Cascaded microring resonators for biomedical applications: Improved sensitivity at large tuning range

The progress in bioanalytics caused a growing demand of innovation in reliable, miniaturized and low cost optical sensor systems based on integrated optical devices. We present a detailed analysis of sensor elements for applications in aqueous solution based on two cascaded microring resonators (MRRs) by using the Vernier effect (VE). This approach is beneficial for ultra-high sensitivity at large fabrication tolerances, aspects of crucial importance for the practical detection of biomolecules such as peptides. The architecture consists of two silicon nitride microrings connected via a bus waveguide. The free spectral range (FSR) of individual rings is slightly different in order to achieve VE. Thereby the external refractive index of the reference ring is fixed; the second one varies due to the presence of the analyte. The precise operation is controlled by using spectral tuning via integrated micro-heaters. Theoretical analysis has been performed for different structu ral parameters. A sensitivity several orders of magnitude higher than in the case of a single ring can be predicted for TE and TM polarization, respectively. The first design of Vernier devices and its experimental characterization will be presented. The devices include tapered grating couplers in order to couple light between fibers and chip at moderate alignment tolerances in a reliable manner. Therefore, by combining the VE and the spectral tuning, cascaded MRRs are an optical configuration very promising for sensing applications.