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Abstract
A silicon-based polarizing beam splitter (PBS) working at the 2 μm wavelength band is proposed and demonstrated experimentally by using a bent directional coupler assisted with a nano-slot waveguide. The nano-slot width is chosen as 180 nm so that the present PBS can be fabricated with MPW foundries. In theory, the designed PBS has extinction ratios (ERs) of >15 dB and >30 dB for TM- and TE- polarizations in the wavelength range of 1825-2020 nm, respectively. For the fabricated PBS, the excess losses (ELs) are low (∼0.5 dB) while the measured results show the ERs are >15 dB and >20 dB for TM- and TE-polarizations in the wavelength band of 1860-1980 nm.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Silicon photonics has been attracting much attention because of the ability for achieving ultra-high integration density and the CMOS compatibility in the past decade. Among various silicon photonic devices, a polarization beam splitter (PBS) for separating/combining TE- and TM- polarizations is regarded as one of the most important fundamental elements in many optical systems, including polarization-transparent photonic integrated circuits [1], coherent optical transceivers [2] and quantum photonics [3]. Currently various waveguide structures have been proposed and demonstrated successfully for realizing on-chip PBSs, such as directional couplers (DCs) [4–6], multimode interferometers [7–10] and subwavelength-grating (SWG)-assisted structures [11–13]. Among them, asymmetric DCs have attracted extensive attention because of the performance excellence and design simplicity. In particularity, an ultra-small PBS based on a bent DC was proposed for the first time in 2011 [5] and was realized experimentally in 2013 [6]. Furthermore, an improved structure with cascaded bent DCs is proposed and realized in 2017 [14]. It shows that the measured bandwidths are as large as ∼95 nm and ∼70 nm for achieving an ER of >25 dB and >30 dB, respectively, which is one of the best results reported for an ultra-compact PBS on silicon. Note that all these reported PBS are developed for the wavelength band of 1550 nm, which is currently the most important window for optical fiber communications.
On the other hand, recently the wavelength band of 2 μm and beyond is becoming more and more interesting as a new wavelength window for optical fiber communications potentially [15]. For example, hollow core photonic band gap fibers (HC-PBGF) and thulium-doped fiber amplifiers have been developed [16]. Furthermore, great success has been achieved for the development of low-loss silicon photonic waveguides [17–19], grating couplers [20], lasers [21], photodetectors [22] as well as modulators [23] operating at 2 μm. As it might be noticed, there are very few results for on-chip polarization-handling devices at 2 μm, which are needed for many systems. In [24], an ultra-broadband Ge-on-Si waveguide polarization rotator was demonstrated to work over a broad wavelength range of 9–11 μm. It can be seen that more efforts should be made for developing high-performance on-chip polarization-handling devices for the new wavelength-band of 2 μm.
In this paper, we propose and demonstrate a broadband high-performance PBS on silicon by utilizing a bent directional coupler assisted with a nano-slot waveguide. In this structure, the minimal nano-slot width is chosen as wslot=180 nm to satisfy the requirement for the standard foundry process. In theory, the designed PBS works very well with a low excess loss (EL) of <0.5 dB and a high extinction ratio (ER) of >15 dB in a broad band of 1825–2020 nm for TM polarization. For TE polarization, the PBS even works better with an EL of < 0.02 dB and an ER of >30 dB in the wavelength range of 1825–2020 nm. For the fabricated PBS, the measured results show that the ELs are low (∼0.5 dB) while the measured results show the ERs are >15 dB for both TM- and TE-polarizations in the wavelength band of 1860-1970nm.
2. Structure and design
Figures 1(a)−1(c) show the schematic configuration of the proposed PBS, which consists of two bent DCs in cascade. Each bent DC is designed with the combination of a silicon-on-insulator (SOI) strip waveguide and a nano-slot waveguide. Here, we choose the SOI wafer with a 220-nm-thick top-silicon layer with a 2-μm-thick buried-oxide-layer. For the SOI nanophotonic waveguides, there is a 1-μm-thick SiO2 upper-cladding. The refractive indices of Si and SiO2 are nSi = 3.453 and nSiO2 = 1.522 at the 1.92 μm, which is chosen regarding that the laser source at the 2 μm wavelength band available in our lab.
