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Abstract
The 2-µm waveband is becoming an emerging window for next-generation high-speed optical communication. To enable on-chip high-speed data transmission, improving the signal-to-noise ratio (SNR) by suppressing the coupling loss of a silicon chip is critical. Here, we report grating couplers for TE and TM polarized light at the 2-µm waveband. With a single-step fully etched process on the 340 nm silicon-on-insulator (SOI) platform, the devices experimentally demonstrate high coupling efficiency of -4.0 dB and 1-dB bandwidth of 70 nm for the TE polarized light, while -4.5 dB coupling efficiency and 58 nm 1-dB bandwidth for the TM polarized light. For comprehensive performance, both of them are among the best grating couplers operating in the 2-µm waveband so far. We also demonstrate 81Gbps high-speed on-chip data transmission using pulse amplitude modulation 8-level (PAM-8) signals.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
To meet the growing demand for high-speed data transmission, exploring new spectra that can afford large capacity and low latency attracts lots of attention. In recent years, due to the emergence of low-loss hollow-core fiber (HCF) [1] and high-gain, low-noise thulium-doped fiber amplifiers [2], the 2-$\mathrm{\mu}\textrm{m}$ waveband has become a potential candidate for fulfilling this mission. Silicon photonics offers low material loss, high integration density, and CMOS compatibility that is ideal for extending the communication windows from the conventional O-band, C-band, and L-band to the emerging 2-$\mathrm{\mu}\textrm{m}$ waveband [3]. In the 2-$\mathrm{\mu}\textrm{m}$ waveband, since the Rayleigh scattering is inversely proportional to the fourth power of the wavelength, the linear optical loss from the sidewall roughness of silicon waveguides is substantially reduced. On the other hand, the standard silicon-on-insulator (SOI) wafer, i.e., with 220 nm and 340 nm thick top silicon layer, works well for both the conventional bands and the 2-$\mathrm{\mu}\textrm{m}$ waveband. In addition, the two-photon absorption (TPA) and associated free-carrier absorption become weak at this waveband, even negligible for wavelengths longer than 2.2 $\mathrm{\mu}\textrm{m}$ [4]. So far, several silicon photonic devices operating in the 2-$\mathrm{\mu}\textrm{m}$ waveband have been demonstrated, such as optical filters [5], microring resonators [6], multimode interferometer splitters [7], and subwavelength grating slot waveguides [8], etc.
Grating couplers are one of the most used devices for coupling light from an optical fiber to on-chip photonic devices. The performance is crucial for on-chip high-speed data transmission since a highly efficient grating coupler can improve signal-to-noise ratio (SNR) which is beneficial for lower bit error rate (BER). It also enables more light-matter interaction which is important for mid-infrared sensing [9–11]. Compared with grating couplers reported in the 1.55-$\mathrm{\mu}\textrm{m}$ waveband, there is still plenty of room for improving couplers’ performance in the 2-$\mathrm{\mu}\textrm{m}$ waveband [12]. Table 1 summarizes the experimentally demonstrated grating couplers in the 2-$\mathrm{\mu}\textrm{m}$ waveband, focusing on the coupling efficiency (CE) and the bandwidth (BW). Among them, Ref. [13] demonstrated the highest CE of −3.8 dB in this region. However, the fabrication process requires additional deposition of 10 nm SiO2 and 160 nm poly-silicon on a 220-nm-thick-top-silicon SOI wafer. In addition, the 1-dB BW is about 46 nm. In the meantime, Ref. [17] reported the widest 1-dB BW of 115 nm by designing an ultra-thin structure. Its CE is about −7.1 dB. Therefore, a grating coupler that could offer high CE and wide BW in the 2-$\mathrm{\mu}\textrm{m}$ waveband is highly desired.
