Optical Gate Switch Using Phase-Change Material and Si Wire Waveguide
Daiki Tanaka1, Yuichiro Ikuma1, Yuya Shoji2, Masashi Kuwahara3, Xiaomin Wang3,Kenji Kintaka2, Hitoshi Kawashima2, Tatsuya Toyosaki1 and Hiroyuki Tsuda1
1: Graduate School of Science and Technology, Keio University
2: Network Photonics Research Center, AIST
3: Photonics Research Institute, AISTABSTRACT
The optical gate switch using phase-change material (PCM) and Si wire waveguide was reported.
We successfully demonstrated switching by laser pulse irradiation. The wavelength of laser pulses
was 660 nm. We used a laser pulse with a width of 400 ns and a peak power of 15 mW for
crystallization, and a laser pulse with a width of 12 ns and a peak power of 70 mW for
amorphization. 'ON' to 'OFF' to 'ON' switching with an averaged extinction ratio of around 7 dB wasobserved seven times at the wavelength of 1550 nm.
Key words: Optical switch, Phase-change material, Optical circuit, Si wire waveguide
INTRODUCTION
The optical switch using phase-change material (PCM) is suitable for flexible and large capacity
network. In past, we proposed and fabricated the optical switch using PCM [1-5]. This switch iscompact, fast, and low power consumption. It can be very small because PCM has large difference incomplex refractive index between the amorphous and crystalline states. Moreover, it has switchingtime of only 20-400 ns and the self-holding characteristics. In this paper, we demonstrated
switching of the optical gate using PCM and Si wire waveguide.
STRUCTURE OF THE OPTICAL GATE SWITCH
Fig. 1 depicts the structure of the optical gate switch using PCM. We used Ge2Sb2Te5 (GST225) as
PCM. The complex refractive indices of GST225 were 4.4 + 0.098i for the amorphous state and 7.1 +0.78i for the crystalline state at the wavelength of 1550 nm, respectively. This switch had a25-nm-thick GST225 layer on Si wire waveguide with a cross section of 450 x 190 nm2. A
50-nm-thick ZnS-SiO2 passivation layer was also deposited on PCM. On the other hand, theinput/output Si wire waveguides with the cross section of 450 x 210 nm2 were covered with thick (~2µm) SiO2 over cladding. The length of switching region was only 5 µm. The scanning electron
microscopy (SEM) image of this switch was shown in Fig. 2.
EXPERIMENTAL
We changed the state of GST225 by laser pulse irradiation. The experimental setup is shown in Fig.

3. The electric pulse from the pulse generator (PG) directly modulated the laser diode (LD). Fig. 4
shows the electric pulse waveforms from PG. We used a pulse with a width of 400 ns and a peak
power of 15 mW for crystallization, and a pulse with a width of 12 ns and a peak power of 70 mW foramorphization. The optical pulse from LD with the lasing wavelength of 660 nm was focused by lenson GST225 layer. The spot diameter was about 1 µm.
The transmission characteristics of the switch were measured with continuous wave emitted from atunable LD with a wavelength of around 1550 nm. This input light was coupled to the Si waveguidewith focusing lens. The output light from the Si waveguide was also collected by the same lens. The
initial state of the as-deposited GST225 layer was considered to be “amorphous”. Thus thecrystallizing pulse was irradiated on to the gate region at first (‘ON’ to ‘OFF’ switching). After that,the amorphizing pulse was irradiated (‘OFF’ to ‘ON’ switching). We repeated these actions andmeasured the transmittance by the power meter. The change of the output power as laser pulse
irradiations is shown in Fig. 5. The output power included the coupling loss occurred in thewaveguide facets. After 9 irradiations, quasi-stable switching with an averaged extinction of 7 dB
was successfully observed 7 times. The output power was dropped by the crystallizing pulse andrecovered by the amorphizing pulse. We can see that the output power decreased gradually. It
resulted from the spread of the crystalline region. The spot of the crystallizing pulse was somewhat
large as compared with that of the amorphizing pulse. The loss due to the unnecessary
crystallization will be improved by using the optical pulse with flat-top profile.
CONCLUSION
We fabricated the optical gate switch using PCM (Ge2Sb2Te5) and Si wire waveguide. 'ON' to 'OFF'
to 'ON' switching operation with an averaged extinction ratio of around 7 dB was successfully
observed seven times.
ACKOWLEDGEMENT
This work was supported by the Strategic Information and Communications R&D Promotion
Programme (SCOPE) from the Ministry of Internal Affairs and Communications, and by the Global
COE Program for the “High-Level global cooperation for leading-edge platform on access spaces”.
REFERENCES[1] H. Tsuda, et al., PCOS2007, Atami, pp. 39-42, 2007.
[2] Y. Ikuma, et al., IEICE Electron. Express, vol. 5, no. 12, pp. 442-445, 2008.
[3] Y. Ikuma, et al., Electron. Lett., vol. 46, no. 5, pp. 368-369, 2010.
[4] Y. Ikuma, et al., IPR 2010, IWA6, July 25-28, Monterey, U.S.A., 2010.
[5] Y. Ikuma, et al., Electron. Lett., vol. 46, no. 21, pp. 1460-1462, 2010.

ZnS-SiO2/GST225/Si core

Laser pulse
Irradiation(660 nm)

SiO2 over cladding
5 µm

Input light (1550 nm)

SiO2 substrate

Fig. 1. Structure of the optical gate switch.

ZnS-SiO2/GST225/Si core

SiO2 over cladding

Fig. 2. SEM image of the optical gate switch.

Optical fiber
LD (660 nm)
Coaxial cable

NA: 0.8

PG

Laser pulse irradiation

LD: Laser diode
PG: Pulse generatorInput light(1550 nm)
Fig. 3. Experimental setup for laser pulse irradiation.

for crystallization

for amorphization

15 mW

70 mW

12 ns

400 nsFig. 4. Electric pulse waveforms from PG (a) for crystallizationand (b) for amorphization.

-20
Output power [dBm]◆ amorphous-30■ crystalline

-40
-50
-60
averaged extinction ratio of 7 dB

-70
05

1015

202530

Number of pulse irradiation
Fig. 5. Change of output power as laser pulse irradiations.