In this paper, an on-chip silicon polarization beam splitter using a particle-swarm-optimized counter-tapered directional coupler is proposed, designed, and fabricated. The coupling length of the proposed device is only 5 µm. As the waveguide width variation ΔW increases from −20 to 20 nm, the simulated polarization extinction ratio larger than 18.67 dB and the corresponding insertion loss lower than 0.17 dB are achieved. Measured experimental results achieved insertion loss <0.50 dB, TE polarization extinction between 16.68 to 31.87 dB, TM polarization extinction between 17.78 to 31.13 dB, over the wavelength range 1525 to 1600 nm.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Owing to high complementary metal–oxide–semiconductor (CMOS) compatibility and strong ability to realize large-scale photonic integration, silicon photonics has attracted much interest [1,2]. In such a promising technology, the silicon-on-insulator (SOI) platform offers an attractive way of developing compact photonic integrated devices and circuits because of the high index contrast. But it would cause strong polarization dependence simultaneously . Thus, to solve this issue, as a common approach, polarization diversity devices including polarization rotators and polarization beam splitters (PBSs) are introduced.
As one of the key components, PBSs splitting or combining two orthogonal polarization modes have been experimentally demonstrated on SOI platform by using various structures, such as Mach-Zehnder interferometers (MZIs), multi-mode interference (MMI) couplers, grating structures, optimized topological structures, hetero-anisotropic metamaterials, and directional couplers (DCs). Although the reported PBSs using MZIs , MMI couplers [5,6], or grating structures [7–10] can have high polarization extinction ratios, their footprints are large. An ultra-compact PBS can be realized by using the optimized topological structure, but the related polarization extinction ratio is limited to around 10 dB within a 32 nm bandwidth . For the reported metamaterial-based PBS , it can provide a compact footprint, high polarization extinction ratio, low insertion loss and large bandwidth, but high-accuracy fabrication is needed. Owing to the simple design, relatively compact size, and high stability, DC-based PBSs have attracted much attention. The reported PBS using a symmetric sinusoidal-bend-based directional coupler with a nominal coupling length of 8.55 µm can have an average extinction ratio no less than 12.0 dB over a bandwidth of 100 nm . The fabricated PBS using a single bent DC has an extinction ratio larger than 6 dB from 1520 to 1600 nm . The performance of this kind of PBS can be further improved by cascading multiple bent DCs or using the triple-bent-waveguide DC [15,16]. Although PBSs using asymmetric straight DCs can achieve high polarization extinction ratios and low insertion losses [17–22], the coupling length and coupling strength are required to be accurately controlled due to the phase matching. Owing to the high fabrication tolerance, the PBSs using tapered DCs have been fabricated. The reported PBS based on tapered straight DC with a coupling length of 29 µm can have a polarization extinction ratio greater than 16 dB and an insertion loss lower than 0.4 dB over a 100 nm spectral bandwidth . For the fabricated PBS based on tapered bend DC, an extinction ratio is no less than 11.98 dB and an insertion loss is no greater than 1.39 dB in 1510-1590 nm . However, how to easily realize an on-chip ultra-compact PBS having high performance is still a challenge, especially for multi-dimensional-multiplexing optical network-on-chip [25–27].
In this paper, we propose and investigate an on-chip silicon PBS using the counter-tapered directional coupler with excellent CMOS compatibility, ultra-compact footprint, low insertion loss (IL), large fabrication tolerance, high polarization extinction ratio (PER), and broad bandwidth (BW). Structural parameters of the proposed silicon PBS are designed and optimized by using particle swarm optimization (PSO) algorithm and finite difference time domain (FDTD) method. We also thoroughly evaluate and analyze the functionalities and properties of the designed device. For our presented silicon PBS, a coupling length of 5 µm is obtained. As the waveguide width variation ΔW changes from −20 to 20 nm, the PER greater than 18.67 dB and the IL less than 0.17 dB are realized for our designed device. The designed silicon PBS can have a PER larger than 16.08 dB and an IL smaller than 0.23 dB when the gap G0 varies from 250 to 310 nm. For the fabricated device operating with the input fundamental transverse-electric (TE0) mode, within a bandwidth from 1525 to 1600 nm, the best PER is up to 31.87 dB, while in the worst case it is 16.68 dB. The corresponding IL varies from 0.003 to 0.498 dB. The measured PER ranges from 17.78 to 31.13 dB and the measured IL changes from 0.001 to 0.462 dB within a bandwidth from 1525 to 1600 nm for the fabricated PBS operating with the input fundamental transverse-magnetic (TM0) mode.