Table 1. Performance comparison of grating couplers operating in the 2-µm waveband
Here, we report two subwavelength-engineered grating couplers for the TE and TM polarizations in the 2-$\mathrm{\mu}\textrm{m}$ waveband. The devices are fabricated by a one-step full-etching process on a silicon-on-insulator (SOI) platform. By adjusting the period and duty cycle of the grating and subwavelength structure waveguide (SSW), the proposed grating couplers experimentally demonstrate high CE of −4.0 dB and wide 1-dB BW of 70 nm for the TE polarized light and −4.5 dB CE and 58 nm 1-dB BW for the TM polarized light at the center wavelength around 2.04 µm. Moreover, focusing structures are employed to reduce the estate of the devices. To the best of our knowledge, our grating couplers claim the best overall performance of the grating couplers reported in the 2-$\mathrm{\mu}\textrm{m}$ waveband so far. We also demonstrate 81 Gbps high-speed on-chip data transmission using pulse amplitude modulation 8-level (PAM-8) signals. Such devices pave the way for SOI-based on-chip applications in the 2-$\mathrm{\mu}\textrm{m}$ waveband.
2. Principle and design
Figure 1(a) shows the three-dimensional (3D) schematic of the proposed device. It is designed on an SOI chip with a 340-nm thick ($h1$) top silicon layer and a 2-$\mathrm{\mu}\textrm{m}$ thick ($h2$) buried oxide (BOX) layer. A magnified picture of the grating coupler is shown in Fig. 1(b), where (${f_x}$, ${p_x}$) and (${f_y}$, ${p_y}$) are the duty cycle and period of the grating and the subwavelength structured waveguide (SSW), respectively. Compared with the fully-etched 1D grating structures which consist of a set of air trenches, replacing the air trenches with the SSWs significantly reduces the refractive index contrast within one grating period. Therefore, it enables utilizing more grating periods for the light coupling between an optical fiber and silicon waveguide, which is beneficial to improve the coupling efficiency [22]. On the other hand, compared with the shallow etched grating coupler, our device requires single-step full-etching, which has fewer processing steps and lower costs. In addition, as shown in Fig. 1(c), the width of the strip waveguide ($w1$) connecting the two grating couplers is 600 nm for single-mode light transmission in the 2-$\mathrm{\mu}\textrm{m}$ waveband.
Fig. 1. (a) Schematic 3D view of the proposed SOI grating coupler; (b) Top view of the grating coupler. Here (${f_x}$, ${p_x}$) and (${f_y}$, ${p_y}$) are the duty cycle and period of the grating and the SSW, respectively; (c) Cross-sectional view of the strip waveguide between the two couplers, where $w1$=600 nm, $h1$=340 nm, ${h_C}$=2 $\mathrm{\mu}\textrm{m}$, and ${h_{BO}}$=2 $\mathrm{\mu}\textrm{m}$.
We first design a grating coupler with two interleaved materials, namely silicon ${n_{si}}$ and SSW equivalent material ${n_{SSW}}$, for TE polarization. It is well-known that the target wavelength of a grating coupler is related to the period ${p_x}$ along the propagation direction, i.e., the x-direction in our case, and the filling factor, ${f_x}$, by the following equation:
where ${\lambda _c}$ is the center wavelength, ${n_c} = 1$ for our case, and $\theta $ is the incident angle. The effective index of the grating (${n_{eff}}$) is determined by ${f_x}$ and the effective indices of the silicon region ${n_{eff - si}}$ and the SSW region ${n_{eff - SSW}}$ as:In this work, we set ${\lambda _c}$=2 $\mathrm{\mu}\textrm{m}$ and $\theta $=10$^\circ $. The effective index of the silicon dioxide-covered silicon region ${n_{eff - si}}$ is numerically calculated to be ∼2.95. By fixing the ${p_y}$ to 500 nm to guarantee the feasibility of the effective medium in the 2-$\mathrm{\mu}\textrm{m}$ waveband, we can calculate the ${n_{ssw}}$ using the second-order approximation of the effective medium theory [23]. Figure 2(a) shows the mode profile of the fundamental TE mode of the SSW in the Y-Z plane at the wavelength of 2 $\mathrm{\mu}\textrm{m}$. The calculated effective indices of the SSW for different duty cycles of the SSW ${f_y}$ are plotted in Fig. 2(b). Note that when ${f_y}$ is less than 0.5 (TE) or 0.3 (TM), no physical mode exists in the SSW due to the low equivalent refractive index.
