A compound optical microresonator design for self-referencing and multiplexed refractive index sensing

We propose a newtype of self-referencing and multiplexed refractive index (RI) sensor based on a compound optical microresonator structure consisting of Fabry-Pérot (FP) resonators coupled with microring resonators. The transmission spectra shows resonant features that are superimposed on a background defined by FP oscillations. The resonances have asymmetric Fano-like non-Lorentzian shapes, which are used as sensing peaks, while the FP oscillations are used as reference peaks for internal self-referencing. The sensing peaks shift linearly with the increased RI of the cladding in the microring resonator, while FP peaks stay constant. When the temperature is increased, both the FP peaks and the Fano resonances shift linearly at the same rate, which eliminates the temperature effect on RI measurements. We theoretically analyzed that the two-mirror FP resonator coupled with a single microring resonator and optimized its sensing performance through finite-difference time-domain simulations. A sensitivity value of 220 nm/RIU and a maximum figure of merit of 4400 RIU-1 were achieved. We also proposed two possible multiplexing schemes consisting of two-mirror and three-mirror FP resonators coupled with two microring resonators of different radii. The proposed sensor concept is simple, easy-to-fabricate, self-calibrating and can be used for simultaneous measurements of different samples.