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Abstract
A novel high-fabrication-tolerance mode demultiplexer (MD) based on an S-bend waveguide is designed, which is used to split TE1 mode and TE0 mode, and convert the TE1 mode to TE0 mode. Based on the MD, a polarization-rotator-splitter (PRS) is demonstrated. The transmission losses of the fabricated PRS are lower than 0.5 dB and 0.6 dB for TE0 mode and TM0 mode, respectively, in the wavelength span of 1520-1630 nm. And the corresponding polarization extinction ratios are larger than 19.5 dB and 17.6 dB, respectively. This MD has the most compact size comparing with other experimentally demonstrated MDs used in PRS.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Silicon photonics is a widely-researched technology for the integrated optical circuit. Because of compatibility with commercial complementary metal-oxide-semiconductor (CMOS) process and the high index contrast, we can integrate hundreds of optical devices on just one square millimeter. In polarization-division-multiplexing (PDM) system and some polarization insensitive system, polarization-rotator-splitter (PRS) is the essential device. The on chip PRS has two kinds of structures, directional coupler with double-layer waveguide in the coupling area [1–3] and TM0-TE1 mode converter connected by a mode demultiplexer (MD) [4–13]. Due to using wider waveguide in the polarization rotation part, the second kind of PRS has advantage of yield. For the TM0-TE1 mode converter, a waveguide with inverted T-shape in cross section (T-shape waveguide) should be employed [14], which can convert the input TM0 to TE1 mode, and maintain the input TE0 mode unchanged. Then the TE0 mode and the converted TE1 mode are input into the MD, which is used to split the TE0 and TE1 mode, and convert the TE1 mode to TE0 mode at the same time. Using the CMOS process, it can be conveniently fabricated for different patterns at different etching depth in the silicon layer. Thus, silicon photonics has the technological superiority of realizing polarization rotation on-chip.
Four types of MD have been reported, asymmetrical directional coupler (ADC) [4–8,15–17], asymmetrical Y-branch [9–11], asymmetrical Mach-Zehnder interferometer (AMZI) [12,13,18], and PhC-like subwavelength structure [19,20]. The ADC needs a long-tapered waveguide to increase its bandwidth. The asymmetrical Y-branch has broad bandwidth, but it needs high-resolution lithography and etching to fabricate the crucial narrow gap between the two output waveguides. AMZI usually has narrow bandwidth and is sensitive to the fabrication error. PhC-like subwavelength structure has the smallest size, but the production cost is quite high. And because it is difficult to collect the power scattered by the holes efficiently, the insertion loss of the PhC-like subwavelength structure is relatively high.
The multimode interferometer (MMI) can be used to split two different modes. Some transformed MMI can have broad bandwidth and low insertion loss, such as the multi-taper cascaded 1 × 2 MMI [21] and mode order converter [22]. This enlighten us that the transformed MMI can be used to realize a high-performance MD.
In this work, we proposed a new MD structure used in the PRS. The MD is based on a compact S-bend waveguide, using multimode interference to realize mode split and mode order convert simultaneously. The insertion loss of the fabricated PRS is lower than 0.6 dB in the wavelength span of 1520-1630 nm. And the polarization extinction ratio is larger than 17.6 dB. Thus, this PRS can work in the optical communication C + L band. The PRS is based on a single waveguide and has no fine structures, which means that it can be easily fabricated.
2. Device design
In the proposed PRS, we use TE1 mode as the transitional mode to convert TM0 mode to TE0 mode. The MD undertakes the function that converting the TE1 mode to TE0 mode. TE1 and TE0 mode have different symmetries. As mention in our previous work [22,23], to realize the interconversion between two modes with different symmetries, the waveguide should use some asymmetrical structure, which can break the input mode symmetry and excite some modes with different symmetry. The multimode interference is a popular method for mode conversion. The mode excitation and multimode interference path can be controlled in the MMI by adjusting the waveguide shape along the direction of light propagation. Therefore, we can control the mode conversion by using the transformed MMI.