Fig. 2. (a) The electric field distribution of the fundamental TE and TM mode of the SSW in the Y-Z plane at 2 $\mathrm{\mu}\textrm{m}$ wavelength; (b) The effective index of the SSW (${n_{eff - SSW}}$) as a function of the SSW duty cycle (${f_y}$) at 2 $\mathrm{\mu}\textrm{m}$ wavelength; Simulated CE and back reflection for (c) TE and (d) TM polarization at 2 $\mathrm{\mu}\textrm{m}$ wavelength.
Benefiting from the above analysis, we significantly reduce the required computation amount and simulate the devices with different ${f_x}$ and ${n_{eff - SSW}}$ using the two-dimensional (2D) finite difference time domain (FDTD) method. As shown in Fig. 2(c), after converting the simulated ${n_{eff - SSW}}$ to ${f_y}$, we obtain the highest CE of −2.5 dB and a low back reflection of −19 dB for the TE polarized grating coupler when ${f_x}$=0.4, ${f_y}$=0.6.
Similarly, we design the grating coupler for the TM polarization. The ${p_y}$ is fixed at 400 nm to satisfy the subwavelength requirement for the TM polarization at the 2-$\mathrm{\mu}\textrm{m}$ waveband. We plot the mode profile of the fundamental TM mode inside the SSW at the wavelength of 2 $\mathrm{\mu}\textrm{m}$ in Fig. 2(a) and the calculated effective indices of the SSW for different ${f_y}$ in Fig. 2(b). As shown in Fig. 2(d), when ${f_x}$=0.5, ${f_y}$=0.3, the device has the highest CE of −4.4 dB and a low back reflection of −26 dB.
3. Optimization
To make the devices more compact, we employ focusing structures by curving the gratings. Following the constructive interference condition, each grating line of the focusing grating should satisfy Eq. (1) along the propagation direction. Therefore, in the right-handed Cartesian x-y coordinate system, with the focal point located in the origin and light propagating along the x direction, we design the grating lines using the phase-match formula [24]:
Fig. 3. Simulated CE and back reflection for (a) TE and (b) TM polarized light as a function of wavelength. Top: the corresponding electric field distribution at the 2 $\mathrm{\mu}\textrm{m}$ wavelength. Simulated TE mode CE spectra for different ${f_x}$ (c) and ${f_y}$ (e). Simulated TM mode CE spectra for different ${f_x}$ (d) and ${f_y}$ (f).
We also plot the fabrication tolerance in terms of duty cycles ${f_x}$ and ${f_y}$ for TE polarized grating couplers in Figs. 3(c) and (e), respectively. And a similar analysis for TM polarized grating coupler is shown in Figs. 3(d) and (f). As ${f_x}$ and ${f_y}$ increase, the center wavelength shifts to longer wavelengths while the bandwidth remains almost unchanged for both TE and TM grating couplers. However, for the peak coupling efficiency, it is relatively robust for TE couplers but is positively related to the duty cycle for TM couplers.
Because of the interference, thickness of the BOX ${h_{BO}}$ and cladding ${h_C}$ layers significantly affect the grating coupling efficiency. Considering a sufficient thick BOX layer to avoid leakage into the bottom handle silicon and maximized coupling efficiency as shown in Figs. 4(a) and (b), we pick ${h_{BO}} = {\; }{h_C} = 2\,\mathrm{\mu}\textrm{m}$ as the thickness of our BOX and cladding layers.
Fig. 4. Simulated CE as a function of the (a) BOX and (b) cladding thickness. Here, the BOX thickness in (a) is fixed at 2 $\mathrm{\mu}\textrm{m}$ while in (b) the cladding thickness is fixed at 2 $\mathrm{\mu}\textrm{m}$.
4. Fabrication and measurement
The devices are fabricated on an SOI chip with a 340 nm thick top silicon layer and a 2 $\mathrm{\mu}\textrm{m}$ thick buried dioxide layer. With a similar process as Refs. [25–27], we use electron-beam lithography (EBL) to define the designed layout on the ZEP-520A resist. After that, the patterns are transferred to the silicon layer using the induced coupled plasma (ICP). Finally, we deposit a 2 µm thick silicon dioxide layer over the devices as a top cladding by plasma-enhanced chemical vapor deposition (PECVD). The scanning electron microscope (SEM) pictures of the fabricated grating couplers for the TE and TM polarizations are plotted in the insets of Figs. 4(a) and (b), respectively.
