P08

Mach-Zehnder Interferometer Optical Switch Using Low-loss N-doped GeTe Thin Film
Yasuro Shimazaki1, Masashi Kuwahara2, Xiaomin Wang2, Hiroyuki Tsuda1
1: Graduate School of Science and Technology, Keio University
E-mail: shimazaki@tsud.elec.keio.ac.jp
3-14-1 Hiyoshi, Kohoku-ku, Yokohama City, 223-8522 Japan
2: National Institute of Advanced Industrial Science and technology
1-1-1 Higahsi, Tsukuba City, 305-8565 Japan
ABSTRACTThe self-holding, small Mach-Zehnder interferometer (MZI) optical switch using N-doped GeTe,
was proposed. N-doped GeTe thin films with various doping concentration were prepared by
sputtering and refractive indices were measured. The switching characteristics of the 2×2 MZI
optical switch using N-doped GeTe were simulated. It was shown that the transmission losses could
be less than 1 dB for both cross and bar operation, and the crosstalks could be less than -21 dB.
Key words: N-doped GeTe, MZI optical switch
1. INTRODUCTION
We had proposed the MZI optical switch using undoped GeTe [1], and demonstrated switching
operation of the optical gate using
and Si wire waveguides [2].
They have self-holding characteristics; therefore, low-power consumption can be achieved.
Moreover, the size of the switch using phase change material (PCM) can be very small because therefractive index changes largely. In order to improve the switching characteristics, it is effective todecrease the absorption of PCM. In this paper, N-doped GeTe thin films with various concentrations
were fabricated and their characteristics were measured. The switching operation of the 2×2 MZI
optical switch using the N-doped GeTe was simulated.
2. N-doped GeTe
-The N-doped GeTe thin films [3] on Si substrate were prepared by reactive sputtering of a GeTe
target using the Ar/N2 gas mixture. The compositions were varied by changing the Ar/N2 ratio. Wechanged the Ar/N2 ratio, 10/0, 9.5/0.5, 9.3/0.7, 9.0/1.0, 8.5/1.5.
-The complex refractive indices for amorphous state and crystal state at a wavelength of 1550 nm
are shown in Table 1. When N2 concentration in the gas was more than 10%, the absorption of theN-doped GeTe was much smaller than that of undoped GeTe.
-We annealed the N-doped GeTe thin film with an Ar/N2 ratio of 9.0/1.0 by 315
and it turned
into the crystalline state. We could perform the amorphization and the recrystallization of theannealed sample by the laser radiation (Figure 1). The wavelength and the spot size of the laser are
660 nm and 1 μm respectively. When performing the amorphization, the width and the power of thatwere 70 ns and 180 mW respectively. When doing the crystallization, they were 400 ns and 80 mW
respectively.
3. Mach-Zehnder Interferometer Optical Switch-Figure 2 shows the schematic illustration of the optical switch. The width and the height of asilicon waveguide were 450 nm and 210 nm respectively. The width, the thickness and the length of
the N-doped GeTe thin film were 450 nm, 50 nm and 6.5 μm, respectively. In the switching
simulation, the N-doped GeTe thin film on one of the arms was switched between the amorphousstate and the crystalline state while that on another arm was maintained in the amorphous state. TheN-doped GeTe film fabricated with an Ar/N2 ratio of 9.0/1.0 was used for calculation. The input light
was divided equally into the two arms by the 3-dB directional coupler. When the N-doped GeTe thinfilms on both arms were in the amorphous state, the light was guided to the cross port (the crossoperation); and when one of the films was in the crystalline state, the light was guided to the bar port(the bar operation).
Table 2 shows the transmission losses from the input port to the bar port and to the cross port forthe cross operation and the bar operation. The transmission losses were less than 1 dB for bothoperations; and the crosstalks were less than -21 dB. Compared to the previous report [1], both thetransmission losses and the crosstalks were reduced because of the decrease in the absorption ofPCM.
4. CONCLUSION
The N-doped GeTe thin films with various doping concentrations were fabricated. When the N2
concentration in the gas was more than 10%, the absorption of the N-doped GeTe was much smaller
than that of undoped GeTe. In the switching simulation, we could obtain less than 1 dB transmission
losses for both operations, with crosstalk of less than -21 dB.
REFERENCES[1] P. Jain, et al., “Mach Zehnder interferometer optical switch using phase-change material,”
Photonics in Switching 2012, Th-S4-P09, Ajaccio, France, Sep. 14, (2012).
[2] D. Tanaka, et al., “Ultra-small, self-holding, optical gate switch using Ge2Sb2Te5 with amulti-mode Si waveguide,” Optics Express Vol. 20, No. 9, pp. 10283-10294 (2012).
[3] Alexander V. Kolobov, et al., “Local Structure of nitrogen in N-doped amorphous and crystalline
GeTe,” Appl. Phys. Lett., 100, 061910-1 (2012).
Table 2 Transmission losses to the bar
port and to the cross port.

Table 1 Complex indices of refraction at awavelength of 1550 nm.
Ar/N2 ratio

Amorphous

Crystal

10/0

4.02-0.09i

5.45-0.14i

9.5/0.5

3.79-0.14i

5.24-0.47i

9.3/0.7

3.60-0.01i

4.67-0.27i

9.0/1.0

3.39-0.00i

4.06-0.04i

8.5/1.5

3.25-0.00i

3.79-0.02i

Bar port

Cross port

Cross operation

23.01dB

0.01dBBar operation

0.97dB

22.22dB(a)(b)

Figure 1 Microscopic photos of the N-doped GeTe thin film; (a) amorphousmark, and (b) erased mark by the crystallization
Bar port
N-doped GeTe

Cross port
Arm 1

Arm 2

3dB coupler
SiO cladding
Input power

Si waveguide
Figure 2 Schematic illustration of switch