Open Access
Add to BibSonomy
Abstract
In this paper, we experimentally demonstrate an ultra-broadband high-performance polarization beam splitter (PBS) based on silicon-on-insulator (SOI) platform. The proposed device is based on a directional coupler consisting of a 70-nm taper-etched waveguide and a slot waveguide with a compact coupling length of 11 microns, the structure of which is suitable for a commercial two-step fabrication process. Benefiting from the preferences of coupling TM mode to slot waveguide and restricting TE mode in taper-etched waveguide, the polarization extinction ratios (PER) for TE and TM polarizations can reach as high as 30 dB and 40 dB at 1550 nm based on experimental results, respectively; besides, an ultra-wide operation bandwidth with PER >20 dB is achieved as ~175 nm from 1450 nm to 1625 nm (covering S, C and L bands), or the bandwidth with PER >25 dB is over ~120 nm from 1462 nm to 1582 nm, which is the largest operation bandwidth to the best of our knowledge. At last, the insertion losses (IL) are −0.17 dB and −0.22 dB for TE and TM polarizations at 1550 nm, respectively.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Silicon-on-insulator (SOI) waveguide devices has attracted much interest recently due to their low power consumption, high-density integration and the complementary metal oxide semiconductor (CMOS) compatibility. High refractive index contrast between the silicon and silicon dioxide enables the waveguide to achieve an ultra-compact footprint of nanometer-scale, which is highly desirable for the system integration. However, this property also results in high birefringence for the silicon waveguides, leading to polarization-dependent properties in the waveguides. Therefore, the polarization diversity devices, such as polarization beam splitters (PBSs), polarization rotators (PRs) [1,2] and polarization splitter-rotators (PSRs) [3], are essential for the silicon photonic integrated circuits (PICs); they also perform as a crucial unit for many applications, including coherent optical communication [4,5] and quantum communication [6].
So far, a number of schemes have been proposed to achieve PBSs, such as PBSs based on multimode interference (MMI) [7], on grating-assistant structure [8,9], on Mach-Zehnder interference (MZI) [10] and on directional coupler (DC) [11–20], etc. Among them, the asymmetric DC (ADC) structure has been considered as a preferred candidate because of its significant birefringence that makes it potential to achieve a very high polarization extinction ratio (PER). However, one major barrier for this type of PBSs is their limited operation bandwidth, due to that the coupling length is sensitive to wavelength variation. To solve this problem, several novel ADC structures have been proposed in recent years. For example, in [18], a silicon PBS based on a three-waveguide directional coupler was proposed and broad bandwidth of 70 nm with PER > 20 dB was experimentally realized. In [15], a broadband PBS was proposed utilizing a triple-bend-waveguide DC, which extends the bandwidth to 90 nm with PER > 20 dB for both polarizations. In [16], the bandwidth of the proposed PBS improves furtherly to ~135nm with PER > 20 dB by carefully designing the cascaded bend DC, which is a remarkable result for the compact PBSs on silicon.
In this paper, we propose an SOI-based high-performance PBS with a very large operation bandwidth. A 70 nm taper-etched waveguide and a 220 nm etched slot waveguide are utilized to form the asymmetric DC structure, as these etching depths are standardly provided by foundries during the multi-project wafer (MPW) services. The coupling length is as compact as 11 microns, besides the structure is robust to the size variation according to the simulation analyses. After the optimization for the structure, the device was then fabricated and measured. The experimental results show that very high PERs of 30 dB and 40 dB for TE and TM polarizations at 1550 nm are achieved, with quite low insertion losses (IL) of −0.17 dB and −0.22 dB, respectively. The operation bandwidth is as large as ~175 nm (~120 nm) for PER > 20 dB (25 dB), which is the largest bandwidth that has been reported for the SOI-based PBS to the best of our knowledge
2. Structure and design
The 3D schematic of our proposed device is shown in Fig. 1(a), which consists of a 70 nm taper-etched waveguide ending with a through output port, and a slot waveguide ending with a cross output port. Between them, the slot waveguide is utilized for preventing the coupling of TE mode [20], and the taper-etched waveguide is utilized for coupling TM mode to the slot waveguide over a broad bandwidth. The device is designed for a SOI wafer which has a 220 nm-thick top silicon layer on a 2 μm-thick buried oxide (BOX) layer, as shown in Fig. 1(b). To keep with the standard integrated photonic design protocols, the whole device is covered with an upper silica cladding.
