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.