基于液晶包层平板光波导的偏振转换器设计

Design of polarization converter based on liquid crystal-clad slab waveguide

基于液晶的动态可调偏振转换器具有大调制幅度、低功耗的优势,在片上集成系统具有重要的研究意义。然而现有的液晶偏振转换器存在传输损耗大、液晶分子难取向的问题。为此,本文提出了一种基于液晶包层平板光波导的偏振转换器。首先基于液晶各向异性与4层波导理论,构建了满足液晶包层平板光波导物理特性的偏振转换分析模型,厘清偏振转换发生的内在机理为液晶分子场致转动引起的波导内模式耦合。而后通过精确求解耦合状态下波导模式的传播常数和光场分布,提出一种适合于液晶包层平板光波导的偏振转换分析方法,总结出偏振转换效率及最小转换长度的理论公式,并掌握了模式耦合对偏振转换效率的影响规律。最后综合优化各层波导参数得到了一组可实现高偏振转换效率的器件参数,分析了芯层厚度、取向层厚度和电极长度偏差对转换效率的影响。结果表明,本文设计的液晶包层平板光波导偏振转换器可在0.2 V的驱动电压变化下实现转换效率在0%~99.98%的连续可调。

The dynamic tunable polarization converter based on liquid crystals,which boastes large modulation amplitude and low power consumption,holds significant research interest in on-chip integrated systems.However,existing liquid crystal polarization converters encounter substantial transmission losses and difficulties in orienting the liquid crystal molecules.In response,this paper introduces a polarization converter based on a liquid crystal-clad slab waveguide.Initially,leveraging the anisotropy of liquid crystals and the four-layer waveguide theory,a polarization conversion analysis model that satisfied the physical characteristics of the liquid crystal-clad slab waveguide was developed.This model elucidated the internal mechanism of polarization conversion as mode coupling within the waveguide,induced by fielddriven rotation of liquid crystal molecules.Subsequently,by precisely solving for the propagation constants and optical field distributions of the waveguide modes under coupled conditions,a polarization conversion analysis method suitable for liquid crystal-clad slab waveguides was proposed.This method summarized the theoretical formulas for polarization conversion efficiency and minimum conversion length,and elucidated the influence of mode coupling on polarization conversion efficiency.Finally,by comprehensively optimizing the parameters of each waveguide layer,a set of device parameters capable of achieving high polarization conversion efficiency was derived.The impact of deviations in the core layer thickness,orientation layer thickness,and electrode length on conversion efficiency was analyzed.The results indicate that the designed liquid crystal-clad slab waveguide polarization converter can achieve continuously adjustable conversion efficiency from 0%to 99.98%with a change in driving voltage of 0.2 V.