A finite-difference eigenmode method (FDE, Lumerical Inc.) was used to calculate the mode field profiles and the propagation constants for all eigenmodes in the slot waveguide and the taper-etched waveguide. Figure 2(a) shows the effective indices of TE mode in the taper-etched waveguide and the slot waveguide at different wavelengths. Three wavelengths of 1450 nm, 1550 nm and 1625 nm were chosen to analyze the device performance relating to the wavelength variation from 1450 nm to 1625 nm, as the value of effective index diminishes continuously when the wavelength increases, leading to a continuous performance variation during this wavelength range. The width of the taper on the taper-etched waveguide (Wt) varies from 450 nm to zero, leading to the height of the waveguide changing from 220 nm to 150 nm; while the height of the slot waveguide is fixed at 220 nm and the width is fixed at 540 nm within which there is a 180-nm-wide gap. As shown in Fig. 2(a), the effective indices of the TE mode in the slot waveguide are below 1.55 for all wavelengths; on the other hand, the effective indices in the taper-etched waveguide are all above 1.91. So, there is a significant phase mismatch of TE mode between the waveguides [20], and the TE mode can be strongly restricted in the taper-etched waveguide and outgoes at the through port when it is injected into the input port. However, as shown in Fig. 2(b), the effective indices of the TM mode in the slot waveguide are fixed at 1.65, 1.60 and 1.57 for wavelengths of 1450 nm, 1550 nm and 1625 nm, respectively; there are intersections of indices for all the wavelengths as Wt varies from 200 nm to 170 nm, which means that phase matching occurs between the slot waveguide and the taper-etched waveguide for TM polarization at the wavelength range from 1450 nm to 1625 nm. Therefore, when TM mode is injected into the input port, it is efficiently coupled into the slot waveguide and outgoes at the cross port. Note that, the effective index difference between TE and TM mode in the taper-etched waveguide is always kept in a high value as Wt varies, by which polarization rotation is prevented in this waveguide.
Fig. 2 The calculated effective indices of (a) TE mode and (b) TM mode for the taper-etched waveguide and slot waveguide as a function of Wt at wavelengths of 1450 nm, 1550 nm and 1625 nm.
The optical power extinction ratios (ERs) between the through port and the cross port as functions of wavelength and main structure parameters (width of the slot waveguides W2, gap of the slot waveguide Ws, gap between slot and taper-etched waveguides Wg, and length of the taper-etched waveguide Lc) were calculated using 3D finite-difference time-domain method (3D FDTD, Lumerical Inc.). During the simulations, other parameters were fixed at the values listed in Table 1, and the width of taper Wt varies from 450 nm to 0 nm in all cases. The calculated ERs are presented in a form of 2D contour maps as shown in Figs. 3(a)-(d). The values listed in Table 1 are marked by the dash lines in the figures, from where we can see that the values are optimal with a balanced consideration of the ERs for both TE and TM. Note that, we can also see from Fig. 3 that, compared to the optimized structure, a high ER of > 20 dB for both polarizations at a broad wavelength range from 1450 nm to 1625 nm is still maintained with a variation for ΔW2 of ± 60 nm, ΔWs of ± 80 nm, ΔWg of ± 30 nm and ΔLc of ± 350 nm, indicating that the device is highly tolerant to the fabrication imperfections.
Table 1. Main parameters of PBS.
Fig. 3 The calculated ER values as functions of wavelength and parameters such as (a) width of the slot waveguides W2; (b) gap of the slot waveguide Ws; (c) gap between slot and taper-etched waveguides Wg; (d) length of the taper-etched waveguide Lc for both TE and TM modes.
The guided power distribution along the designed device for TE and TM modes are shown in Figs. 4(a) and 4(b), respectively. We can see that TE mode passes through the DC and outputs via the through port, whereas the TM mode gradually couples into the slot waveguide and outputs via the cross port. When TE (or TM) mode is injected, the transmission of corresponding TE (or TM) mode from either through or cross port over the band from 1450 to 1625 nm has been calculated and shown in Fig. 4(c), where the black and red lines indicate the transmissions of TE-to-TE mode out from the through port and the cross port, respectively; the blue and pink lines indicate the transmissions of TM-to-TM mode out from the through port and the cross port, respectively. We can see that transmissions for both modes at their corresponding ports are higher than −0.6 dB, and the transmissions from the undesired ports are lower than −25 dB.