We use a 2-µm amplified spontaneous emission (ASE) as our light source to measure the coupling efficiency. After converting to TE/TM polarized light by passing through a polarizer, the light transmits through a polarization-maintaining (PM) fiber and into the grating coupler. The input and output grating couplers are connected with a 1 $\textrm{mm}$ long single-mode waveguide (600 nm wide). The output signal is collected by a single-mode fiber and sent to an optical spectrum analyzer (OSA). Ignoring the propagation loss of the single-mode waveguide and assuming identical CE of the two couplers, we plot the measured CE for the wavelength range from 1.95 $\mathrm{\mu}\textrm{m}$ to 2.1 $\mathrm{\mu}\textrm{m}$ in Fig. 5. Figure 5(a) is the result for the TE grating coupler with a maximum coupling efficiency of −4.0 dB at the wavelength of 2 $\mathrm{\mu}\textrm{m}$ and a 1-dB bandwidth of 70 nm. The peak coupling efficiency for the TM grating coupler, Fig. 5(b), is about −4.5 dB at the wavelength of 2.032 $\mathrm{\mu}\textrm{m}$, and the 1-dB bandwidth is about 58 nm. The experimental results are consistent with the simulation with slightly lower CEs, narrower BWs, and roughly 40 nm center wavelength shifts, which may cause by the slightly under-etching of the devices.
Fig. 5. Measured CEs of the grating coupler for (a) TE and (b) TM polarizations. The corresponding SEM images are shown in the insets.
We also confirm the communication application of the proposed grating coupler by demonstrating a high-speed signal system for the TE grating coupler at the wavelength of 2 $\mathrm{\mu}\textrm{m}$. The setup is plotted in Fig. 6. PAM-8 signals are generated by an arbitrary waveform generator (AWG) with a 120 GSa/s sampling rate, amplified by an electric amplifier (EA), and then converted to an optical signal by a Mach-Zehnder modulator (MZM). The variable optical attenuator (VOA) is used to control the power of the output optical signal. After amplified by a thulium-doped fiber amplifier (TDFA), the output optical signal is converted to an electrical signal by a photodetector (PD), and the data is collected by an oscilloscope.
We plot the measurement results in Fig. 7. In this experiment, the maximum transmission speed is 81 Gbps. When coupling the high-speed signal through the chip using a pair of grating couplers with a coupling loss of −4.0 dB/coupler, the BER is $8.613 \times {10^{ - 3}}$. From Fig. 7(a), we find the transmission speed decreases with increasing coupling loss at the approximately same BER. At the same time, with the same transmission speed (27 Gbaud/s PAM8), the BER increases rapidly as the coupling loss increases. Compared with the coupler with −4.0 dB coupling loss, the BER increases 6.4 times when the grating coupler has a −7 dB coupling loss due to the higher coupling loss induced worsened SNR. Hence, the grating coupler with lower coupling loss significantly improves the BER and transmission speed, which is important for next-generation on-chip communications.
Fig. 7. (a) Measured transmission speed (black line) and BER (red line) versus coupling loss. (b) Measured eye diagram of 81 Gbps PAM8 signal using our TE grating couplers (−4.0 dB coupling loss per grating coupler).
5. Conclusion
In summary, the 2-$\mathrm{\mu}\textrm{m}$ waveband becomes essential for next-generation optical communication. Here, we report two grating couplers for the TE and TM polarizations in the 2-$\mathrm{\mu}\textrm{m}$ waveband based on the SOI platform with a 340-nm-thick top silicon layer. The devices experimentally demonstrate high coupling efficiencies of −4.0 dB for the TE polarized light with a 1-dB bandwidth of 70 nm and −4.5 dB for the TM polarized light with a 1-dB bandwidth of 58 nm. Higher coupling efficiency can support devices with better communication performance. With the focusing design, the grating couplers feature a small footprint. In the meantime, the devices only need single-step lithography and etching, significantly reducing the cost. For the overall performance, including both the peak CE and the 1-dB BW, our grating couplers claim the best grating couplers in the 2-$\mathrm{\mu}\textrm{m}$ waveband. Our devices provide key building blocks for developing silicon photonic integrated circuits in the 2-$\mathrm{\mu}\textrm{m}$ waveband.
