基于三波导定向耦合器的紧凑型偏振分束器的设计

Design of Compact Polarization Beam Splitter Based on Triple-waveguide Directional Coupler

为了提高偏振分束器的消光比,利用混合等离子体波导和条形介质波导之间的非对称定向耦合特性,设计了由两个尺寸不同的硅波导和一个中间输入混合等离子体波导组成的偏振分束器,采用三维时域有限差分法对偏振分束器的结构和性能进行优化。实验结果表明:混合等离子体波导中TE和TM模式的双折射率对比通过介质加载波导增强,可获得超紧凑的结构,实验耦合长度仅有4.6μm;在1.55μm的中心波长处,TE模式的偏振消光比为-38.9 dB,插入损耗为0.5 dB,TM模式的消光比为-34.7 dB,插入损耗为0.45 dB,设计的偏振分束器具有高消光比;在波导制作公差范围内,模式的插入损耗小于2.8 dB,消光比低于-22.5 dB。

In recent years,the silicon-on-insulator platform has attracted much interest in implementing integrated optical circuits. Due to the high refractive index contrast between the waveguide and cladding,the silicon-on-insulator waveguide has a strong light confinement ability,which helps the device to achieve a compact structure size and large-scale integration. However,this high refractive index contrast causes silicon nanophotonic devices to have high polarization sensitivity so that TE and TM modes have different propagation characteristics in an identical silicon-on-insulator waveguide,which may disrupt the optical signals in optical interconnections and quantum communications. To tackle this problem,one solution is using a polarization beam splitter to polarize the TM mode and TE mode light. Considering future ultradense photonic integrated circuits,a compact PBS with high performance is desired. Therefore,the hybrid plasma waveguide is introduced into the design of the polarization beam splitter. The hybrid plasmonic waveguide consists of a high refractive index dielectric layer(e.g.,Si),a metal cap(e.g.,Ag),and a thin low-index material layer(e. g.,SiO2) between the Si layer and the metal layer. Compared with the traditional dielectric waveguide, it has a significant birefringence effect, which can maintain the performance of the device and further reduce the size of the device.In this paper,we propose and optimize a novel hybrid plasmonic polarization beam splitter utilizing asymmetrical directional coupling between the dielectric waveguides and a hybrid plasmonic waveguide on the silicon-on-insulator platform,which is ultracompact, and low-loss. Employing phase-matching conditions and super-mode theory, this polarization beam splitter is elaborately designed. The device is a three-waveguide asymmetric directional coupler. The middle hybrid plasmonic waveguide,which has a dielectric-loaded structure to enhance the refractive index contrast,is defined as the input waveguide. And different silicon-based waveguides on both sides are used to couple and separate TE and TM modes. Compared with the traditional polarization beam splitter based on the directional coupler composed of the same waveguide,this device facilitates two different types of waveguides to enhance the structural birefringence,which provides the device with an ultra-short coupling length and high extinction ratio. In addition,the dependence of TE and TM modes on the middle waveguide is also weakened.In this design of the device,it is necessary to ensure that the input waveguide and the TM coupling waveguide are phase-matched in the TM mode,but not phase-matched in the TE mode. In the case of the TE coupling waveguide,the principle of phase matching for TE mode is similar. The finite element method is used to calculate the mode field distribution and effective refractive index of silicon waveguides and a hybrid plasmonic waveguide,and then determine the size of each waveguide according to the phase matching conditions,which provides a basis for device modeling and simulation analysis. And the 3 D finite difference time domain method is used to study the mode characteristics and optimize the structure of the polarization beam splitter to obtain better performance parameters. The experimental results show that the coupling lengths for TE and TM modes are 4.2 μm and4.6 μm,respectively,and the polarization conversion efficiency of the modes can reach 94.7% and 95.5%,respectively. It can be seen from the light propagation of the polarization beam splitter that when the TE polarization mode is launched from the input side of the hybrid plasmonic waveguide,it is gradually coupled to the TE coupling port in the coupling region,with scarcely any modes mixing. Similarly,when the TM polarization mode is input into the hybrid plasmonic waveguide,it is finally coupled to the TM coupling port. For the TE mode,the polarization extinction ratio value is-38.9 dB and the insertion loss is-0.5 dB at the wavelength of 1.55 μm. For the wavelength range from 1.48 μm to 1.62 μm,the insertion loss of the TE mode is less than 0.92 dB,and the polarization extinction ratio is lower than-28.3 dB. For the TM mode,the polarization extinction ratio is-34.7 dB,and the insertion loss is 0.45 dB at the wavelength of1.55 μm. In the 140 nm wavelength range centered at 1.55 μm,the insertion loss of the TM mode is less than 0.89 dB,and the polarization extinction ratio is lower than-34.6 dB. The above experimental results mean that the performance of the polarization beam splitter is not sensitive to the wavelength. In addition,the influence of fabrication tolerance of waveguide in polarization beam splitter on device performance is also studied. Within the fabrication tolerance range of waveguides,the insertion loss of the mode is less than 2.8 dB,and the extinction ratio is lower than-22.5 dB. The results show that the device has high fabrication tolerances. In summary,the polarization beam splitter designed in this paper has a high extinction ratio,low insertion loss,and the coupling length is only 4.6 μm. It has great potential application value in the ultra-small silicon-based optical integrated circuits in the future.