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90°光混频器是无线相干光通信系统接收端的关键器件, 在提升接收端灵敏度和抗干扰能力方面发挥着重要作用. 传统的90°光混频器存在对精度要求高、体积大、受限于模式失配、偏振敏感和功能单一等缺点. 为解决上述问题且进一步实现器件的多功能化, 在铌酸锂平台基于多模干涉(multimode Interference, MMI)结构, 设计了兼具90°光混频和模式分离功能的多功能集成器件. 该器件在功能上具备良好的可扩展性, 在性能上均优于传统结构, 具有低损耗、高精度和宽带宽的特点, 并通过容差分析验证了器件在较大工艺误差范围内仍能保持优异的性能, 展现了较高的工艺容差性和可靠性. 该器件可同时应用于大规模集成光学中, 为高性能片上光通信系统提供了新技术.90°optical mixer, as an essential part of coherent optical communication and heterodyne detection, improves polarization discrimination and anti-interference capabilities, increases receiver sensitivity, and permits demodulation of higher-order modulation forms. The disadvantages of traditional 90° optical mixers, however, include their high precision needs, size, mode mismatch restrictions, polarization sensitivity, and single functionality. Utilizing a lithium niobate platform, a multimode interference (MMI) structure, and a micro-thermoelectric electrode array, and with the help of the finite difference time domain (FDTD) method, a multipurpose device that combines 90° optical mixing and mode separation capabilities is designed in this work. According to the results, when no voltage is applied across the micro-thermoelectric electrodes, the multipurpose device acts as a 90° optical mixer. The common-mode rejection ratios of all four outputs are all above –30 dB, phase errors are below 4°, and the losses in the wavelength range of 1520—1580 nm exceed –13.862 dB. When a voltage is applied across the micro-thermoelectric electrodes, TE0, TE1, TE2, and TE3 modes are separated by the multipurpose device acting as a mode splitter. In addition to controlling crosstalk fluctuation within 8.8 dB, the minimum loss divergence between modes is less than 0.024 dB. Research findings indicate that the physical characteristics of optical field interference within the MMI structure enable perfect phase matching and energy distribution across a wide spectrum range, even when no voltage is supplied across the micro-thermoelectric electrode terminals. By controlling the interference superposition process inside the multi-mode region and improving broadband 90° optical mixing parameters, the stable phase-matching conditions are maintained across the wide spectrum. The lithium niobate-based linear electro-optic effect (Pockels effect) modifies the waveguide refractive index distribution through an external electric field when a voltage is applied across the micro-thermoelectrodes. By changing the light field's coupling path and propagation mode inside the MMI structure, the mode-separating integrator can precisely achieve mode separation, thereby confirming the efficiency of the electro-optic effect in optical functional control, which meets the isolation requirements for various mode optical signals. Furthermore, a systematical tolerance analysis of the device's width and length is carried out, demonstrating how structural dimensional deviations affect the mode coupling efficiency and optical field interference circumstances. The integrated broadband 90° optical mixer and mode splitter device described in this paper has excellent process tolerance properties.
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图 1 宽带90°光混频器的示意图 (a) 三维图; (b) 俯视图
Fig. 1. Schematic diagram of a wideband 90° optical mixer: (a) Three-dimensional diagram; (b) top view.
图 2 性能随结构参数变化曲线 (a) 耦合效率随输出波导间隔gap_ln的变化; (b) 损耗随输出端波导长度Lout-ln的变化; (c) 损耗随多模波导长度LMMI的变化
Fig. 2. Performance curves with structural parameters: (a) Curve of coupling efficiency with output waveguide gap_ln; (b) variation of loss with output waveguide length Lout-ln; (c) variation of loss with multimode waveguide length LMMI.
图 3 基于LiNbO3平台宽带90°光混频器单束光输入光强图 (a) 单束信号光输入; (b) 单束本振光输入
Fig. 3. Single-beam optical input light intensity diagram of a wideband 90° optical mixer based on the LiNbO3 platform: (a) Single beam signal light input; (b) single-beam local oscillator optical input.
图 4 各模式传输光场图 (a) 输出TE0模式; (b) 输出TE1模式; (c) 输出TE2模式; (d) 输出TE3模式
Fig. 4. Transmission light field diagram of each mode: (a) Outputs TE0 mode; (b) outputs TE1 mode; (c) output TE2 mode; (d) output TE3 mode.
