⇦Chinese Physics Letters, 2019, Vol. 36, No. 5, Article code 054204 Improved Performance of a Wavelength-Tunable Arrayed Waveguide Grating in Silicon on Insulator * Pei Yuan (袁配)1,2†, Xiao-Guang Zhang (张晓光)3†, Jun-Ming An (安俊明)1,2, Peng-Gang Yin (殷鹏刚)3, Yue Wang (王玥)1**, Yuan-Da Wu (吴远大)1,2** Affiliations 1State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083 2Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049 3Key Laboratory of Bio-inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beihang University, Beijing 100083 Received 23 January 2019, online 17 April 2019 *Supported by the National Key R&D Program of China under Grant No 2016YFB0402504.
†Pei Yuan and Xiao-Guang Zhang contributed equally to this work.
**Corresponding author. Email: wy1022@semi.ac.cn; wuyuanda@semi.ac.cn Citation Text: Yuan P, Zhang X G, An J M, Yin P G and Wang Y et al 2019 Chin. Phys. Lett. 36 054204    Abstract The improved performance of a wavelength-tunable arrayed waveguide grating (AWG) is demonstrated, including the crosstalk, insertion loss and the wavelength tuning efficiency. A reduced impact of the fabrication process on the AWG is achieved by the design of bi-level tapers. The wavelength tuning of the AWG is achieved according to the thermo-optic effect of silicon, and uniform heating of the silicon waveguide layer is achieved by optimizing the heater design. The fabricated AWG shows a minimum crosstalk of 16 dB, a maximum insertion loss of 3.91 dB and a wavelength tuning efficiency of 8.92 nm/W, exhibiting a $\sim $8 dB improvement of crosstalk, $\sim $2.1 dB improvement of insertion loss and $\sim $5 nm/W improvement of wavelength tuning efficiency, compared to our previous reported results. DOI:10.1088/0256-307X/36/5/054204 PACS:42.81.Qb, 42.82.Et, 42.79.Sz © 2019 Chinese Physics Society Article Text Silicon photonics,[1–3] which is compatible with mature complementary metal-oxide semiconductors, is a promising and cost-effective solution for future high-speed large-capacity optical communication systems because silicon nanowire waveguides can permit extremely small bending radii of several micrometers and therefore high-level integration of different optical functions.[4–12] However, the mode refractive index of waveguides with high index contrast is very sensitive to their dimensional fluctuation because a tiny dimensional fluctuation in the fabrication process will introduce a great mode index change. In interference devices such as arrayed waveguide grating (AWG)[13–15] multiplexers/de-multiplexers, an increase of sensitivity can cause large phase distortion of the fabricated grating arms. Because of this random phase error, the crosstalk of AWG will become worse and the peak wavelength of each channel will drift as well. In our previous work,[16] widened arrayed waveguides were introduced to reduce the mode index sensitivity of waveguides with high index contrast and then to decrease the crosstalk of AWG. The error analysis of AWG with width fluctuation has been worked out using the transfer function method, which theoretically shows that the crosstalk of AWG will decrease by $\sim $12 dB when the waveguides in the straight section of arrayed waveguides are widened from 500 nm to 800 nm. To solve the peak wavelength drift of each channel, wide heaters of 10 µm with spacing of 10 µm were used in our previous work, and 1.1 nm wavelength tunability was achieved at 276 mW power consumption. However, the crosstalk of AWG is still too high for an application in optical communication systems and interconnection systems, and the structure of the heater needs to be optimized because the temperature distribution in the core silicon layer with heater spacing of 10 µm is not uniform. In this work, an AWG with lower crosstalk and insertion loss has been demonstrated by optimizing the boundary structures between the free propagation regions (FPRs) and arrayed waveguides, and larger wavelength tunability of AWG is achieved by optimizing the heater structure. Generally speaking, the high crosstalk of an AWG is the result of beam defocusing at the output waveguides due to unequal phase shift of the arrayed waveguides, which is partly generated by the fabrication process. As stated in the introduction section, silicon AWGs with high index contrast usually perform worse on crosstalk than AWGs based on low-index-contrast waveguides,[17] because waveguides with high index contrast are more vulnerable to the dimension deviation during the fabrication process. The fabricated dimensions which deviate from the designed values and the rough side walls of the waveguides will both change the mode index of the waveguides and then make phase shift of the arrayed waveguides unequal. To some degree, shallow etched waveguides can decrease the possibility of dimension deviation and side wall roughness because shallow etching is more controllable during the fabrication process, thus a shallow etching depth is preferred in this study. However, shallow etched waveguides need a larger bending radius, which will increase the footprint of the device. On the other hand, the insertion loss of AWG is mainly caused by the mode mismatching between the single-mode waveguides and the FPRs with multi-modes. To solve this problem, tapers are usually used to connect the single-mode waveguides and the FPRs. These tapers are expected to be long enough for low loss mode conversion and wide enough to receive as much light as possible to decrease optical loss. However, they may also cause the degradation of crosstalk arising from the narrow gap and long coupling distances between adjacent tapers.
cpl-36-5-054204-fig1.png
Fig. 1. The structure of the wavelength-tunable AWG: (a) the overall mask layer of the AWG, and (b) the detailed structure of bi-level tapers between the FPR and the input/output/arrayed waveguides.
