We propose and demonstrate an ultra-broadband polarization splitter-rotator (PSR) on the Silicon-On-Insulator (SOI) platform. The proposed PSR consists of a straight multi-mode waveguide, an asymmetrical directional coupler and a bent directional coupler. The multi-mode waveguide enables highly-efficient TM0-TE1 polarization rotation. The excited TE1 mode is then converted to be TE0 mode by the asymmetrical directional coupler. The remained TM0 mode is filtered out by the bent directional coupler. On the other hand, the incident light of TE0 mode goes through the PSR with negligible conversion and coupling. Only one-step etching is required for the proposed PSR. The fabricated PSR shows a high extinction ratio > 30.82 dB and a low loss < 0.57 dB at the central wavelength. The extinction ratio is > 20 dB over an ultra-broad wavelength band > 85 nm.
© 2017 Optical Society of America
The Silicon-On-insulator (SOI) is considered to be a promising platform to develop the ultra-compact integrated optical devices taking advantages of the CMOS compatibility and the high index contrast between the Si (nSi ~3.46) and SiO2 (nSiO2 ~1.45). However, the birefringence is significant for the silicon nanowire waveguide and the SOI-based devices are usually polarization dependent. The polarization independent devices are preferred, but such devices are usually difficult to design and realize. Over the recent years, the polarization diversity circuits have been proposed to solve this problem. The two polarizations are firstly separated into two independent circuits, so that the devices could work at single polarization state, and finally the two polarization states are combined. Numerous polarization handling devices have been proposed and demonstrated, including the polarizers [1,2], the polarization beam splitters (PBSs) [3,4], the polarization rotators (PRs) [5,6] and the polarization splitter-rotators (PSRs) [7–12]. Among them, the PSR combines the advantages of the PBS and PR. By utilizing the PSR, the TE and TM components could be separated into two individual waveguides. Moreover, one of the polarization states will be converted to the orthogonal one, so that the beams in the two output waveguides are at the same polarization state. Thus, the whole circuits can be designed for working at only one polarization state.
Several PSRs have been theoretically and experimentally demonstrated, including the ones based on the cut-cornered structures [7,8], the asymmetrical directional couplers (ADCs) [9,10] and the adiabatic taper structure [11,12]. The cut-cornered structures could provide low losses and high extinction ratios. However, two-step etching is required for this kind of PSR, which makes the fabrication quite complex. Only one-step etching is required for the ADC-based PSRs, but the application of the ADC-based PSR is limited by the relatively narrow wavelength band. The adiabatic taper structures are commonly used in the PRs and the PSRs for the low losses and easy fabrication. The major drawbacks for the taper-based PSRs are the large device size and limited bandwidth.
In this paper, we propose and demonstrate an ultra-broadband PSR based on the multi-mode waveguide. The proposed PSR consists of a straight multi-mode waveguide, an asymmetrical directional coupler (ADC) and a bent directional coupler (BDC), as shown in Fig. 1(a). The linearly tapered waveguides are used as connections between each section. The first multi-mode waveguide section serves as a polarization rotator where the incident TM0 mode is converted to be the TE1 mode. After that, an ADC is cascaded to convert the excited TE1 mode to be TE0 mode and couple out from the cross (CRO) port. Another BDC is cascaded after the ADC to filter out the residual TM0 mode. On the other hand, the input light of TE0 mode will go through the PSR and couple out from the through (TRU) port with negligible conversion and coupling due to the phase mismatch shown in Fig. 1(b). By applying such cascaded structure, the working wavelength band could be greatly enhanced without introducing much additional loss. Furthermore, only one-step etching is required for the proposed PSR, which indicates an easy fabrication process.