Funding
Natural Science Foundation of Shanghai (21ZR1443100); National Natural Science Foundation of China (61705099, 62175080); National Key Research and Development Program of China (2022YFB2803600).
Acknowledgment
We thank the ShanghaiTech University Quantum Device Lab (SQDL) for technique support.
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. W. Shen, J. Du, L. Sun, C. Wang, Y. Zhu, K. Xu, B. Chen, and Z. He, “Low-Latency and High-Speed Hollow-Core Fiber Optical Interconnection at 2-Micron Waveband,” J. Lightwave Technol. 38(15), 3874–3882 (2020). [CrossRef]
2. Z. Li, A. M. Heidt, J. M. Daniel, Y. Jung, S. U. Alam, and D. J. Richardson, “Thulium-doped fiber amplifier for optical communications at 2 µm,” Opt. Express 21(8), 9289–9297 (2013). [CrossRef]
3. Y. Zou, S. Chakravarty, C.-J. Chung, X. Xu, and R. T. Chen, “Mid-infrared silicon photonic waveguides and devices,” Photonics Res. 6(4), 254 (2018). [CrossRef]
4. J. Huang, H. Han, A. Liu, H. Wang, X. Liu, Y. Zou, M. Lu, and Y. Chen, “Efficient Second Harmonic Generation by Mode Phase Matching in a Silicon Waveguide,” IEEE Photonics J. 9(3), 1–7 (2017). [CrossRef]
5. D. Liu, H. Wu, and D. Dai, “Silicon Multimode Waveguide Grating Filter at 2 µm,” J. Lightwave Technol. 37(10), 2217–2222 (2019). [CrossRef]
6. C. Zhong, H. Ma, C. Sun, M. Wei, Y. Ye, B. Tang, P. Zhang, R. Liu, J. Li, L. Li, and H. Lin, “Fast thermo-optical modulators with doped-silicon heaters operating at 2 µm,” Opt. Express 29(15), 23508–23516 (2021). [CrossRef]
7. M. S. Rouifed, C. G. Littlejohns, G. X. Tina, Q. Haodong, T. Hu, Z. Zhang, C. Liu, G. T. Reed, and H. Wang, “Low Loss SOI Waveguides and MMIs at the MIR Wavelength of 2 µm,” IEEE Photonics Technol. Lett. 28(24), 2827–2829 (2016). [CrossRef]
8. Z. Ruan, L. Shen, S. Zheng, A. Wang, Y. Long, N. Zhou, and J. Wang, “Subwavelength grating slot (SWGS) waveguide at 2 µm for chip-scale data transmission,” Nanophotonics 7(5), 865–871 (2018). [CrossRef]
9. J. Zhou, Z. Zhang, B. Dong, Z. Ren, W. Liu, and C. Lee, “Midinfrared Spectroscopic Analysis of Aqueous Mixtures Using Artificial-Intelligence-Enhanced Metamaterial Waveguide Sensing Platform,” ACS Nano 17(1), 711–724 (2023). [CrossRef]
10. A. Z. Subramanian, E. Ryckeboer, A. Dhakal, et al., “Silicon and silicon nitride photonic circuits for spectroscopic sensing on-a-chip [invited],” Photonics Res. 3(5), B47–B59 (2015). [CrossRef]
11. Y. Ma, B. Dong, and C. Lee, “Progress of infrared guided-wave nanophotonic sensors and devices,” Nano Convergence 7(1), 12–34 (2020). [CrossRef]
12. X. Wang, Y. Liu, Z. Zhang, J. Wang, Y. Yao, Q. Song, and K. Xu, “Research progress in 2 µm waveband on-chip photonic integrated devices (Invited),” Infrared Laser Eng. 51(3), 20220087 (2022). [CrossRef]
13. N. Hattasan, B. Kuyken, F. Leo, E. M. P. Ryckeboer, D. Vermeulen, and G. Roelkens, “High-efficiency SOI fiber-to-chip grating couplers and low-loss waveguides for the short-wave infrared,” IEEE Photonics Technol. Lett. 24(17), 1536–1538 (2012). [CrossRef]
14. W. Zhou and H. K. Tsang, “Dual-wavelength-band subwavelength grating coupler operating in the near infrared and extended shortwave infrared,” Opt. Lett. 44(15), 3621–3624 (2019). [CrossRef]
15. J. Li, L. Liu, W. Sun, X. Wen, K. Xu, and Q. Song, “The 2-µm fully-etched silicon grating coupler,” in 2017 Conference on Lasers and Electro-Optics Pacific Rim (CLEO-PR), 1–3 (2017).
