S22

Compact optical switch using phase-change material
for transparent photonic network
Hiroyuki Tsuda(1), Takumi Moriyama(1), Paridhi Jain(1), Daiki Tanaka(1), Masashi Kuwahara(2), Xiaomin Wang(2),and Hitoshi Kawashima(2)(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, Japantsuda@elec.keio.ac.jp
ABSTRACT
We have fabricated optical switches using Si waveguides and phase-change material. The switch is based on an
asymmetric Mach-Zehnder interferometer, and thin film Ge2Sb2Te5 is deposited onto the Si waveguides. We observe
the change in transmission spectra of the switch by laser pulse irradiations repeatedly. The phase change of theGe2Sb2Te5 film with only 1-μm-square is sufficient for π-phase shift and switching.
Key words: GeSbTe, Optical Switch, Si Waveguide

1. INTRODUCTION
Recently, with the rapid development of digital coherent technology, transmission capacity per single core of an
optical fiber is greater than 100 Tbit/s. On the other hand, optical signal routing in an optical node has a bottle neck ofelectronic signal processing and switching performances. The use of optical technology may be a solution for such aproblem. In particular, large-scale, low-power consumption optical switches will play an important role to realize
high-throughput optical nodes. The application of phase change material (PCM) to optical switches is very attractive
because the size of the switch can be very small due to the large refractive index difference of the crystalline state andthe amorphous state, compared to the switch using other index control mechanism. Moreover, it has self-holding
characteristics and energy efficient. Therefore, it is very suitable for an optical node in transparent photonic network.
The number of switches can be integrated and the switching times for various kinds of optical switches are shown in
Fig. 1. The PCM switches potentially can be very large scale and operated at relatively high-speed; it will be one of
the enabling technologies of fast reconfigurable optical network system.

Num ber of switches

Slowly reconfigurable NWFast reconfigurable NW

10 6
10 510 4
10 3
10 2ME MS
LCOS

Phasechange
Si withplasm aeffect

Si withthermooptic effect
Silica10 1LN, SOA,PLZT

msμs

All-opticalnsSwitching tim eFig. 1. Number of switches can be integrated and the switching times for various kinds of optical switches.

Previously, we had proposed optical switches using PCM [1-3], and showed optical gate operation [4-5]. They could
be switched more than two thousand times [6].
In this paper, an optical switch based on asymmetric Mach-Zehnder interferometer [7] was fabricated andswitching characteristics were measured.
2. DEVICE STRUCTURE
Figure 2(a) and 2(b) show the schematic structure of the optical switch and the enlarged structure of the switchingregion. The typical waveguide width was 0.45 μm, and the height was 0.21 μm, respectively. The optical switch isbased on an asymmetric Mach-Zehnder interferometer (MZI) with an arm length difference, ΔL. The free spectral
range (FSR) of the asymmetric MZI was about 4 nm, which was corresponding to ΔL of 150 μm. Multimodeinterference (MMI) couplers were used for splitting input light equally and combining lights propagating two arms.
The switching region consisted of taper waveguides with a length of 100 μm, 5-μm straight waveguide, and twosquare patterns of the thin film Ge2Sb2Te5 deposited on, as shown in Fig. 2(b). The thickness of Ge2Sb2Te5 film was
30 nm.

(a)

Switching region

MMI couplerΔL
2

Switching region(b)

100 μm0.45 μm

5 μm
2 μm

1 μm
Ge2Sb2Te5Fig. 2. Schematic structure of (a) optical switch and (b) enlarged structure of the switching region.
The fabrication process of the optical switch is as follows. A silicon-on-insulator (SOI) wafer with a top Si layer of
210 nm-thick was used for the substrate. The Si waveguides were formed in the top Si layer by reactive ion etching(RIE). A resist pattern for the thin Ge2Sb2Te5 film was formed by EB lithography and Ge2Sb2Te5 was deposited bysputtering. Then, square Ge2Sb2Te5 patterns were formed by a lift-off process. Finally, a 2 μm-thick SiO2 overcladding layer was deposited by plasma-enhanced chemical vapor deposition (PCVD). The photos after patterning of
the resist film to form Ge2Sb2Te5 are shown in Fig. 3(a) and 3(b).

(a)(b)

Si waveguideGe2Sb2Te5

Si waveguide

Test patterns

Areas for Ge2Sb2Te5
deposition

Fig. 3. (a) Photo after patterning of the resist film to form Ge2Sb2Te5, and (b) enlarged photo near switching region.

3. EXPERIMENTAL RESULTS
The experimental setup is shown in Fig. 4. For optical switching operation, the laser diode (LD) with a lasing
wavelength of 660 nm was directly modulated. Generated optical pulses were guided by single mode fiber (SMF) andfocused on to Ge2Sb2Te5 film with a spot-size of about 1 μm using a lens with a numerical aperture (NA) of 0.8. The
transmittance of the optical switch was measured by scanning the lasing wavelength of the tunable laser source (TLS).
The transverse electric (TE) mode of the light was used. We optimized the pulse conditions, and decided to use apulse with a width of 400 ns and a peak power of 70 mW for crystallization, and a pulse with a width of 70 ns and apeak power of 160 mW for amorphization.

Fig. 4. Experimental setup for measuring optical switching characteristics.

Transm ittance (dB)Figure 5 shows transmission spectra when Ge2Sb2Te5 films on both arms were crystalline (red curve); and when
Ge2Sb2Te5 films on the one-side arm were crystalline and Ge2Sb2Te5 films on the other side arm were amorphous(blue curve). The interference peaks shifted by about a half of FSR. If the input light wavelength was tuned to one ofthe peak, the extinction ratio would be 15 dB.

-40-45-50-55-60-65
1540

1545

1550

1555

1560

W avelength (nm )Fig. 5. Transmittance spectra change by controlling the phases of Ge2Sb2Te5 films. Red curve; all of the Ge2Sb2Te5films were crystalline. Blue curve; Ge2Sb2Te5 films on the one-side arm were crystalline and those on the other side
arm were amorphous.

4. CONCLUSION
A small-sized optical switch using Ge2Sb2Te5 thin films was designed and fabricated. Phase changes of twoGe2Sb2Te5 thin films of 1-μm-square were sufficient for π phase shift and switching. The loss of the device was stillhigh due to coupling losses from optical fibers and Si waveguides, and to large absorption in crystalline Ge2Sb2Te5
thin films.
ACKOWLEADGMENT
This work is supported in part from the Ministry of Education, Culture, Sport, Science, and Technology in Japan.
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