2. Design principles
In this paper, the device is designed on the Silicon-On-Insulator (SOI) platform with a 220-nm thick silicon top layer and a 2-μm thick oxide buffer layer. The cladding is chosen to be air for the demand of symmetry breaking. The TM0-TE1 polarization conversion is realized by using a straight multi-mode waveguide. The TM0 mode and the TE1 mode could be hybridized in the multi-mode waveguide with an ultra-short beat length . By carefully choosing the width W1 and length L2 of the multi-mode waveguide, the input TM0 mode could be completely converted to TE1 mode and output from the second taper section. Figure 1(b) shows the dispersion curves for the TE0, TE1 and TM0 modes. The effective indices are calculated at the central wavelength of 1.55 μm by using Finite Element Method (FEM). It can be seen that the effective index difference between TM0 and TE1 mode is minimum at the width of 660 nm. Thus, the multi-mode waveguide width is chosen to be W1 = 660 nm in order to enhance the mode hybridization.
The input waveguide width is chosen to be W0 = 400 nm to satisfy the single-mode condition. In the ADC region, the multi-mode waveguide width is chosen to be W2 = 810 nm to obtain an efficient coupling. The length of the second taper is chosen to be L3 = 2 μm to ensure a lossless connection. The first taper length L1 and the multi-mode waveguide length L2 are then optimized by utilizing the Eigen Mode Expansion (EME) method [shown in Figs. 2(a) and 2(b)]. It could be seen from Fig. 2(b) that the residual power of TM0 mode is as small as ~-33.33 dB with L1 = 2.5 μm and L2 = 6.1 μm, while the TM0-TE1 conversion efficiency is as high as ~95% [see Fig. 2(a)]. Thus, the first taper length and the multi-mode waveguide length are chosen to be L1 = 2.5 μm and L2 = 6.1 μm, respectively. Figures 2(c) and 2(d) show the light propagation profiles in the rotator section for TE0 and TM0 modes, respectively. It could be observed that the launched TM0 mode could be efficiently converted to TE1 mode, while the input TE0 mode could maintain the polarization and propagate through the rotator with negligible loss. The ADC is then exploited to realize the TE1-TE0 mode conversion. The single-mode waveguide width is chosen to be W3 = 400 nm according to the phase match condition [see Fig. 1(b)]. The gap width in the ADC region is chosen to be Wg,1 = 175 nm to ensure an efficient coupling as well as an easy fabrication. The coupling length L4 is then optimized by using the 3D Finite-Difference Time Domain (FDTD) method, and finally chosen to be L4 = 22.2 μm. An adiabatic taper with L5 = 3 μm connects the coupling region and the output single-mode waveguide.
By cascading the multi-mode waveguide section and the ADC section, the input TM0 and TE0 mode could be efficiently separated, and the TM0 mode could be converted to TE0 with low loss and high extinction ratio. However, the wavelength band is still quite limited mainly due to the strong wavelength sensitivity for the SOI waveguide. When the working wavelength deviates from 1.55 μm, the effective indices of TM0 and TE1 mode are not perfectly matched anymore, thus, the TM0-TE1 mode conversion could be incomplete . Consequently, part of the TM0 mode would propagate into the TRU port, which could lead to the decrease of the working bandwidth.
To reduce the TM0 mode output at the TRU port and further improve the performance of the PSR, a BDC-based filter is cascaded after the ADC. For the proposed BDC, the light of TM0 mode goes across the coupler and is then coupled into the radiation mode by a short taper with the length of L7 = 2.5 μm. The coupling is inhibited for the TE0 mode because of the phase mismatch between the two centrically bent waveguides in the BDC. The input waveguide width is chosen to be W4 = 400 nm for single-mode propagation. The bending radius for the input waveguide is chosen to be R1 = 16 μm to ensure low bending loss. The width and bending radius for the coupling waveguide are then determined to be W5 = 480 nm and R2 = 15.39 μm according to the phase match condition nTM0(W4)R1 = nTM0(W5)R2, where nTM0(W4) and nTM0(W5) denote the effective indices for the TM0 modes in the input waveguide and coupling waveguide, R1 and R2 denote the bending radii of the input waveguide and coupling waveguide, respectively. The gap width in the BDC is then calculated to be Wg,2 = 170 nm. The loss and extinction ratio of the BDC is calculated as functions of the arc angle θ using 3D FDTD, as shown in Fig. 3(a). The arc angle is optimized to be θ = 12° to obtain an extinction ratio > 20 dB and a low loss < 0.2 dB. The light propagation profiles and transmittance spectra are then calculated using 3D-FDTD, and the results are shown in Figs. 3(b)-3(d). One could observe that the TM mode is efficiently filtered out by the BDC, while the TE mode goes through with negligible loss. From the transmittance spectra, the extinction ratio is > 15 dB over a ~200 nm wavelength span.
