. Chan, M. E.
Simon, A. Gasparyan, H. Shea, V. Lifton, C. Bolle, S. Arney, R.
Frahm, M. Paczkowski, M. Haueis, R. Ryf, D. T. Neilson, J. Kim, C.
Randy Giles, and D. Bishop, “Beam-steering micromirrors for large
optical cross-connects,” IEEE J. of L. T., Vol. 21, No. 3, pp. 634-642,
2003.
[5] R. Sawada, E. Higurashi, A. Shimizu, and T. Maruno, “Single crystalline mirror actuated electrostatically by terraced electrodes withhigh-aspect ratio torsion spring,” in Proc. of Opt. MEMS 2001, PaperC-1, pp. 23-24, 2001.
[6] J. A. Walker, “The future of MEMS in telecommunications networks,” J. of Micro-mechanics and Microeng., Vol. 10, No. 3, R1-7,
2000.
[7] C. M. Schroeder, “Accurate silicon spacer chips for an optical-fiber
cable connector,” The Bell Sys. Tech. J., Vol. 57, No. 1, pp. 91-97,
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[8] G. Proudley, C. Stace, and H. White, “Fabrication of two-dimensional fiber optic arrays for an optical crossbar switch,” Opt. Eng., Vol.
33, No. 2, pp. 627-645, 1994.
[9] T. Yamamoto, J. Yamaguchi, and Y. Uenishi, “Low-cost and highly
accurate two-dimensional optical fiber arrays for large-scale 3D
MEMS optical switches,” in Proc. of PS2002, paper-PS.WeC11, pp.
184-186, 2002.
[10] T. Satake, N. Kashima, and M. Oki, “Very small single-mode tenfiber connector,” IEEE J. of L. T., Vol. 6, No. 2, pp. 269-272, 1988.
[11] J. Yamaguchi, N. Takeuchi, A. Shimizu, T. Yamamoto, E. Higurashi,R. Sawada, and Y. Uenishi, “Characteristics and control of MEMS
mirrors for optical cross-connect switch,” in Proc. of MIPE03, PaperOD-04, 2003.

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Tsuyoshi Yamamoto
Research Engineer, Network Hardware Integration Laboratory, NTT Microsystem Integration Laboratories.
He received the B.E. degree in electrical engineering from Kansai University, Osaka in 1991.
In 1991, he joined the NTT Communication
Switching Laboratories, where he engaged inresearch and development on digital free-space
optical switches and large-scale free-space optical interconnection systems. In 1998-99, he wasa visiting research engineer at the Department ofElectrical and Computer Engineering, McGill
University, Quebec, Canada. Recently, he has
been involved in research and development of 3D MEMS optical switches for large-scale opticalcross-connects. He is a member of IEEE.

Johji Yamaguchi
Senior Research Engineer, Network Hardware
Integration Laboratory, NTT Microsystem Integration Laboratories.
He received the B.E., M.E., and Ph.D. degrees
in mechanical engineering, all from Tokyo Institute of Technology, Tokyo in 1988, 1990, and1993, respectively. He joined NTT Interdisciplinary Research Laboratories, where he is engaged
in research on optical cross-connect systems.
Recently, he has been involved in research on 3D MEMS optical switches for large-scale opticalcross-connects. He is a member of the Japan
Society of Mechanical Engineers and the Japan
Society for Precision Engineering (JSPE).

42Renshi Sawada
Senior Research Engineer, Supervisor, Network Hardware Integration Laboratory, NTT
Microsystem Integration Laboratories.
He received the B.E., M.E., and Ph.D. degrees
in mechanical engineering from Kyushu University, Fukuoka, in 1976, 1978, and 1995, respectively. In 1978, he joined the Electrical Communication Laboratories of NTT in Tokyo. He has
been engaged in research on the polishing of Si
substrates, gettering of Si crystalline defects, fabrication of dielectrically isolated Si substrates,and optical MEMS such as a micromirror array,
integrated optical displacement sensors, and an
integrated optical blood flow sensor. He is amember of JSPE, the Institute of Electrical Engineers of Japan, and the Society of Instrument andControl Engineers. He received awards fromJSPE in 1981 and 1991.

Yuji Uenishi
Senior Manager, Department V, NTT.
He received the B.E., M.S., and the Ph.D.
degrees in applied physics from Osaka University, Osaka in 1982, 1984, and 1997, respectively.
In 1984, he joined the NTT Electrical Communication Laboratories, where he had been engaged
in research and development on micro-optical
integrated devices. He studied MEMS technology as a visiting researcher at Case WesternReserve University, Ohio, U.S.A. from 1992 to1993. Since returning to NTT Labs, he has been
studying the technology for integrating MEMS
and photonic devices, and has performed pioneering work on optical MEMS in Japan. He is asenior member of IEEE and a member of theJapan Society of Applied Physics. He received
JSPE paper award in 1995 and the Micro Optics
Conference paper award in 1997.

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