For the bent DCs, the widths (w1 and w2) of the strip waveguide and the nano-slot waveguide are determined according to the phase-matching condition for TM polarization [9], i.e., neff1 R1= neff2 R2, where R1 and R2 are the bending radii for the bent strip waveguide and the bent nano-slot waveguide, neff1 and neff2 are their effective indices. In this way, efficient cross-coupling can be achieved possibly for the TM-polarization mode when the angle-lengths (θ1 and θ2) of the coupling region are chosen appropriately. For TE polarization, on the other hand there is significant phase-mismatch between two bent waveguides since the nano-slot waveguide has a much lower effective index than the strip waveguide.
In order to guarantee sufficiently low bending loss in the bent DC, we calculate the bending losses for the TE-polarization mode in the strip waveguide and the TM-polarization mode in the nano-slot waveguide when choosing their core widths as w1=1.1 μm and w2=1.21 μm, 1.43 μm, 1.6μm with the nano-slot width wslot = 0.1 μm, 0.18 μm, 0.25 μm, respectively (which are choosing to keep the same phase-matching condition for TM polarization). Here, a simulation software (Lumerical MODE solution) was used to calculate the bending losses and the effective indices. Figures 2(a) and 2(b) show the calculated results as the bending radius R varies. Here, the nano-slot width is chosen as wslot=180 nm regarding the minimal feature size allowed by the foundry. It can be seen that the bending loss is less than 1 dB/cm for the TE-polarization mode in the strip waveguide as well as the TM-polarization mode in the nano-slot waveguide when choosing the radius as R>53 μm. As an example, we chose R1 = 60 μm.
Fig. 2. Calculated bending losses as the radius R varies (a) the TE-polarization mode in the strip waveguide; (b) the TM-polarization mode in the nano-slot waveguide.
Figures 3(a) and 3(b) show the calculated results (R1neff1 and R2neff2) for the TM- and TE-polarization modes of the strip waveguide and the nano-slot waveguide. Here, we choose R2 = 61.5 μm, while the corresponding gap width wgap is given by wgap =(R2−R1)−(w1+w2)/2, where w1 and w2 are the core widths for the strip waveguide and the nano-slot waveguide (as shown in Fig. 1(b)). According to the phase-matching condition for TM polarization (see Fig. 3(a)), the core-widths (w1 and w2) for the strip waveguide and the nano-slot waveguide are chosen to be w1=1.1 μm and w2=1.43 μm as an example, respectively. Correspondingly, the gap in the coupling region is wgap=235 nm, which is sufficiently large for the foundry. Meanwhile, the TE polarization modes in the strip waveguide and the nano-slot waveguide has significant phase-mismatch because the two bent waveguides have different birefringence. As a result, the TE-polarization mode launched from the input port just goes through the coupling region without coupling almost. In this way, the launched TE- and TM-polarization modes output from the through port and the cross port, respectively, and a PBS can be realized.
For the designed PBS, a three-dimensional finite-difference time-domain (3D-FDTD) method with non-uniform grid sizes (Lumerical FDTD) was then used to simulate the light propagation. Figures 4(a) and 4(b) show the calculated transmissions of TE- and TM-polarization modes in DC #1 operating in the wavelength band of 1.82–2.02 μm as the arc-length θ1 varies. From these figures, it can be seen that the transmission of the TE polarization mode is insensitive to the arc-length θ1 and there is little cross-coupling, as expected in theory. The designed PBS works very well for TE polarization in a broad bandwidth.
In contrast, the transmission of the TM polarization mode has some wavelength-dependence, which is expected for an evanescent coupling system. When choosing θ1=15.2°, the PBS works well with a high extinction ratio of 39.3 dB at the central wavelength of 1920 nm. The bandwidth BWER>20dB for achieving an ER of >20 dB is about ∼70 nm. In order to improve the PBS performance, another bent DC (#2) in cascade is introduced, as shown in Fig. 1(a). In this way, the residual power at the through port for TM-polarization can be filtered out. In order to separate the coupling region, here the angle θ2 is chosen to be larger than θ1. For example, we choose θ2 = 1.6θ1=24.3°. To make the PBS more compact, the bending radius R3 for the strip waveguide in DC #2 is chosen as R3=38 μm according to R3=R1(sinθ1/sinθ2), and one has R4=R3+(R2− R1) = 39.5 μm accordingly.