2. Principle and design
The structure of the introduced silicon PBS based on the counter-tapered directional coupler is described in Fig. 1(a). Figure 1(b) illustrates a detailed drawing of the coupling region. The cross-sectional view of the coupling region (along line A-A’) is shown in Fig. 1(c). Figure 1(d) depicts the effective refractive indexes of TE0 and TM0 modes changing with the waveguide width for the case of the silicon strip waveguide with a height of Hc=220 nm. In the simulation, Si and SiO2 are given by Lumerical FDTD Solutions software. As seen in Fig. 1(d), the effective refractive index of the TM0 mode goes through a relatively gentle change with the increase of the waveguide width. Hence, the introduced silicon PBS is designed as a TM0-mode coupled PBS. When TE0 and TM0 modes are launched into the Input port, the TE0 mode goes straight and comes out from the Bar port, while the TM0 mode is coupled into the adjacent waveguide and emerges from the Cross port. In order to efficiently separate the input TM0 and TE0 modes in a short coupling length, structural parameters for the coupling region are carefully designed.
As illustrated in Fig. 1(b), the taper in the coupling region is split into M equal segments of length Ls and each segment’s width is denoted by Wm (m=0, 1, 2, …, M). To realize a compact PBS with low insertion loss and high polarization extinction ratio, the width Wm and the number of the segments M are optimized by using the PSO algorithm and FDTD method. In the optimization process, the length Ls and the widths W0, Wa, and Wt are respectively set to be 0.50 µm, 450 nm, 200 nm, and 450 nm. Each PSO is starting from the previous optimal value. The optimization figure of merit (FOM) is defined as FOM= (PTE0_Bar + PTM0_Cross) for the case of 1≤m≤(M-3), while the definition of FOM is given by FOM= (PTE0_Bar/PTE0_Cross + PTM0_Cross/PTM0_Bar) for the case of (M-2)≤m ≤ M. Here, Pmode X_port Y represents the optical power of mode X obtained from the port Y. By using Eqs. (1) and (2), the width of the segment and the variation range of segment’s width regarded as the particle's position and velocity can be updated .
- (I) Assign the variable M and set m=0.
- (II) Initialize particles’ states, such as the velocity and position.
- (III) Assign the variable m = m+1 and set FOM= (PTE0_Bar + PTM0_Cross) if m < M is satisfied. Otherwise, set M = M +1 and go back to step I.
- (IV) Set FOM= (PTE0_Bar/PTE0_Cross + PTM0_Cross/PTM0_Bar), if m≥(M-2) is satisfied. Otherwise, go to step V.
- (VI) Go to step VII if the maximum number of iterations is reached. Otherwise, go back to step V.
- (VII) Stop the optimization process if FOM>500 is satisfied. Otherwise, go back to step III.
Figure 2 shows the corresponding flow chart. In the simulation, r1 and r2 are set to be r1 = r2 = 2. During each iteration, the variation that the particle’s position ps can take belongs to [0.30, 0.45]. Table 1 lists the gap G0, the corresponding optimum width of each segment, and the performance of the designed PBS. As seen in Table 1, a minimum PER of 26.48 dB and a maximum IL of 0.10 dB can be obtained when the gap G0 is selected to be 270 nm and the widths W1, W2, W3, W4, W5, W6, W7, W8, W9, and W10 are respectively optimized to be 350 nm, 390 nm, 400 nm, 420 nm, 430 nm, 400 nm, 380 nm, 360 nm, 340 nm, and 330 nm. In the calculation, definitions of PER and IL are given by PERTE0,TM0 = 10log (TBar,Cross/TCross,Bar) and ILTE0,TM0 = −10log (TBar,Cross), where TBar stands for the transmittance of the input beam to the Bar port and TCross represents the transmittance of the input beam to the Cross port.