Based on the above analysis, an asymmetrical MMI (AMMI) can be used to accomplish the function of the desired MD. The input TE0 mode and TE1 mode have different field distribution. Thus, they will experience different mode excitation and have different multimode interference path in the AMMI. By adjusting the shape of the AMMI, both of the TE1 and TE0 mode can convert to TE0 mode in the same waveguide. Meanwhile, the imaging point (also is the output port) of the two mode can be different. Comparing with other structures, the S-bend multimode waveguide has higher efficiency in mode exciting. Meanwhile, it is easy to design the output multi-ports by using S-bend. Thus, we use an S-bend multimode waveguide to design the MD. Figure 1 shows the structure of the proposed MD. The device is based on silicon-on-insulator (SOI) channel waveguide covered by SiO2, and the thickness of the silicon layer is 220 nm.
Fig. 1. Schematic of the proposed MD. The dash-dot line is the center line of the S-bend with a constant waveguide width. The red TE0 at output port is converted from TE1 mode and the blue TE0 at output port is converted from TE0 mode.
The S-bend composes of two bends, up bend and down bend, as shown in Fig. 1. The two bends have the same center radius R and bent angle θ. Before optimization, the width of the S-bend waveguide is constant. To optimize the shape of the S-bend, we divide both of the two bends into four parts with equal bent angle. We change the inner and outer radius at each equal diversion point during optimizing the S-bend shape. And between two equal diversion points, the inner and outer radius change linearly. That is equivalent to adjusting the width of the S-bend. The parameters are shown in Fig. 1. For the up bend, the inner radiuses at the equal diversion points are set to be R-Rdn, n = 1, 2…5, and the outer radiuses at the equal diversion points are set to be R + Run, n = 1, 2…5. For the down bend, the inner radiuses at the equal diversion points are set to be R-Run, n = 5, 6…9, and the outer radiuses at the equal diversion points are set to be R + Rdn, n = 5, 6…9. Based on those settings, we optimize R, θ, Run and Rdn of the S-bend to realize the MD.
We use FDTD-solutions to simulate the MD. It needs to optimize the transmission of two modes at the same structure. Thus, we use the API function of FDTD-solutions to connect other soft which can do the optimization. We write a particle swarm optimization algorithm with immune function in the soft to optimize the parameters of the MD. The optimized result is shown in the Table 1.
Table 1. Optimized result of the MD
The simulated results of the optimized MD are shown in Fig. 2. From Fig. 2(a) and 2(b), we can see that the input TE0 mode and TE1 mode have different multimode interference paths in the S-bend. The input TE0 mode converts to TE0 mode again at the end of the S-bend. And the converted TE0 mode outputs from P2 port. The input TE1 mode also converts to TE0 mode at the end of the S-bend. But the converted TE0 mode outputs from P1 port. Thus, the S-bend can realize the mode splitting and converting at the same time. Figure 2(c) and 2(d) show the transmission losses and the extinction ratios of the two output ports. In the wavelength span of 1520-1630 nm, while using TE0 mode input, the transmission loss is lower than 0.3 dB, and the extinction ratio is larger than 24 dB. While using TE1 mode as input, the transmission loss is lower than 0.3 dB, and the extinction ratio is larger than 26 dB in the corresponding wavelength range.
Fig. 2. Simulated results of the proposed MD. (a) and (b) Electric field distributions in the MD with TE0 mode input and TE1 mode input, respectively. (c) and (d) Transmission efficiencies at different output ports with TE0 mode input and TE1 mode input, respectively.
Then we investigate the fabrication tolerance of the optimized MD. We simulate the transmission loss and extinction ratio of the MD versus the waveguide width deviation in wavelength span of 1520 - 1630 nm. The width deviation of the waveguide is set to be ± 30 nm. As the insert shown in Fig. 3(a), the solid line shows the positive deviation, and the dash line shows the negative deviation. Figure 3(a) and 3(b) show the largest transmission loss and minimum extinction ratio of the MD varied with the waveguide width deviation, separately, in the wavelength span of 1520 -1630 nm. It can be seen, within ± 30 nm waveguide width deviation, the largest transmission loss is always less than 0.78 dB, and the minimum extinction ratio is always larger than 20 dB. The results indicate that the proposed MD has high fabrication tolerance.
