Self-holding optical switch using phase-change material for energy
efficient photonic network
Hiroyuki Tsuda(1), Daiki Tanaka(1), Masashi Kuwahara(2), Xiaomin Wang(2),and Hitoshi Kawashima(2)Department of Electronics and Electrical Engineering,(1) Graduate School of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama-shi,
Kanagawa-ken 223-8522, Japan(2) National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba-shi, Ibaraki-ken
305-8568, Japan
Tel. +81-45-566-1627, Fax. +81-45-566-1529, E-mail Tsuda@elec.keio.ac.jp
Abstract:
We have proposed and fabricated the optical switch using Si waveguides and phase-change material (PCM). The
multimode interference (MMI) type optical gate switch using Ge2Sb2Te5 is fabricated and it shows wide wavelength
range operation of more than 100 nm with an averaged extinction ratio of 12.6 dB. The switching time from thecrystalline state to the amorphous state is 130 ns and the switching time from the amorphous state to the crystalline stateis 400 ns. We have also designed a 2x2 optical switch based on directional coupling. The size of switch is only 2 µm x
10 µm.
Introduction:
The demand for network capacity grows steadily due to the dramatic increase in communication traffic of the Internet.
Figure 1 shows internet traffic in Japan and corresponding energy consumption. They increase at a 30% annual rate.

10000

1014

1000

1013

100101210101111010

0.12000 2005 2010 2015 2020 2025 2030 2035

Energy consumption （kWh/Y）

Traffic in Japan （Tbit/s）

Without any technological development, the energy consumption in the network will be a critical issue.

YearFig. 1. Increase in internet traffic and corresponding energy consumption in the network in Japan. Circles are cited fromMIC white papers.
Photonic switching technology is the key to realize energy efficient, large capacity networks. In particular, high-speed,
broadband, large-scale and low-power consumption optical switches are required. Thanks to the large index differencebetween an amorphous state and a crystalline state, the size of the switch with PCM like Ge2Sb2Te5 can be very small
compared to the switch using other index control mechanism, as shown in Table 1. Moreover, it has relatively

high-speed switching and self-holding characteristics.
Table 1 Comparison of optical switches with various materials.
Material

Switching
MechanismIndex ChangeSwitchingTimeSwitchSize

SilicaThermo-optic

0.02%

50 ms

500×10000 µm

Si

Thermo-optic

0.5%

50 µs

100×200 µm

LiNbO3

Electro-optic

0.02%

ps～ns

50×20000 µm

III-V/Si

Plasma Effect1%

ns

20×100 µm

GeSbTePhase Change> 30%

400 ns

5×20 µm

Self-holding
Characteristics

Available

In this paper, we have proposed an MMI type optical gate switch [1-5] and a 2x2 optical switch [6, 7] using PCM andSi waveguides.
MMI-type optical gate switch:
With large change of absorption between amorphous state and crystalline state, very small-sized optical gate switch canbe configured. The MMI structure was used to relax tolerance of fabrication because typical Si waveguide width is 450nm, and it is difficult to deposit a PCM film onto the correct position. Figure 2(a) shows a schematic of an MMI typeoptical gate switch. The MMI waveguide has one input port and one output port, as shown in Fig. 2(b). The input lightexcites many propagating modes and they are focused at the center of the MMI waveguide, and again focused at theoutput waveguide. The PCM film is located on the focusing spot in the MMI waveguide to enhance the interaction ofthe propagating light and the PCM film.