Fig. 4 Optical field distribution when (a) TE mode or (b) TM mode is injected into the PBS. (c) Transmissions of TM (or TE) mode from either through or cross port over the band from 1450 to 1625 nm when TM (or TE) mode is injected. (d) Transmissions of TE (or TM) mode from either through or cross port over the band from 1450 to 1625 nm when TM (or TE) mode is injected.
When TE (or TM) mode is injected, the transmission of TM (or TE) mode from either through or cross port over the band has been calculated and shown in Fig. 4(d), where the black and red lines indicate the transmissions of TE-to-TM mode out from the through port and the cross port, respectively; the blue and pink lines indicate the transmissions of TM-to-TE mode out from the through port and the cross port, respectively. We can see few polarization rotation of TE-to-TM mode as the output power of the TE-to-TM modes from both ports are below −30 dB when TE is injected. Whereas when the TM mode is launched, a little part of the mode rotates its polarization direction, coverts to a TE-like mode and outgoes from the cross port, as shown in Fig. 5(a). The power of the TM-to-TE mode from the cross port is ~-18 dB, much higher than the TM-to-TE mode from the through port of below −30 dB.
Fig. 5 Affection of the filter. When TM mode is injected, the propagation of TE polarization (a) without filter or (b) with filter; and the propagation of TM polarization (c) without filter or (d) with filter. (e) The comparison for the transmission of rotated polarization with/without filter when TM mode is injected. (d) Affection on the PER and IL when filter is added.
To eliminate the TM-to-TE mode, a simple way is adding a filter next to the slot waveguide. The filter is actually a rectangular waveguide with a height of 150 nm (70-nm etched), a width of 270 nm and a length of 9 μm, with which TE mode in the slot waveguide can couple to the filter and cuts off at the end facet, as shown in Fig. 5(b). Meanwhile, there is few influence on the propagation of TM mode because of the phase mismatch, as shown in Figs. 5(c)-(d).
The transmissions of TM-to-TE mode out from through and cross port with/without filter were calculated, as shown in Fig. 5(e), where the red and black lines indicate the change of TM-to-TE mode at the cross port and at the through port, respectively. By utilizing this filter, the power of the TM-to-TE mode at the cross port decreases from −18 dB to −25 dB, meanwhile the power of the TM-to-TE mode at the through port is still lower than −30 dB, indicating that there is a significant effect when the filter is added. Figure 5(f) shows the influence on PER and IL when the filter is added. The red and yellow lines indicate the change of PER and IL for TE polarization, we can see that there is negligible influence on the device for the TE delivery with/without filter. However, a significant change of PER for TM mode occurs when the filter is added, and the IL increases about 0.3 dB during the whole band, which are indicated by the black and blue lines, respectively. The increasement of IL is due to the reason that the TM-to-TE mode is eliminated by the filter, leading to a decline of the total transmission power. However, the decline of the PER for TM mode is mainly due to the change of TM transmission out from through port (namely crosstalk). The crosstalk is generated by the slight coupling effect between the slot waveguide and the strip waveguide with height of 150 nm (including the S-bend) after the TM mode has been coupled from the taper-etched waveguide to the slot waveguide. When the filter is added, there is an impact on the coupling between the taper-etched waveguide and the slot waveguide, which would furtherly influence the coupling between the slot waveguide and strip waveguide, leading to a power variation of crosstalk. By calculation, the variation of crosstalk is quite low with values of around −30 dBm, but a significant change of PER can be induced by this variation because the PER is very high. For example, the PER decreases from 47 dB to 32 dB at 1515 nm with a crosstalk increasement of −32.42 dBm, while the PER decreases by only 3 dB at 1625 nm with a larger crosstalk increasement of −27.22 dBm, due to the lower PER. However, when the filter is added, the PER for TM mode is still higher than 20 dB, and the IL is below −1 dB during the whole band.