Fig. 3. In the wavelength span of 1520 - 1630 nm, (a) the simulated largest transmission loss of the MD varied with the waveguide width deviation, (b) the simulated minimum extinction ratio of the MD varied with the waveguide width deviation. The insert in the (a) shows the width deviation of the waveguide, the solid line shows the positive deviation, and the dash line shows the negative deviation.
We connect the optimized MD to a T-shape waveguide by a tapered waveguide. The T-shape waveguide is used to convert TM0 mode to TE1 mode and maintain the TE0 mode unchanged. The schematic of the proposed PRS is shown in Fig. 4. The thicknesses of the two layers for the T-shape waveguide are 150 nm and 220 nm, respectively. At the input-port and output-port of the T-shape waveguide, the two layers have the same waveguide widths. The widths of the T-shape waveguide for the input-port and the output-port are 450 nm and 1.2 μm, respectively. The T-shape waveguide is divided into two sections with both the lengths L1 and L2 of 50 μm. In the middle of the T-shape waveguide, the width of the 220-nm-thickness waveguide Wu is 450 nm, and the width of the 150-nm-thickness waveguide Wd is 900 nm. Those settings guarantee that the mode hybridization region, which is the key structure for polarization rotation [14], is formed in the T-shape waveguide.
Fig. 4. The schematic of the proposed PRS. Left is the overhead view and right is a 3-demension view.
Figure 5 shows the simulated results of the proposed PRS. In the simulation, we connect 500-nm-wide S-bend waveguides to the P1 and P2 ports to increase the gap between the two output ports. From Fig. 5(a) and 5(b), we can find that the input TE0 mode remains unchanged in the T-shape waveguide, while the input TM0 mode converts to TE1 mode. Combining with the MD, we finally designed the PRS. From Fig. 5(c) and 5(d), it can be seen that, in the wavelength span of 1520-1630 nm, the transmission losses of the PRS are lower than 0.31 dB and 0.44 dB for TE0 mode input and TM0 mode input, respectively. And the corresponding polarization extinction ratios are larger than 20.2 dB and 21.4 dB. The detected powers of TM0 modes at the output ports are slight and can be neglected.
Fig. 5. Simulated results of the proposed PRS. (a) and (b) Electric field distributions in the PRS with TE0 and TM0 mode input, respectively. (c) and (d) Transmission efficiencies at different output ports with TE0 and TM0 mode input, respectively.
3. Device measurement
The PRS is fabricated with the commercial silicon photonics process. Figure 6(a) shows the optical micrographs of the fabricated PRS. Figure 6(b) shows the testing optical circuit of the PRS. In the circuit, we cascade four polarization splitters [24] to filter out the unwanted polarization mode and remain only one polarization mode. We use the polarization filter at both input port and output port, as shown in Fig. 6(b). Thus, we can separately measure the output TE0 mode and TM0 mode at the P1 and P2 ports by connecting TE0 mode pass filter and TM0 mode pass filter with the PRS, respectively. In the Fig. 6(b), it only shows the TM0 mode pass filter connecting with the output ports of the PRS. It is used to test the output TM0 mode. The circuit used to test the output TE0 mode has the similar structure. To measure the PRS in a broad band, the edge coupler is used to couple the optical beam between optical fiber and SOI chip. The measured coupling efficiency of the edge coupler is about -2 dB/facet. And the polarization dependent loss of the edge coupler is 0.4 dB/facet. We use 1×2 MMI to split the input beam equally into two outputs. One of the outputs is set as the reference, and the other is input into the PRS, shown in the Fig. 6(b). We measured the transmission spectrum of each port in the test circuit and the transmission spectra of the polarization filters. Then we normalized the transmission spectra of the PRS by the transmission spectra of edge coupler, the transmission spectra of polarization filters, and the transmission spectra of the reference ports. The normalized measured results are shown in Fig. 7.