In addition to the widened arrayed waveguide structure, bi-level tapers have been used to decrease the insertion loss and crosstalk, as shown in Fig. 1. Shallow etched waveguides (first-level tapers) with 70 nm etching depth are adopted at the boundary between FPRs and the input/output/arrayed waveguides, deep etched waveguides with 150 nm etching depth are applied especially at the bending waveguides, and second-level tapers are used as the transition of the above two waveguide structures. Figure 1(a) shows the overall mask layer of the wavelength-tunable AWG, which consists of the input/output/arrayed waveguides, the two FPRs, heaters made of titanium nitride and pads made of aluminum. Figure 1(b) shows the detailed structure of bi-level tapers between the FPR and the input/output/arrayed waveguides. The dense wavelength division multiplexing (DWDM) system requires large channels and small channel spacing, which needs to strictly control the wavelength drift of the AWG. In this work, heaters are used to realize the precise positioning of AWGs on the ITUT grid. According to the thermo-optic effect, the peak wavelength shift ($\Delta \lambda $) arising from the change of temperature ($\Delta T$) can be calculated from $$\begin{align} \Delta \lambda =\,&\frac{\Delta n_{\rm eff}}{n_{\rm g}}\cdot \lambda_{0},~~ \tag {1} \end{align} $$ $$\begin{align} \Delta n_{\rm eff} =\,&\frac{\partial n_{\rm eff}}{\partial n_{\rm Si}}\cdot \Delta n_{\rm Si} +\frac{\partial n_{\rm eff}}{\partial n_{\rm SiO_{2}}}\cdot \Delta n_{\rm SiO_{2}},~~ \tag {2} \end{align} $$ where $\Delta n_{\rm Si} /\Delta n_{\rm SiO_{2}}$ represents the change in refractive index of the Si/SiO$_{2}$ caused by the change of temperature, and therefore the effective index of the waveguide ($n_{\rm eff}$) is changed as well; $n_{\rm g}$ is the group index of the waveguide; and $\lambda_{0}$ is the peak wavelength of a certain channel. The thermo-optic coefficient ($dn/dT$) of Si equals $1.84\times 10^{-4}$/K, and that of SiO$_{2}$ equals $1.0\times 10^{-5}$/K.[18]
cpl-36-5-054204-fig2.png
Fig. 2. Heat simulation: (a) the simulated heat distribution with 4-µm-wide heater spacing at 240 mW power consumption, (b) and (c) the temperature distributions along the $x$-axis under different heater spacings and under different power consumptions, respectively.
Simulated heat distribution is shown in Fig. 2. Figure 2(a) shows the simulation structure and the heat distribution with 4-µm-wide heater spacing at 240 mW power consumption. Then we have selected 280 points with equal distance along the $x$-axis of the silicon waveguide layer, and probed the temperature of each point. Figure 2(b) shows the temperature of the 280 points under different heater spacings ($D_{\rm h}$), from which it can be seen that as the heater spacing becomes smaller, the temperature of the silicon layer becomes higher and the temperature distribution becomes more uniform. Figure 2(c) shows the temperature of the 280 points with 4-µm-wide heater spacing under different power consumptions, from which it can be seen that the heat distribution is uniform under different power consumptions, which ensures the uniform tunability of the device. The designed parameters of the wavelength-tunable AWG are listed in Table 1.
Table 1. The designed parameters of the wavelength-tunable AWG. SOI: silicon on insulator.
Parameter Symbol Value
Thickness of top silicon of SOI $H$ 220 nm
Thickness of shallow etched slab waveguides $h_{\rm s}$ 70 nm
Thickness of deep etched slab waveguides $h_{\rm d}$ 150 nm
Width of input/output waveguides $w$ 500 nm
Width of arrayed waveguides $W$ 800 nm
Central wavelength $\lambda_{0}$ 1.55 µm
Channel spacing $\Delta \lambda$ 1.6 nm
Number of input channels $N_{\rm i}$ 5
Number of output channels $N_{\rm o}$ 25
Pitch width of arrayed waveguides $d$ 1 µm
Width of tapers $W_{\rm t}$ 0.8 µm
Pitch width of input/output waveguides $\Delta x_{\rm i}/\Delta x_{\rm o}$ 1.5 µm
Heater spacing $D_{\rm h}$ 4 µm
Heater width $W_{\rm h}$ 10 µm
The wavelength-tunable AWG is fabricated on an 8-inch silicon-on-insulator chip with a 220-nm-thick top silicon layer. The thickness of the buried oxide is 2 µm. The waveguides with 70 nm and 150 nm etching depths are formed by a double silicon dry etching process. After a 1.5-µm-thick SiO$_{2}$ layer is deposited by plasma-enhanced chemical vapor deposition, titanium nitride is deposited by sputtering and lift-off technology to form the heater. Lastly, aluminum as the electrode and via material is deposited after 1-µm-thick SiO$_{2}$ is deposited as the upper cladding. The micrographs of the fabricated wavelength-tunable AWG are shown in Fig. 3, and the size of the device is $1.5\times 0.23$ mm$^{2}$.