Figures 5(a) and 5(b) show the transmissions at the cross- and through-ports of the present PBS with DC #1 and DC #2 in cascade. Theoretically speaking, DC #2 does not introduce any influence on the transmission at the cross port for the launched TM- and TE-polarization modes. From Figs. 5(a) and 5(b), it can be seen that no notable excess loss is observed for the transmission of TE polarization at the through port when DC #2 is introduced. Meanwhile, the bandwidth BWER > 20dB for TM polarization is broadened from 70 nm to 135 nm, which is useful for the applications at the 2 μm wavelength band. Figure 5(c) shows the simulated light propagation for the launched TE- and TM-polarization modes in the present PBS when operating at λ=1820 nm, 1920 nm and 2020 nm, respectively. It can be seen DC #2 can effectively filter out the residual power of the TM polarization mode while the TE-polarization mode can go through DC #2 almost without any loss.
Fig. 5. Calculated results of the designed PBSs with DC #2. (a) TE; (b) TM. (c) Light propagation for TE and TM polarization in different wavelengths.
Finally, the nano-slot waveguide at the cross port is converted to a strip waveguide by using a mode converter based on waveguide tapers. Here, we chose the taper length as l1=16 μm, so that the nano-slot waveguide is converted to a 1-μm-wide strip waveguide adiabatically. In this way, the mode conversion loss is negligible. With such a design, the present PBS consisting of two bent DCs (#1 and #2) and a nano-slot-strip converter has a footprint of ∼10 × 48 μm2. The footprint can be shrunk by merging the mode converter and the bending section, which will be considered in the future work.
Here, we give an analysis for the fabrication tolerance of the designed PBS by assuming that there are variations (Δw, Δh) of the core widths and the core height. Basically speaking, the present PBS is insensitive to the dimension variations when working with TE polarization because the significant phase-mismatch remains even with some fabrication errors. Therefore, we focus on the analysis for the PBS working with TM polarization. Figure 6(a) shows the simulation results for the PBS working with TM polarization. From this figure, it can be seen that the PBS still works well even when Δw=±20 nm. Figure 6(b) shows the results for the PBS working with TM polarization when Δh=±10 nm. In this case, one can still achieve a bandwidth as broad as >150 nm for achieving an ER of >15 dB.
Fig. 6. Analysis for the fabrication tolerance of the present PBSs there are variations (Δw, Δh) of the core widths (a) and the core height (b).
3. Fabrication and measurement
The designed PBS was fabricated by the CUMEC (Chongqing United Microelectronics Center, China) with standard processes of deep-UV lithography and inductively coupled plasma dry-etching. A silica upper-cladding was deposited on the top. Figure 7(a) shows the microscope image of the fabricated PBS together with some test structures. The zoom-in view of the TE-type grating coupler, the PBS, and the TM-type grating coupler are shown in Figs. 7(b)–7(d), respectively. Here, the TE-type grating couplers are shallowly etched with an etching depth of 70 nm, while the TM-type grating couplers are fully etched.
Fig. 7. (a) Microscope images of the fabricated PBSs and test structures. Zoom-in view of (b) TE-type grating coupler, (c) the PBS, (d) and the TM-type grating coupler.
A broadband amplified spontaneous emission (ASE) light source (1810 ∼ 2010 nm) was used as the source and an optical spectrum analyzer (OSA) was applied as the receiver for the measurement of the transmissions at the output port. Figures 8(a)–8(b) show the measured transmissions at the through- and cross-ports of the fabricated PBS. Here, the measured transmissions were normalized with respect to the measured transmissions of the straight waveguide on the same chip. The straight waveguide at the bottom is with the TE-type grating and the one in the middle is with the TM-type grating. From Figs. 8(a)–8(b), it can be seen that the excess losses for both TE- and TM- polarization modes are very low (<0.5 dB) in the whole measured wavelength range, as expected by the theoretical simulation results. The ER of the TE polarization mode for the PBS becomes reduced when the wavelength is less than 1880 nm. This is due to the limitation of the detection sensitivity of the OSA and the bandwidth of the grating couplers. Nevertheless, the bandwidth BWER>20dB for TE-polarization is about 90 nm (1880-1970 nm). For TM polarization, note that the central wavelength with the highest ER of ∼28 dB is around 1950 nm, which has a red shift compared to the theoretical design. This is possibly attributed to some fabrication error and imperfect filling of the SiO2 upper-cladding in the nano-slot region. For the present PBS, the measured bandwidths BWER>15dB and BWER>20dB are ∼120 nm and ∼95 nm for TM polarization, respectively. The PBS performance can be improved in the future by further optimizing the fabrication process.
