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回音壁模式微腔的色散调控是克尔光频梳生成的关键.然而回音壁模式微腔色散设计主要是通过改变微腔结构调控模场分布,方式较为单一.本文将径向分布的梯度折射率n (r)引入回音壁模式微腔,提出使用改变折射率分布调控梯度折射率微腔色散.通过数值计算和有限元仿真结果表明,折射率梯度的约束使微腔模场的位置远离微腔边缘,梯度折射率微腔具有零几何色散特性.基于设计不同折射率分布提出两种色散调控方式：修饰微腔边缘几何外形和构建双势阱.并且研究了微腔半径、楔角大小、离子扩散和塑形工艺顺序、双势阱宽度和间距对色散的影响.仿真结果表明两种方式均可以得到通讯波段较大范围的反常色散,梯度折射率微腔色散调控方式十分灵活,在非线性光学应用领域具有极大潜力.Kerr optical frequency combs based on whispering-gallery mode (WGM) microresonators have great potential for applications in various fields such as precision measurement, spectral analysis, optical communication, and quantum technology. The interaction between dispersion and nonlinearity is crucial for determining the stability and bandwidth performance of optical frequency combs. In particular, the Kerr bright soliton optical frequency comb requires a suitable anomalous group velocity dispersion (GVD) to maintain the dissipative system. Therefore, designing the dispersion of the WGM microresonator is essential for generating the Kerr optical frequency comb. However, WGM microresonators typically have normal and fixed material dispersion, and their dispersion design is primarily based on modulating the mode field distribution by changing the microresonator structure to achieve anomalous dispersion, which offers limited flexibility.
In this paper, we introduce a radially distributed gradient refractive index n(r) into WGM microresonators and propose using the refractive index profile to control the dispersion of gradient-index (GRIN) microresonators. Numerical simulations and finite element analysis demonstrate that the refractive index gradient constrains the mode field, pushing it away from the cavity edge, resulting in near-zero geometric dispersion in the GRIN microresonator.Two dispersion modulation methods are explored: modifying the microresonator’s geometric shape and constructing a dual potential well. The effects of microresonator radius, wedge angle, ion diffusion sequence, and potential well width and spacing on dispersion are systematically investigated. Simulation results show that both methods can achieve a wide range of anomalous dispersion within the communication band. In the first method, mode field leakage in the bilateral wedge-shaped GRIN microresonator produces anomalous dispersion, while no leakage results in normal dispersion. When the mode field is pushed away from the edge, near-zero dispersion is achieved. In the second method, energy coupling between the inner and outer modes in the dual potential well structure leads to anomalous dispersion in the inner mode and normal dispersion in the outer mode.
Our findings highlight the flexibility of GRIN microresonator dispersion control, offering great potential for nonlinear optical applications. -
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