高密度相位光栅的偏振选择性

高密度光栅具有与传统光栅不同的性质,其衍射特性往往是偏振相关的。本文针对1550nm波长TE/TM偏振入射光和0.5的光栅占空比,利用严格耦合波分析数值计算了不同光栅周期下0级及-1级的衍射效率。研究表明,相比周期为1550nm的光栅,当周期为1200nm时,偏振相关衍射效应明显增强,当光栅周期为890nm时,TE偏振光的衍射效率随着光栅深度呈正余弦变化,而TM偏振光的衍射效率始终集中在0级,具有偏振选择性。通过模式方法,利用模式中的有效折射率概念,研究了不同周期下被入射光所激发的两种光波模式通过光栅区域传播所累积的相位差;基于双光束干涉,模拟了0级和-1级的衍射效率。结果表明,利用严格耦合波分析的数值计算结果符合模式方法的理论预期,对于高密度相位光栅的偏振选择性给予了合理的物理机制解释。

Polarization-selectivity of high-density phase gratings

Diffraction properties of high-density gratings are polarization-dependent, which are different from those of conventional gratings. For TE- and TM-polarized incident waves with a wavelength of 1 550 nm and a duty cycle of 0.5, diffraction efficiencies in the 0th and-1st orders are calculated using Rigorous Coupled-wave Analysis(RCWA). It indicates that more polarization-dependent characteristics are shown for the period 1 200 nm compared with 1 550 nm. Especially, with the period 890 nm, the diffraction efficiency of TE-polarized wave changes with grating depth sinusoidally, while the efficiency of TM-polarized wave which has polarization-selectivity is always concentrated on the 0th order. According to modal method, modes excited by the incident wave are investigated for different periods and the accumulated phase differences of the two modes with different effective indices are calculated when they propagate through the grating region. Moreover, diffraction efficiencies in the 0th and-1st orders are simulated based on two-beam interference. Experimental result shows that the numerical calculation result using RCWA coincides well with the theoretical predictions of modal method, which can explain the physical mechanism of polarization-selectivity of high-density phase gratings.