Fig. 8. Measured transmissions at the through- and cross-ports of the fabricated PBS for (a) TE polarization; (b) TM polarization.
4. Conclusions
In this paper, a PBS at the wavelength band of 2 μm has been proposed and experimentally demonstrated by utilizing a bent directional coupler assisted with a nano-slot waveguide. The core widths and the bending radii are chosen appropriately according the phase matching condition for the TM polarization modes. In this design, there is significant phase mismatch for TE polarization modes. For the designed PBS, the footprint is ∼10 × 48 μm2. Such a PBS has shown high performance with an EL of <0.02 dB and an ER of >30 dB in the wavelength range of 1825–2020 nm for TE polarization. For TM polarization, the PBS shows some wavelength dependence due to the intrinsic property of a DC, while it still works well with a low EL of <0.5 dB and a high ER of >15 dB in a broad band of 1825–2020 nm. The measured results have shown that the fabricated PBS has low ELs of ∼0.5 dB and decent ERs of >15 dB for both polarizations in the wavelength band of 1860-1980 nm. To the best of our knowledge, this is the first high-performance silicon PBS working at the wavelength band of 2 μm. In particular, the nano-slot width is chosen as wslot=180 nm due to the requirement of the standard foundry.
Funding
National Major Research and Development Program (2018YFB2200200); National Science Fund for Distinguished Young Scholars (61725503); National Natural Science Foundation of China (61961146003, 91950205).
Disclosures
The authors declare no conflicts of interest.
References
1. Y. Yin, Z. Li, and D. Dai, “Ultra-broadband polarization splitter-rotator based on the mode evolution in a dual-core adiabatic taper,” J. Lightwave Technol. 35(11), 2227–2233 (2017). [CrossRef]
2. X. Tu, M. Li, J. Xing, H. Fu, and D. Geng, “Compact psr based on an asymmetric bi-level lateral taper in an adiabatic directional coupler,” J. Lightwave Technol. 34(3), 985–991 (2016). [CrossRef]
3. K. Tan, Y. Huang, G. Q. Lo, C. Lee, and C. Yu, “Compact highly-efficient polarization splitter and rotator based on 90 degrees bends,” Opt. Express 24(13), 14506–14512 (2016). [CrossRef]
4. C. Li and D. Dai, “Compact polarization beam splitter for silicon photonic integrated circuits with a 340-nm-thick silicon core layer,” Opt. Lett. 42(21), 4243–4246 (2017). [CrossRef]
5. D. Dai and J. E. Bowers, “Novel ultra-short and ultra-broadband polarization beam splitter based on a bent directional coupler,” Opt. Express 19(19), 18614–18620 (2011). [CrossRef]
6. J. Wang, D. Liang, Y. Tang, D. Dai, and J. E. Bowers, “Realization of an ultra-short silicon polarization beam splitter with an asymmetrical bent directional coupler,” Opt. Lett. 38(1), 4–6 (2013). [CrossRef]
7. X. Guan, H. Wu, Y. Shi, and D. Dai, “Extremely small polarization beam splitter based on a multimode interference coupler with a silicon hybrid plasmonic waveguide,” Opt. Lett. 39(2), 259–262 (2014). [CrossRef]
8. W. Yang, Y. Xu, Y. Li, X. Wang, and Z. Wang, “A compact and wide-band polarization beam splitter based on wedge-shaped MMI coupler in silicon-on-insulator,” in Optical Fiber Communication Conference. Optical Society of America, 2015, p. W2A.12.