Thus, the parameters in the present polarization splitter-rotator are summarized as: W0 = 400 nm, W1 = 660 nm, W2 = 810 nm, W3 = 400 nm, W4 = 400 nm, W5 = 480 nm, Wg,1 = 175 nm, Wg,2 = 170 nm, L1 = 2.5 μm, L2 = 6.1 μm, L3 = 2 μm, L4 = 22.2 μm, L5 = 3 μm, L6 = 1 μm, L7 = 2.5 μm, R1 = 16 μm, R2 = 15.39 μm and θ = 12°. The total size of the present PSR is ~47.5 × 5 μm2. The light propagation profiles and transmittance spectra of the whole device with the above parameters are simulated by using 3D-FDTD. From the profiles shown in Figs. 4(a) and 4(b), the TE0 mode propagates directly into the TRU port, while the TM0 mode is firstly converted to be TE1 mode, then converted to be TE0 mode and coupled out from the CRO port. From the transmittance spectra shown in Figs. 4(c) and 4(d), the extinction ratio is as high as and 43.60 dB for TE polarization and 41.50 dB for TM polarization, respectively. The losses are 0.21 dB for TE polarization and 0.34 dB for TM polarization, respectively. Over an ultra-broad wavelength span from 1.45 μm to 1.58 μm, the extinction ratio is > 20 dB for both TE and TM polarization. The transmission response for the PSR without BDC filter is also calculated as a comparison, as shown in Figs. 4(c) and 4(d). It can be seen that by exploiting the BDC-based filter, the 20-dB bandwidth could be greatly broadened. According to the further 3D-FDTD simulations, the device with a 20-nm width variation or a 15-nm thickness variation will still have > 20 dB extinction ratio and < 1 dB excess loss. Thus, the proposed PSR can be easily realized by today’s advanced fabrication technologies.
3. Fabrication and characterization
The designed PSR was fabricated on the SOI platform with a 220-nm thick silicon layer and a 2-μm thick oxide layer. An E-beam lithography process (Raith 150 II) was carried out to define the pattern. After that, the silicon top layer was fully etched by utilizing the inductively coupled plasma reactive-ion-etching (ICP-RIE) to transfer the pattern onto the silicon top layer. The focused grating couplers were also fabricated at each input/output port for fiber-chip coupling and polarization selectivity . Straight waveguides with same device lengths were also fabricated on the same chip for normalization. In order to get the transmission responses of the fabricated PSR, two sets of the PSRs with different grating couplers were fabricated on the same chip, as shown in Fig. 5(a). One should note that the PSRs are fabricated close enough so that could be considered to be with the same parameters. Figure 5(b) shows the scanning electron microscopy (SEM) of the fabricated PSR.