Fig. 6. (a) Optical micrographs of the fabricated PRS. (b) Optical micrographs of the testing circuit for the fabricated PRS. The polarization filter composed of four polarization splitters connects with the output ports of the PRS. Figure (b) only shows the situation of TM0 mode pass filter connecting with the output ports of the PRS. The circuit with TE0 mode pass filter connecting with the PRS has the similar structure.
Fig. 7. Normalized measured transmission efficiencies at the two output ports of the PRS, (a) TE0 mode input, (b) TM0 mode input.
In Fig. 7(a), we can see that, when TE0 mode inputs, the transmission loss (TE0 mode output from P2 port) is lower than 0.5 dB in the wavelength span of 1520-1630 nm. And the corresponding polarization extinction ratio is larger than 19.5 dB. In Fig. 7(b), when TM0 mode inputs, the transmission loss (TE0 mode output from P1 port) is lower than 0.6 dB. And the corresponding polarization extinction ratio is larger than 17.6 dB.
In Fig. 7(b), it can see a resonance when detecting the TE0 mode outputs from P2 port. That is because, as shown in Fig. 6(b), the location of TM0-mode-input branch is close to the input port. Part of the TE-polarization light that is not coupled into the edge coupler transmits in the box and cladding layer, and couples into the output P2 port again. Meanwhile, during TE1 mode transmits in the MD, part of the power is scattered into the P2 port. The above two beams have the similar power but different phases. Thus, we can see an obvious interference of the two beams in the P2 port. The TE0-mode-input branch relatively far away from the input edge coupler. Therefore, the light coupling into P1 port from the box and cladding is much weaker than the light comes from the scattering of the input TE0 mode in the MD. Thus, as shown in Fig. 7(a), there is no resonance when detecting the TE0 mode outputs from P1 port.
It can be seen that the polarization extinction ratio of the fabricated PRS is less than the extinction ratio of the MD fabrication tolerance study shows. There are three major reasons. First, the output mode of the PR part is note pure at some wavelength. Those modes, except the TE0 mode converted from the input TE0 mode and the TE1 mode converted from the input TM0 mode, will decrease the extinction ration of the PRS. Second, except the MD part, the structure parameters of the PR part, such as the alignment error between the two layers and the waveguide widths of the two layers, affect the final ER performance of the PRS. Third, the sidewall of the fabricated waveguide is not smooth. Part of the light will be scattered by the rough sidewall when it transmits in the waveguide. It causes some unwanted crosstalk.
Table 2 shows the comparison of reported PRSs with different MD structure. We can see that our PRS has the most compact MD size compared with other experimentally demonstrated PRSs. Meanwhile, the measured performance of it is comparable to the best reported ones.
Table 2. Comparison of reported PRSs with different MD structure
4. Conclusion
We design and fabricate a PRS based on a novel compact MD. In the transformed S-bend MD, different input modes will experience different mode excitation and different multimode interference path. By piecewise adjusting the shape, the proposed MD efficiently splits the input TE0 mode and TE1 mode. And meanwhile, it converts the TE1 mode to TE0 mode at the output. The simulation results show the S-bend MD has high fabrication tolerance. Connecting the MD with a T-shape waveguide, we design the proposed PRS. The PRS is fabricated with the commercial silicon photonics process. In the wavelength span of 1520-1630 nm, the measured transmission losses of the PRS are lower than 0.5 dB and 0.6 dB for TE0 mode input and TM0 mode input, respectively. And the corresponding polarization extinction ratios are larger than 19.5 dB and 17.6 dB, respectively. It means that the proposed PRS can work in a broadband wavelength range which covers the C + L band. The minimum dimension of the proposed PRS is 200 nm. Containing no fine structure guarantees that the proposed PRS can be fabricated in almost all existing silicon photonics process. Thus, the proposed PRS also has advantage of production.
Funding
National Key Research and Development Program of China (2019YFB2205201, 2019YFB2205203); Hubei Technological Innovation Special Fund (2019AAA054).
Disclosures
D. G. Chen, M. Liu, Y. G. Zhang, L. Wang, X. Hu, P. Feng, X. Xiao and S. H. Yu: China Information and Communication Technologies Group Corporation (CICT) (F,I,E,P).
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