9. L. Xu, Y. Wang, A. Kumar, D. Patel, E. El-Fiky, Z. Xing, R. Li, and D. V. Plant, “Polarization beam splitter based on MMI coupler with SWG birefringence engineering on SOI,” IEEE Photonics Technol. Lett. 30(4), 403–406 (2018). [CrossRef]
10. L. Xu, Y. Wang, E. El-Fiky, D. Mao, A. Kumar, Z. Xing, M. G. Saber, M. Jacques, and D. V. Plant, “Compact broadband polarization beam splitter based on multimode interference coupler with internal photonic crystal for the SOI platform,” J. Lightwave Technol. 37(4), 1231–1240 (2019). [CrossRef]
11. H. Xu, D. Dai, and Y. Shi, “Ultra-broadband and ultra-compact on-chip silicon polarization beam splitter by using hetero-anisotropic metamaterials,” Laser Photonics Rev. 13(4), 1800349 (2019). [CrossRef]
12. C. Li, M. Zhang, J. E. Bowers, and D. Dai, “Ultra-broadband polarization beam splitter with silicon subwavelength-grating waveguides,” Opt. Lett. 45(8), 2259–2262 (2020). [CrossRef]
13. Y. Xu and J. Xiao, “Compact and high extinction ratio polarization beam splitter using subwavelength grating couplers,” Opt. Lett. 41(4), 773–776 (2016). [CrossRef]
14. H. Wu, Y. Tan, and D. Dai, “Ultra-broadband high-performance polarizing beam splitter on silicon,” Opt. Express 25(6), 6069–6075 (2017). [CrossRef]
15. R. Soref, “Enabling 2 μm communications,” Nat. Photonics 9(6), 358–359 (2015). [CrossRef]
16. N. Ali and R. Kumar, “Mid-infrared non-volatile silicon photonic switches using nanoscale Ge2Sb2Te5 embedded in silicon-on-insulator waveguides,” Nanotechnology 31(11), 115207 (2020). [CrossRef]
17. R. Soref, “Mid-infrared photonics in silicon and germanium,” Nat. Photonics 4(8), 495–497 (2010). [CrossRef]
18. N. Singh, A. Casas-Bedoya, D. D. Hudson, A. Read, E. Magi, and B. J. Eggleton, “Mid-ir absorption sensing of heavy water using a silicon-on-sapphire waveguide,” Opt. Lett. 41(24), 5776–5779 (2016). [CrossRef]
19. H. Lin, Z. Luo, T. Gu, L. C. Kimerling, K. Wada, A. Agarwal, and J. Hu, “Mid-infrared integrated photonics on silicon: A perspective,” Nanophotonics 7(2), 393–420 (2017). [CrossRef]
20. A. Malik, E. J. Stanton, J. Liu, A. Spott, and J. E. Bowers, “High performance 7-8 ge-on-si arrayed waveguide gratings for the midinfrared,” IEEE J. Sel. Top. Quantum Electron. 24(6), 1–8 (2018). [CrossRef]
21. J. Margetis, S. Al-Kabi, W. Du, W. Dou, Y. Zhou, T. Pham, P. Grant, S. Ghetmiri, A. Mosleh, B. Li, J. Liu, G. Sun, R. Soref, J. Tolle, M. Mortazavi, and S.-Q. Yu, “Si-based gesn lasers with wavelength coverage of 2–3 μm and operating temperatures up to 180 k,” ACS Photonics 5(3), 827–833 (2018). [CrossRef]
22. Y. Yin, R. Cao, J. Guo, C. Liu, J. Li, X. Feng, H. Wang, W. Du, A. Qadir, H. Zhang, Y. Ma, S. Gao, Y. Xu, Y. Shi, L. Tong, and D. Dai, “High-speed and high-responsivity hybrid silicon/black-phosphorus waveguide photodetectors at 2 µm,” Laser Photonics Rev. 13(6), 1900032 (2019). [CrossRef]
23. W. Cao, D. Hagan, D. J. Thomson, M. Nedeljkovic, C. G. Littlejohns, A. Knights, S.-U. Alam, J. Wang, F. Gardes, W. Zhang, S. Liu, K. Li, M. S. Rouifed, G. Xin, W. Wang, H. Wang, G. T. Reed, and G. Z. Mashanovich, “High-speed silicon modulators for the 2 μm wavelength band,” Optica 5(9), 1055 (2018). [CrossRef]
24. K. Gallacher, R. W. Millar, U. Griškevičiūtė, M. Sinclair, M. Sorel, L. Baldassarre, M. Ortolani, R. Soref, and D. J. Paul, “Ultra-broadband mid-infrared ge-on-si waveguide polarization rotator,” APL Photonics 5(2), 026102 (2020). [CrossRef]