cpl-36-5-054204-fig3.png
Fig. 3. The micrographs of the fabricated wavelength-tunable AWG: (a) the overall structure of the AWG, and (b) the structure of the bi-level tapers.
The measured output spectra of the 25-channel silicon AWG before heating (solid lines) and after heating with 240 mW power consumption (dashed lines) are shown in Fig. 4(a). Figure 4(b) shows the insertion loss and crosstalk of each channel and Fig. 4(c) shows the peak wavelength of 25 channels and the wavelength tunability. From Fig. 4 it can be seen that the channel spacing is 1.6 nm, the maximum loss is 3.91 dB, and the lowest crosstalk of the AWG is about 16 dB. The wavelength tunability in 25 channels is basically uniform from 1.9 nm to 2.2 nm and the wavelength tuning efficiency is from 7.92 nm/W to 9.17 nm/W.
cpl-36-5-054204-fig4.png
Fig. 4. (a) The measured output spectra of the silicon AWG before heating (solid lines) and after heating with 240 mW power consumption (dashed lines). (b) Insertion loss and crosstalk of 25 channels of the AWG. (c) The peak wavelength of 25 channels and the corresponding channel tunability.
cpl-36-5-054204-fig5.png
Fig. 5. The measured output spectra of the 13th channel of the AWG under different bias powers.
Figure 5 shows the measured output spectra of the 13th channel of the silicon AWG under different bias powers, and the detailed wavelength tuning parameters are listed in Table 2. The wavelength tuning efficiency is about 9 nm/W. Comparisons have been made with our previous work, and the performance results are listed in Table 3. Compared with our previous work, the AWG in this work has optimized structures, including the bi-level tapers and the heater that can reach uniform temperature distribution. From Table 3, it can be seen that the AWG in this work has improved performance in terms of channel number, footprint, insertion loss (IL), crosstalk (CT) and tuning efficiency.
Table 2. The wavelength tuning parameters of the AWG.
Power consumption (mW) Peak wavelength (nm) Wavelength tunability (nm) Tuning efficiency (nm/W)
0 1556.14 0 0
57 1556.66 0.52 9.12
240 1558.28 2.14 8.92
Table 3. Comparisons to our previous work.
Channel number Footprint (mm$^{2})$ IL (dB) CT (dB) Tuning efficiency (nm/W)
Ref.  [16] 16 $1.4 \times 1$ 6 7.5 3.902
This paper 25 $1.5 \times 0.23$ 3.91 16 8.92
In summary, a thermo-optically tunable silicon AWG with improved performance has been demonstrated. A reduced impact of the fabrication process on the AWG is achieved through the design of bi-level tapers. Uniform heating is achieved by optimizing the heater design. Comparisons have been made with our previous work, which show that the AWG in this work has improved performance in terms of channel number, footprint, insertion loss, crosstalk and tuning efficiency. References Silicon Nanophotonic Integrated Devices for On-Chip Multiplexing and SwitchingThe Past, Present, and Future of Silicon PhotonicsThe commercialization of silicon photonicsDesign of a C-band polarization rotator-splitter based on a mode-evolution structure and an asymmetric directional couplerLow-energy, high-performance lossless 8 × 8 SOA switch3D integrated hybrid silicon laserHigh gain-bandwidth product Ge/Si tunneling avalanche photodiode with high-frequency tunneling effectSuspended polarization beam splitter on silicon-on-insulatorSilicon photonics beyond silicon-on-insulatorDesign and fabrication of two kind of SOI-based EA-type VOAsA high-efficiency grating coupler between single-mode fiber and silicon-on-insulator waveguideCrosstalk analysis of silicon-on-insulator nanowire-arrayed waveguide gratingLow-loss and low-crosstalk 8 × 8 silicon nanowire AWG routers fabricated with CMOS technologyOptimized Silicon AWG With Flattened Spectral Response Using an MMI ApertureDesign and fabrication of wavelength tunable AWGs based on the thermo-optic effectThe 8×10 GHz Receiver Optical Subassembly Based on Silica Hybrid Integration Technology for Data Center InterconnectionThermo-Optically Tunable Silicon AWG With Above 600 GHz Channel Tunability
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