MMI WaveguideControl light

Output

Ge2Sb2Te5

SiO2 Cladding

Si Waveguide

Input

Ge2Sb2Te5
SOI Substrate

MMI WaveguideLightFig. 2 (a) Schematic of an MMI type optical gate switch. (b) Light propagation in an MMI waveguide.
A Si waveguide had a width of 450 nm and a height of 210 nm and it was formed on SiO2 under cladding. The length ofmulti-mode section is 15.3 µm and the width was 2.25 µm. The circular Ge2Sb2Te5 film with a diameter of 1 µm wasdeposited on the center of MMI and covered with ZnS-SiO2 layer. The thicknesses were 35 nm and 37 nm,

respectively. Waveguides were varied with upper cladding of SiO2.
Control optical pulses with a wavelength of 660 nm were generated by directly modulating a laser diode. They werefocused onto the Ge2Sb2Te5 film and the spot size was about 1 µm. A laser pulse with a width of 40 ns and a peak powerof 160 mW was used for amorphization, and a laser pulse with a width of 400 ns and a peak power of 50 mW was usedfor crystallization. The probe light with a wavelength of about 1550 nm was input by focusing lens and was coupled toTE mode in the Si waveguide. The output light was guided to the photo diode connected to the oscilloscope. Figure 3(a)and 3(b) show switching responses of amorphization and crystallization, respectively. The rise time of the MMI typegate switch was 130 ns, and the fall time was 400 ns. It showed wide wavelength range operation of more than 100 nm
with an averaged extinction ratio of 12.6 dB. In addition, we observed thousand repetitions of switching cycles with an130 ns
10.50-1000100 200 300 400

Time (ns)Optical output (a. u.)

Optical output (a. u.)

extinction ratio of 9.2 dB.

400 ns10.50-100 0 100

300500700

Time (ns)Fig. 3(a) Switching from crystalline state to amorphous state. (b) Switching from amorphous state to crystalline state.
2x2 directional coupling optical switch:
A structure of proposed 2×2 optical switch is shown in Fig. 4(a). This switch is based on directional coupling and hasa triple Si waveguide in a switching region. A Ge2Sb2Te5 thin film is placed on the center waveguide.

Position (µµ)

Bar output

Input

0 2 4 6 8 10

Cross output

Ge2Sb2Te5
SOI Substrate

0 2 4 6 8 10

1B5Position (µµ)

Si Waveguide

1B00B50-0B5-1B0-1B5Fig. 4(a) Schematic structure of 2x2 directional coupling optical switch. (b) FDTD calculation results; cross(amorphous) state (left), and bar (crystalline) state (right).
Three optical supermodes are existed in a triple waveguide for TE polarization. An input light excites these modes andthey are interfered with each other and change power distribution in the waveguides. When Ge2Sb2Te5 is in amorphousstate, light is output from a cross port; and when Ge2Sb2Te5 is in crystalline state, light is output from a bar port. Theswitch structure was optimized using FDTD (finite-difference time-domain) simulation method. The examples of lightpropagation in the switch are shown in Fig. 4(b). This switch had only 2×10 µm2 dimensions. Switching operation wassuccessfully demonstrated by FDTD method. The loss of 0.5 dB and the crosstalk of -20.5 dB were achieved for thecross-state with an optimized structure. The loss of 3.0 dB and the crosstalk of -7.8 dB were achieved for the bar-state.
Conclusion:
We proposed optical switches using phase change material. The MMI type optical gate switch was fabricated and fastswitching, wide operating wavelength range, and more than two thousand switching cycles were confirmed. A 2x2
directional coupling optical switch was designed and the characteristics were simulated with FDTD method.
Acknowledgements:
Authors are very grateful to Dr. Yuya Shoji of Tokyo Institute of Technology for fabrication of devices. This work is
partially supported by research and development program for innovative energy efficiency technology program (theNew Energy and Industrial Technology Development Organization).
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[7] Daiki Tanaka, Yuichiro Ikuma, Hiroyuki Tsuda, Masashi Kuwahara, Hitoshi Kawashima, and Xiaomin Wang,“Ultracompact 2×2 Directional Coupling Optical Switch with Si Waveguides and Phase-Change Material,” Photonics inSwitching 2012, We-S22-O03, Ajaccio, France, Sep. 11-14, (2012).