Moreover, after being separated by the proposed PBS, the TE/TM mode should be injected into the standard single mode waveguide of 220 nm height for further processing. Since the height of the cross port is fixed as 220 nm, its width is designed to be tapered from 540 nm to 450 nm for single mode propagation. However, the height of the through port is 150 nm, hence there is an abrupt change of height between the through port and standard output waveguide, this affection on the TE/TM propagation is also analyzed by simulation. Figure 6(a) shows the top view of the optical distribution when TE mode propagates from the waveguide of 450 nm wide and 150 nm height to the waveguide of 220 nm height. Figure 6(b) shows the side view of the propagation. The effective indices (Neff) of TE mode propagating in the waveguide with different height of 150 nm and 220 nm are calculated as 2.01 and 2.35 at 1550 nm, respectively. The transmission and reflection of TE mode were calculated and shown in Fig. 6(c), where we can see that the propagation loss is ~-0.2 dB and the reflection is as low as around −30 dBm over the whole band, indicating that the influence of the height step is slight for TE mode. Figure 6(d) shows the top view of the optical distribution for the TM propagation, and Fig. 6(e) shows the side view of the propagation as well. The effective indices of TM mode propagating in the waveguide with different height are calculated as 1.51 and 1.76 at 1550 nm, respectively. The transmission and reflection of TM mode were calculated and shown in Fig. 6(f), where we can see that the propagation loss is above −3 dB and the loss increases in the long wavelength range, due to the low index when TM mode is propagating in the lower waveguide. However, since the abrupt change of height takes place at the through port, which is used to output the TE mode, the influence of height step on the device performance is negligible. Besides, TM mode out from the through port can be eliminated furtherly by the lower waveguide according to the simulation results.
Fig. 6 Analysis for the affection of the abrupt change at height. When TE mode is injected, (a) the top view and (b) the side view of the propagation. (c) The calculated transmission and reflection of the propagation for TE mode. When TM mode is injected, (d) the top view and (e) the side view of the propagation. (f) The calculated transmission and reflection of the propagation for TM mode.
3. Fabrication and measurement
The proposed device was fabricated by the commercially available MPW service, and it was processed on an SOI wafer with a 220-nm top silicon layer and a 2-μm buried oxide layer. The deep ultraviolet (UV) photolithography was used to define the pattern of the devices and the inductively coupled plasma (ICP) was used to etch the silicon core layer. Finally, a silica upper-cladding was deposited on the structure by a plasma enhanced chemical vapor deposition (PECVD) process. Figure 7(a) shows the optical micrograph of the fabricated PBS, and the Fig. 7(b) shows the detailed structure of the device. Grating couplers are used at the ends of waveguides to couple input and output light beams to SMF fibers, the periods of the grating couplers for the TE and TM modes are 620 nm and 984 nm, respectively. Fill factor and etching depth of the grating couplers are 0.5 and 70 nm, respectively.
Fig. 7 (a) Optical micrograph of the fabricated PBS and (b) the detailed structure. (c) The coupling loss for TE and TM couplers and the output spectrum of TL. (d) Optical micrograph of the reference waveguide.
A tunable laser (TL, from 1440 nm to 1640 nm) was used as the source and the wavelength of input light was tuned from 1450 nm to 1625 nm with a step of 2 nm. An optical spectrum analyzer (OSA) was used to measure the optical power at certain wavelength, and the measured power out from TL is shown as the blue line in Fig. 7(c). We can see that there is an attenuation of input power from 8.5 dBm to 0 dBm when the wavelength decreases from 1550 nm to 1450 nm. The reference single mode waveguides, which are shown in Fig. 7(d), were utilized to characterize the transmission of TE and TM couplers, as shown by the black and red lines in Fig. 7(c), respectively. The peak coupling efficiency for both couplers are located at 1560 nm, with a coupling loss of 5 dB/facet and 7 dB/facet for TE and TM couplers, respectively. In particular, the transmission power from reference waveguides drops to below −30 dB when the wavelength is beyond a wavelength range from1490 nm to 1610 nm for both couplers, due to the wavelength sensitivity of the grating structure. In order to characterize the transmission of the fabricated device, four identical PBSs were fabricated but connecting with different combinations of TE and TM-type grating couplers (TE-to-TE, TM-to-TM, TE-to-TM, and TM-to-TE). Between them, the PER for TE or TM mode can be obtained by measuring the devices with TE-to-TE/TM-to-TM couplers, and the power of rotated polarization can be analyzed by measuring the devices with TE-to-TM/TM-to-TE couplers. Here, to eliminate the influence of the power attenuation of TL and the coupling efficiency of grating couplers, the transmission power at each wavelength was calculated as