The simulated light propagation in the designed silicon PBS is depicted in Fig. 3. From Figs. 3(a) and 3(b), it is seen that the designed device can perform well. Figure 4 illustrates the effect of the wavelength on the PER and IL of the designed silicon PBS. As shown in Figs. 4(a) and 4(b), the PER of the designed silicon PBS is larger than 20.05 dB and the corresponding IL is lower than 0.17 dB, when the wavelength varies from 1521 to 1565 nm. The fabrication tolerance of the designed silicon PBS is also discussed. Figure 5 describes the impact of the waveguide width variation ΔW and the gap G0 on the PER and IL of the designed silicon PBS at 1550 nm. Note that in Figs. 5(a) and 5(b), as the waveguide width variation ΔW increases from −20 to 20 nm, the PER changes from 18.67 to 30.11 dB and the IL is lower than 0.17 dB in the case of the input TE0 mode, while the PER ranges from 19.86 to 35.13 dB and the IL is less than 0.16 dB in the case of the input TM0 mode. From Figs. 5(c) and 5(d), it can be seen that, as the gap G0 varies from 250 to 310 nm, the PER ranges from 25.04 to 26.57 dB and the IL is less than 0.073 dB in the case of the input TE0 mode, while the PER ranges from 16.08 to 26.63 dB and the IL is smaller than 0.23 dB in the case of the input TM0 mode.
3. Fabrication and characterization
The designed silicon PBSs are fabricated by using the silicon photonics multi-project wafer service of IMEC (ISIPP50G) . Figure 6 shows the microscope image of the fabricated devices. As seen in Fig. 6, identical PBSs with different grating couplers are fabricated on the same chip to characterize the performance in the cases of TE and TM polarizations.
The fabricated silicon PBSs are characterized by utilizing a broadband (1525nm-1600nm) amplified spontaneous emission (ASE) light source and an optical spectrum analyzer (YOKOGAWA AQ6317B). The transmission spectra at the Bar and Cross ports are measured as the light beam is launched into the Input port. The transmission of the straight waveguide with TE-type or TM-type grating couplers on the same chip is used to normalize the corresponding measured optical power transmission of the fabricated silicon PBSs with TE-type or TM-type grating couplers. The measured transmission spectra at the Bar and Cross ports of the fabricated PBSs operating in TE and TM polarizations are shown in Fig. 7. As illustrated in Fig. 7, for the fabricated device, the measured PER can vary from 16.68 to 31.87 dB within a bandwidth from 1525 to 1600 nm in the case of the TE polarization, while the measured PER ranges from 17.78 to 31.13 dB within a bandwidth from 1525 to 1600 nm in the case of the TM polarization. When the wavelength increases from 1525 to 1600 nm, the IL of the fabricated PBS operating in TE polarization varies from 0.003 to 0.498 dB and the corresponding IL of the fabricated PBS operating in TM polarization changes from 0.001 to 0.462 dB. It can also be found that the measurement results are not completely consistent with the simulation one. The main reasons are given below: firstly, the bandwidth of the light source used in the measurement is limited, and secondly, the widths of the fabricated waveguides could be not in keeping with the optimum widths of the designed one owing to fabrication errors. Table 2 lists a comparison of our device with other reported DC-based PBSs experimentally demonstrated on SOI platform. From Table 2, it can be seen that our fabricated PBS can realize an ultra-low insertion loss, a high polarization extinction ratio, and a broad bandwidth in an ultra-compact coupling length. The polarization extinction ratio and bandwidth can be further improved by cascading multiple coupling regions [15,16].
In conclusion, we have proposed, designed, and experimentally demonstrated an on-chip silicon PBS using the counter-tapered directional coupler. By taking advantage of PSO algorithm and FDTD method, structural parameters of our introduced silicon PBS are optimized. The coupling length of the introduced silicon PBS is only 5 µm. For the designed device, the calculated PER and IL are respectively greater than 18.67 dB and less than 0.17 dB when the waveguide width variation ΔW increases from −20 to 20 nm, and the calculated PER is larger than 16.08 dB and the IL is lower than 0.23 dB when the gap G0 changes from 250 to 310 nm. Our measurement reveals that, within a bandwidth from 1525 to 1600 nm, the PER ranges from 16.68 to 31.87 dB and the IL varies from 0.003 to 0.498 dB for the fabricated PBS operating in TE polarization. For the fabricated PBS operating in TM polarization, the PER varies from 17.78 to 31.13 dB and the IL changes from 0.001 to 0.462 dB within a bandwidth from 1525 to 1600 nm. Due to excellent CMOS compatibility, ultra-compact footprint, low insertion loss, large fabrication tolerance, high polarization extinction ratio, and broad bandwidth, we believe our design of the silicon PBS using the counter-tapered coupler can offer an attractive option for constructing multi-dimensional-multiplexing optical network-on-chip.
National Natural Science Foundation of China (61875098, 61874078, 61475137); Zhejiang Provincial Natural Science Foundation of China (LY20F050003, LY20F050001); Natural Science Foundation of Ningbo (2018A610133, 2019A610078); and the K. C. Wong Magna Fund in Ningbo University.
The authors declare no conflicts of interest.
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