Modeling and Characterization of Deep-Etched Multilayer Resonators Under Partial Coherent Excitation Using Multimode Optical Fibers

Recently, there has been a resurgence of interest in multimode optical fibers illuminated by a white light source. Largely, in the anticipation of many integrated applications in the biomedical domain and spectral sensing benefiting from the broad spectral range and high numerical aperture. Along these lines, the output light from these fibers can be captured by the physics of partially coherent sources. Yet, the sheer complexity arising from the interplay between partial coherence and microstructure transfer function has posed fundamental challenges in deciphering their response. In this work, we present a numerical model and experimental characterization for the performance of multilayer optical resonators, fabricated by high aspect ratio etching of silicon, under partial coherent optical source excitation. The model studies the effects of optical fiber numerical aperture, Bragg mirror order, cavity length, and surface roughness of the microstructures on the output of the resonator. The results show that the response under standard multimode fiber (partial coherent source) has lower insertion loss, more asymmetry versus wavelength, and larger full width at half maximum (FWHM) than the standard single mode fiber (full coherent source). A MEMS chip is fabricated using 125 µm deep etching of silicon for Bragg mirrors with 2.25, 3, and 3.25 µm silicon layer width and a different number of layers. The structures are characterized using a multimode fiber of 62.5 µm core diameter illuminated by an infrared white light source. The theoretical results have been compared with the experimental results and a good agreement has been obtained.

近年来，采用白光源照明的多模光纤（multimode optical fiber）再度引发了学界的研究热潮。这一趋势主要源于其在生物医学领域及光谱传感领域的诸多集成应用前景——这类光纤凭借宽光谱范围与高数值孔径的特性，可充分满足相关应用需求。基于此，这类光纤的出射光可通过部分相干源物理模型进行分析与描述。然而，部分相干效应与微结构传递函数之间的相互作用带来了极高的复杂度，使得解析这类光纤的响应特性面临根本性挑战。
本研究针对采用高深宽比硅刻蚀工艺制备的多层光学谐振器，建立了其在部分相干光源激发下的性能数值模型，并完成了实验表征。该模型分析了光纤数值孔径、布拉格反射镜（Bragg mirror）阶数、谐振腔长度以及微结构表面粗糙度对谐振器输出特性的影响。
结果显示，相较于标准单模光纤（single mode fiber，全相干光源激发）的情况，标准多模光纤（部分相干光源激发）下的谐振器响应具备更低的插入损耗、更显著的波长不对称性，以及更大的半高全宽（full width at half maximum, FWHM）。
本研究采用125 μm深硅刻蚀工艺制备了MEMS芯片（MEMS chip），该芯片的布拉格反射镜硅层宽度分别为2.25 μm、3 μm与3.25 μm，且具备不同的反射镜层数。实验中采用芯径为62.5 μm的多模光纤搭配红外白光源对所制备的微结构进行表征。将理论仿真结果与实验测试结果进行对比，二者展现出良好的一致性。