where POSA, PTL and Lcoupler represent the measured power by OSA, the reference power of TL and the coupling loss of grating coupler, respectively. Since the detection sensitivity of OSA is around −65 dBm, the measured power of crosstalk would be higher than true value when the coupling loss becomes very huge, leading to a decline of PER when the wavelength is far away from 1560 nm.The measurement results are shown in Fig. 8 and Fig. 9. Figure 8(a) shows the measured transmission spectra of the beams output from the through and cross waveguides for TE-to-TE / TM-to-TM polarized lights, while Fig. 8(b) shows the corresponding PER over the band from 1450 nm to 1625 nm. It can be seen that the PER at 1550 nm is ~30 dB and ~40 dB for TE and TM polarizations, and the measured ILs are −0.17 dB and −0.22 dB, respectively. For both polarizations, the PER declines when the wavelength is beyond a wavelength range from1490 to 1615 nm, which is partly due to the limitation of the detection sensitivity of the OSA and the bandwidth of the grating couplers. However, the bandwidth for a PER of > 20 dB (25 dB) is still large as ~175 nm (~120 nm), with the IL of below −2 dB (−1 dB). Besides, the measured PER for TM polarization is high as above 30 dB from 1475 nm to 1625 nm. A comparison for the silicon-based high-performance PBSs demonstrated in recent years is given in Table 2, from where we can see that the largest bandwidth for PER > 20 dB and PER > 25 dB with compact footprint has been achieved in this work.
Fig. 8 (a) Measured transmissions for TE and TM modes from their corresponding ports. (b) Measured PER over the whole band when TE or TM mode is injected.
Table 2. Comparison of high-performance PBSs demonstrated in recent years.
Figure 9 shows the transmission spectra for TE-to-TM/TM-to-TE lights. The power of the TM-to-TE mode out from cross port is ~-20 dB and slightly stronger than other rotated polarizations, which agrees with the simulation results. The measured power of rotated polarizations for TE-to-TM (from both output ports) and TM-to-TE (from through port) increase when the wavelength is beyond a wavelength range from1490 nm to 1615 nm, which is also due to the limitation of the detection sensitivity of the OSA and the bandwidth of the grating couplers. However, all the rotated polarizations are weaker than −20 dB over most of the band, which means the effect of rotated polarizations is quite small. To relieve the crosstalk, a filter mentioned in previous analyses would be used in our further work.
4. Conclusions
We have proposed and experimentally demonstrated an ultra-broadband high-performance PBS based on the asymmetric DC structure consisting of a 70-nm taper-etched waveguide and a slot waveguide. The coupling length is compact as 11 μm, and the measured PER at 1550 nm is ~30 dB and ~40 dB for TE and TM polarizations, respectively. The bandwidth for a PER of > 20 dB, 25 dB are large as ~175 nm and ~120 nm, with the IL of below −2 dB and −1 dB, respectively. To the best of our knowledge, this is the best results reported for the SOI-based PBS. The broadband PBS is expected to have significant applications in photonic integratable optical communication links and optical interconnection networks.
Funding
National Natural Science Foundation of China (NSFC) (61471054, 61875020, 61335009, 61505011, 61475022, 61331008); Program 863 (2015AA015503); Program 973 (2014CB340100).
References
1. A. Barh, B. M. A. Rahman, R. K. Varshney, and B. P. Pal, “Design and Performance Study of a Compact SOI Polarization Rotator at 1.55 μm,” J. Lightwave Technol. 31(23), 3687–3693 (2013). [CrossRef]
2. Y. Kim, D. W. Kim, M. Lee, M. H. Lee, D. E. Yoo, K. N. Kim, S. C. Jeon, and K. H. Kim, “Demonstration of integrated polarization rotator based on an asymmetric silicon waveguide with a trench,” J. Opt. 18(9), 095801 (2016). [CrossRef]
3. Y. Xiong, D. X. Xu, J. H. Schmid, P. Cheben, S. Janz, and W. N. Ye, “Fabrication tolerant and broadband polarization splitter and rotator based on a taper-etched directional coupler,” Opt. Express 22(14), 17458–17465 (2014). [CrossRef] [PubMed]
4. P. Dong, X. Liu, S. Chandrasekhar, L. L. Buhl, R. Aroca, and Y. K. Chen, “Monolithic silicon photonic integrated circuits for compact 100 + Gb/s coherent optical receivers and transmitters,” IEEE J. Sel. Top. Quantum Electron. 20(4), 150–157 (2014). [CrossRef]
5. P. Dong, Y. K. Chen, G. H. Duan, and D. T. Neilson, “Silicon photonic devices and integrated circuits,” Nanophotonics 3(4–5), 215–228 (2014).