图 5 各模式透射率随电压变化曲线
Fig. 5. Variation curves of transmittance with voltage in each mode.
图 6 各模式性能指标随电压变化曲线 (a) 模式分离器损耗; (b) 模式分离器串扰
Fig. 6. Curves of performance index of each mode with voltage: (a) Mode separator loss; (b) mode separator crosstalk.
图 7 基于LiNbO3平台的宽带90°光混频器光强图
Fig. 7. Light intensity map of a wideband 90° optical mixer based on LiNbO3 platform.
图 8 性能指标随波长变化情况 (a) 90°光混频器损耗随波长变化曲线; (b) 90°光混频器共模抑制比随波长变化曲线; (c) 不同输出端口之间的相位偏差随波长变化曲线
Fig. 8. Variation of performance index with wavelength: (a) 90-degree hybrid loss versus wavelength curves; (b) variation curves of common mode rejection ratio of 90-degree optical with wavelength; (c) variation curves of phase deviation between different output ports with wavelength.
图 9 模式分离器性能指标 (a) 模式分离器损耗; (b) 模式分离器串扰
Fig. 9. Performance index of mode separator: (a) Mode separator loss; (b) mode separator crosstalk.
图 10 宽度容差下共模抑制比性能分析 (a) MMI宽度容差范围为±0.1 μm; (b) MMI容差范围宽度为±0.2 μm
Fig. 10. Performance analysis of common mode rejection ratio under width tolerance: (a) The tolerance range of MMI width is ±0.1 μm; (b) the width of MMI tolerance range is ±0.2 μm.
图 12 宽度容差下相位偏差性能分析 (a) MMI宽度为13 μm; (b) MMI宽度为13.1 μm; (c) MMI宽度为13.3 μm; (d) MMI宽度为13.4 μm
Fig. 12. Performance analysis of phase deviation under width tolerance: (a) MMI width is 13 μm; (b) MMI width is 13.1 μm; (c) MMI width is 13.3 μm; (d) MMI width is 13.4 μm.
图 13 长度容差下共模抑制比性能分析 (a) MMI长度容差范围为±2 μm; (b) MMI长度容差范围为±4 μm
Fig. 13. Performance analysis of common mode rejection ratio under length tolerance: (a) The tolerance range of MMI length is ±2 μm; (b) the tolerance range of MMI length is ±4 μm.
图 14 长度容差下相位偏差性能分析 (a) MMI长度为138 μm; (b) MMI长度为140 μm; (c) MMI长度为144 μm; (d) MMI长度为148 μm
Fig. 14. Performance analysis of phase deviation under length tolerance: (a) MMI length is 138 μm; (b) MMI length is 140 μm; (c) MMI length is 144 μm; (d) MMI length is 148 μm.
图 16 宽度容差下损耗性能分析 (a) MMI宽度为13 μm; (b) MMI宽度为13.1 μm; (c) MMI宽度为13.3 μm; (d) MMI宽度为13.4 μm
Fig. 16. Analysis of loss performance under width tolerance: (a) MMI width is 13 μm; (b) MMI width is 13.1 μm; (c) MMI width is 13.3 μm; (d) MMI width is 13.4 μm.
图 17 宽度容差下串扰性能分析 (a) MMI宽度为13 μm; (b) MMI宽度为13.1 μm; (c) MMI宽度为13.3 μm; (d) MMI宽度为13.4 μm
Fig. 17. Analysis of crosstalk performance under width tolerance: (a) MMI width is 13 μm; (b) MMI width is 13.1 μm; (c) MMI width is 13.3 μm; (d) MMI width is 13.4 μm.
图 18 长度容差下损耗性能分析 (a) MMI长度为138 μm; (b) MMI长度为140 μm; (c) MMI长度为144 μm; (d) MMI长度为148 μm
Fig. 18. Analysis of loss performance under length tolerance: (a) MMI length is 138 μm; (b) MMI length is 140 μm; (c) MMI length is 144 μm; (d) MMI length is 148 μm.
图 19 长度容差下串扰性能分析 (a) MMI长度为138 μm; (b) MMI长度为140 μm; (c) MMI长度为144 μm; (d) MMI长度为148 μm
Fig. 19. Analysis of crosstalk performance under length tolerance: (a) MMI length is 138 μm; (b) MMI length is 140 μm; (c) MMI length is 144 μm; (d) MMI length is 148 μm.
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