16. Z. Zhang, G. I. Ng, H. Qiu, W. Wang, X. Guo, M. S. Rouifed, C. Liu, and H. Wang, “Compact microring resonators integrated with grating couplers working at 2 µm wavelength on silicon-on-insulator platform,” Appl. Opt. 56(19), 5444–5449 (2017). [CrossRef]
17. R. Guo, H. Gao, T. Liu, and Z. Cheng, “Ultra-thin mid-infrared silicon grating coupler,” Opt. Lett. 47(5), 1226–1229 (2022). [CrossRef]
18. W. Chen, J. Wu, D. Wan, J. Wang, J. Wang, Y. Zou, Z. Cheng, and T. Liu, “Grating couplers beyond silicon TPA wavelengths based on MPW,” J. Phys. D: Appl. Phys. 55(1), 015109 (2022). [CrossRef]
19. H. Ma, H. Yang, B. Tang, M. Wei, J. Li, J. Wu, P. Zhang, C. Sun, L. Li, and H. Lin, “Passive devices at 2 µm wavelength on 200 mm CMOS-compatible silicon photonics platform,” Chin. Opt. Lett. 19(7), 071301 (2021). [CrossRef]
20. J. Li, Y. Liu, Y. Meng, K. Xu, J. Du, F. Wang, Z. He, and Q. Song, “2−µm Wavelength Grating Coupler, Bent Waveguide, and Tunable Microring on Silicon Photonic MPW,” IEEE Photonics Technol. Lett. 30(5), 471–474 (2018). [CrossRef]
21. W. Chen, D. Wan, J. Wang, Z. Yi, Z. Cheng, and T. Liu, “Mid-infrared subwavelength grating coupler,” in 2021 Asia Communications and Photonics Conference (ACP), Shanghai, China, pp. 1–3 (2021).
22. Y. Zou, H. Subbaraman, S. Chakravarty, X. Xu, A. Hosseini, W. C. Lai, P. Wray, and R. T. Chen, “Grating-coupled silicon-on-sapphire integrated slot waveguides operating at mid-infrared wavelengths,” Opt. Lett. 39(10), 3070–3073 (2014). [CrossRef]
23. L. Sun, Y. Zhang, Y. He, H. Wang, and Y. Su, “Subwavelength structured silicon waveguides and photonic devices,” Nanophotonics 9(6), 1321–1340 (2020). [CrossRef]
24. Z. Cheng, X. Chen, C. Y. Wong, K. Xu, and H. K. Tsang, “Apodized focusing subwavelength grating couplers for suspended membrane waveguides,” Appl. Phys. Lett. 101(10), 101104 (2012). [CrossRef]
25. A. Zhang, L. Xia, T. Li, C. Chang, P. Zhou, X. Xu, and Y. Zou, “Ultra-compact polarization-independent 3 dB power splitter in silicon,” Opt. Lett. 46(19), 5000–5003 (2021). [CrossRef]
26. W. He, Y. Sun, P. Zhou, L. Xia, T. Li, Q. Yi, L. Shen, Z. Cheng, and Y. Zou, “Subwavelength structure engineered passband filter for the 2-µm wave band,” Opt. Lett. 48(3), 827–830 (2023). [CrossRef]
27. Y. Liu, L. Xia, T. Li, Y. Sun, P. Zhou, L. Shen, and Y. Zou, “High-efficiency mid-infrared on-chip silicon grating couplers for perfectly vertical coupling,” Opt. Lett. 48(2), 239–242 (2023). [CrossRef]