6. L. T. Feng, M. Zhang, Z. Y. Zhou, M. Li, X. Xiong, L. Yu, B. S. Shi, G. P. Guo, D. X. Dai, X. F. Ren, and G. C. Guo, “On-chip coherent conversion of photonic quantum entanglement between different degrees of freedom,” Nat. Commun. 7(1), 11985 (2016). [CrossRef] [PubMed]
7. X. Sun, J. S. Aitchison, and M. Mojahedi, “Realization of an ultra-compact polarization beam splitter using asymmetric MMI based on silicon nitride / silicon-on-insulator platform,” Opt. Express 25(7), 8296–8305 (2017). [CrossRef] [PubMed]
8. Y. Zhang, Y. He, J. Wu, X. Jiang, R. Liu, C. Qiu, X. Jiang, J. Yang, C. Tremblay, and Y. Su, “High-extinction-ratio silicon polarization beam splitter with tolerance to waveguide width and coupling length variations,” Opt. Express 24(6), 6586–6593 (2016). [CrossRef] [PubMed]
9. H. Qiu, J. Jiang, P. Yu, J. Yang, H. Yu, and X. Jiang, “Broad bandwidth and large fabrication tolerance polarization beam splitter based on multimode anti-symmetric Bragg sidewall gratings,” Opt. Lett. 42(19), 3912–3915 (2017). [CrossRef] [PubMed]
10. T. K. Liang and H. K. Tsang, “Integrated polarization beam splitter in high index contrast silicon-on-insulator waveguides,” IEEE Photonics Technol. Lett. 17(2), 393–395 (2005). [CrossRef]
11. D. W. Kim, M. H. Lee, Y. Kim, and K. H. Kim, “Planar-type polarization beam splitter based on a bridged silicon waveguide coupler,” Opt. Express 23(2), 998–1004 (2015). [CrossRef] [PubMed]
12. Z. Ying, G. Wang, X. Zhang, H. P. Ho, and Y. Huang, “Ultracompact and broadband polarization beam splitter based on polarization-dependent critical guiding condition,” Opt. Lett. 40(9), 2134–2137 (2015). [CrossRef] [PubMed]
13. T. Zhang, X. Yin, L. Chen, and X. Li, “Ultra-compact polarization beam splitter utilizing a graphene-based asymmetrical directional coupler,” Opt. Lett. 41(2), 356–359 (2016). [CrossRef] [PubMed]
14. 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] [PubMed]
15. J. R. Ong, T. Y. L. Ang, E. Sahin, B. Pawlina, G. F. R. Chen, D. T. H. Tan, S. T. Lim, and C. E. Png, “Broadband silicon polarization beam splitter with a high extinction ratio using a triple-bent-waveguide directional coupler,” Opt. Lett. 42(21), 4450–4453 (2017). [CrossRef] [PubMed]
16. H. Wu, Y. Tan, and D. Dai, “Ultra-broadband high-performance polarizing beam splitter on silicon,” Opt. Express 25(6), 6069–6075 (2017). [CrossRef] [PubMed]
17. F. Zhang, H. Yun, Y. Wang, Z. Lu, L. Chrostowski, and N. A. Jaeger, “Compact broadband polarization beam splitter using a symmetric directional coupler with sinusoidal bends,” Opt. Lett. 42(2), 235–238 (2017). [CrossRef] [PubMed]
18. Y. Kim, M. H. Lee, Y. Kim, and K. H. Kim, “High-extinction-ratio directional-coupler-type polarization beam splitter with a bridged silicon wire waveguide,” Opt. Lett. 43(14), 3241–3244 (2018). [CrossRef] [PubMed]
19. C. Errando-Herranz, S. Das, and K. B. Gylfason, “Suspended polarization beam splitter on silicon-on-insulator,” Opt. Express 26(3), 2675–2681 (2018). [CrossRef] [PubMed]
20. D. Dai, Z. Wang, and J. E. Bowers, “Ultrashort broadband polarization beam splitter based on an asymmetrical directional coupler,” Opt. Lett. 36(13), 2590–2592 (2011). [CrossRef] [PubMed]
