光学学报, 2023, 43 (16): 1623020, 网络出版: 2023-08-15  

基于超表面结构的紧凑型铌酸锂波导模式转换器 下载: 856次

Compact Lithium Niobate Waveguide Mode Converter Employing Metasurface Structure
作者单位
之江实验室智能网络研究院,浙江 杭州 311121
摘要
模式转换器是片上模式复用(MDM)系统的关键器件。为在绝缘体上铌酸锂(LNOI)中实现结构紧凑的模式转换,本文提出了一种基于超表面结构的模式转换器。该模式转换器可以实现TE0-TE1、TE0-TE2的转换。为了在模式之间实现高效转换,采用逆向设计方法,结合3D电磁场仿真,优化器件倾斜周期性亚波长条形蚀刻结构参数,以同时符合沿传播方向的相位匹配要求,以及符合短耦合长度的横向方向折射率分布要求。仿真结果表明,器件在1400~1700 nm波段内,TE0-TE1转换的插入损耗<0.8 dB,模间串扰<-10 dB,模式转换区长度~20 μm。且该器件具有较好的高阶模式转换扩展性以及工艺容差性,是未来LNOI的高密度集成MDM系统中模式转换器的良好备选方案。
Abstract
Objective

With the development of informatization, there is a growing demand for information transmission and processing. In this situation, it becomes urgent to increase the channel capacity. Currently, there are multiple multiplexing techniques available to enhance channel capacity, such as wavelength division multiplexing, polarization multiplexing, and mode division multiplexing (MDM). Among them, MDM utilizes different modes and polarizations of light to carry data and parallelly transmits multiple data channels in optical waveguides or fibers using only a single wavelength laser source, which can be seen as a new dimension to expand the capacity of optical fiber communication. The lithium niobate-on-insulator (LNOI) platform, with its strong electro-optic effect, low material loss, and wide transparent window, can achieve high-speed electro-optic modulators and optical nonlinear devices while providing high refractive index contrast waveguides, which thus makes it capable of manufacturing high-speed and high-density on-chip optical devices. However, there are few reports on MDM devices on LNOI platforms, and they mainly focus on the principle of phase matching of directional couplers loaded with LNOI platforms with silicon nitride. Although the mode converter based on the principle of phase matching of directional couplers has low processing difficulty and good scalability, it may have the problem of a relatively large footprint, which is not conducive to large-scale on-chip integration.

Methods

According to the coupled mode theory, when light propagates in a medium, the energy of the light field can be coupled from one mode to another mode by designing the appropriate medium structure. In this process, the perturbation of the medium structure not only satisfies the phase matching requirement of the two converting modes along the propagation direction of the z-axis but also has an appropriate refractive index distribution in the lateral direction to obtain an appropriate coupling coefficient and achieve a shorter coupling length. Metasurfaces are two-dimensional artificial materials with subwavelength features that can manipulate the phase, amplitude, and polarization of light waves at subwavelength scales through a special refractive index distribution. Integrating metasurfaces into optical waveguides can help deal with the relatively large footprint of converters based on the principle of phase matching of directional couplers, which is not conducive to large-scale on-chip integration. By leveraging the subwavelength-scale manipulation of light waves offered by metasurface structures, this study proposes a compact lithium niobate (LN) waveguide mode converter that can achieve TE0-TE1 or TE0-TE2 conversion in LN waveguides. In order to achieve coupling between modes, a reverse design method is adopted, and three-dimensional (3D) electromagnetic field simulation is utilized to optimize the subwavelength periodic stripe etching structure parameters of the device, so as to meet the phase matching requirements along the propagation direction and the lateral direction refractive index distribution with a high coupling coefficient.

Results and Discussions

Figure 3 shows the top view of the designed TE0-TE1 LN waveguide mode converter, the simulated modal field distribution, insertion loss, and crosstalk between modes, as well as the modal field distribution of the cross-sectional planes that are perpendicular to the propagation directions at the input and output. It can be seen that the energy of the TE0 mode gradually decreases during propagation and is gradually converted into that of the TE1 mode. Within the bandwidth of 1400-1700 nm, the insertion loss is less than 0.8 dB, with a minimum of 0.3 dB at 1520 nm; the crosstalk between modes is less than -10 dB, with a minimum of -38 dB at 1473 nm, and the extinction ratio is 37.3 dB. The low crosstalk between modes means that most of the energy of the TE0 mode is converted into the energy of the TE1 mode, and the energy loss is not significant, thus making the mode converter suitable for the field of optical communication. To demonstrate the scalability of this design, the TE0-TE2 mode conversion design is also presented in Fig. 4. It shows that the energy of the TE0 mode gradually decreases during propagation and is gradually converted into that of the TE2 mode. Within the bandwidth of 1400-1700 nm, the insertion loss is less than 2.4 dB, and the crosstalk between modes is less than -10 dB. The low crosstalk between modes means that most of the energy of the TE0 mode is converted into the energy of the TE2 mode, and the energy loss is not significant. In order to evaluate the effect of process errors on the performance of the designed structure and ensure the reproducibility of the device, finite-difference time-domain (FDTD) simulations of the insertion loss and crosstalk between modes in TE0-TE1 and TE0-TE2 mode conversions are carried out for processing errors of the etching groove width d and etching groove sidewall angle α (Figs. 5-8). It can be seen that the designed device has good tolerance to process errors in d and α.

Conclusions

This study proposes a compact LN waveguide mode converter based on metasurface structures, which can achieve TE0-TE1 and TE0-TE2 conversions. In order to achieve efficient coupling between modes, a reverse design method is adopted, and 3D electromagnetic field simulation is utilized to optimize the parameters of the tilted periodic sub-wavelength stripe etching structure of the device, which complies with the phase matching requirements of mode conversion along the propagation direction and the transverse refractive index distribution requirements of short coupling length. Simulation results show that the device has an insertion loss of less than 0.8 dB and crosstalk between modes of less than -10 dB in the wavelength range of 1400-1700 nm for TE0-TE1 conversion, with a conversion length of about 20 μm. In addition, the device has good scalability and process tolerance for higher-order mode conversion, making it a good candidate for mode converters in high-density integrated MDM systems in future LNOI.

王琳, 高阳, 石昊, 张磊, 尹坤. 基于超表面结构的紧凑型铌酸锂波导模式转换器[J]. 光学学报, 2023, 43(16): 1623020. Lin Wang, Yang Gao, Hao Shi, Lei Zhang, Kun Yin. Compact Lithium Niobate Waveguide Mode Converter Employing Metasurface Structure[J]. Acta Optica Sinica, 2023, 43(16): 1623020.

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