An amplified spontaneous emission source (ASE) and an optical spectrum analyzer (OSA) were used to measure the transmittance spectra of the fabricated PSRs over a wide wavelength band ranging from 1.52 μm to 1.61 μm. One could choose the input port I1/I4 and output port O1/O4 to measure the losses for the TE-/TM-type grating coupler. To measure the transmission spectra for TE polarization, one could choose I2 as the input port, and get the CRO/TRU port transmission from O2/O3. To measure the transmission spectra for TM polarization, one could choose I3 as the input port, and get the CRO/TRU port transmission from O4/O5. For the PSR launched with the TE0 mode, the polarization rotation is negligible, so that all the grating couplers are chosen to be TE-type. For the PSR launched with the TM0 mode, the TM0 mode could not go across the ADC due to the large phase mismatch, so the port O4 is connected with TE-type grating coupler. Since the residual TE1 mode can’t be supported by the single-mode waveguide and only TM0 mode could propagate through, the port O5 is connected with TM-type grating coupler . The losses of each grating coupler are then obtained from the measured spectra. Figures 5(c) and 5(d) show the normalized transmittance spectra of the fabricated PSRs. From the spectra, the loss is 0.54 dB for TE polarization and 0.57 dB for TM polarization, respectively. The extinction ratio is as high as 39.88 dB for TE polarization and 30.82 dB for TM polarization, respectively. The extinction ratio is > 20 dB over an ultra-broad wavelength band that is > 85 nm. One might find that the measured extinction ratios are a little bit lower than the simulated ones, which is mainly due to the working wavelength deviation caused by the fabrication errors, especially the multi-mode waveguide width. The performance could be improved further by utilizing the advanced fabrication technology such as deep UV lithography.
Several reported PSRs are summarized and compared with present one, as shown in Table 1. It can be found that our demonstrated PSR could have low loss and large working bandwidth with a relative small device size and easy fabrication.
In conclusion, we have theoretically and experimentally demonstrated a PSR with an ultra-broad wavelength band. The proposed PSR consists of a multi-mode waveguide, an asymmetrical directional coupler and a bent directional coupler. Only one-step etching is required for the proposed PSR. The fabricated device shows a high extinction ratio that is > 30.82 dB and a low loss that is < 0.57 dB. The extinction ratio is measured to be > 20 dB over a > 85 nm wavelength band. We believe that the proposed PSR device could find its application for the future densely integrated polarization diversity circuit.
National Natural Science Foundation of China (Grant No. 61675178 & 61377023); National Key Research and Development Program (2016YFB0402502).
References and links
1. X. Guan, P. Chen, S. Chen, P. Xu, Y. Shi, and D. Dai, “Low-loss ultracompact transverse-magnetic-pass polarizer with a silicon subwavelength grating waveguide,” Opt. Lett. 39(15), 4514–4517 (2014).
2. H. Xu and Y. Shi, “On-chip Silicon TE-pass Polarizer Based on Asymmetrical Directional Couplers,” IEEE Photonics Technol. Lett. 1, 861–864 (2017).
3. 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).
4. 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).
5. M. Aamer, A. M. Gutierrez, A. Brimont, D. Vermeulen, G. Roelkens, J.-M. Fedeli, A. Hakansson, and P. Sanchis, “CMOS Compatible Silicon-on-Insulator Polarization Rotator Based on Symmetry Breaking of the Waveguide Cross Section,” IEEE Photonics Technol. Lett. 24(22), 2031–2034 (2012).
6. H. Xu and Y. Shi, “Ultra-compact and highly efficient polarization rotator utilizing multi-mode waveguides,” Opt. Lett. 42(4), 771–774 (2017).
7. 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).
8. H. Guan, A. Novack, M. Streshinsky, R. Shi, Q. Fang, A. E. Lim, G.-Q. Lo, T. Baehr-Jones, and M. Hochberg, “CMOS-compatible highly efficient polarization splitter and rotator based on a double-etched directional coupler,” Opt. Express 22(3), 2489–2496 (2014).
9. Y. Zhang, Y. He, X. Jiang, B. Liu, C. Qiu, Y. Su, and R. A. Soref, “Ultra-compact and highly efficient silicon polarization splitter and rotator,” APL Photonics 1(9), 91304 (2016).
10. A. Barh, A. B. M. 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).
11. D. Dai, Y. Tang, and J. E. Bowers, “Mode conversion in tapered submicron silicon ridge optical waveguides,” Opt. Express 20(12), 13425–13439 (2012).
12. D. Dai and H. Wu, “Realization of a compact polarization splitter-rotator on silicon,” Opt. Lett. 41(10), 2346–2349 (2016).
13. D. Taillaert, P. Bienstman, and R. Baets, “Compact efficient broadband grating coupler for silicon-on-insulator waveguides,” Opt. Lett. 29(23), 2749–2751 (2004).