Robust highly stable multi-resonator refractive index sensor

Here we show that a serial array of three evanescently coupled microring resonators can achieve detection limits for refractive index changes in the surrounding environment that are more than 40% better than a single resonator. The improved performance of the three rings occurs when only the central resonator is exposed to the refractive index change and is due to the narrower linewidth that results from the off-resonant coupling to the adjacent resonators. Unlike a single resonator sensor that is usually operated close to critical coupling to maximize the sensitivity, the three resonator configuration is able to achieve superior detection limits over a wide range of inter-resonator couplings. We use this to show that random variations of the couplings resulting from manufacturing variations has a minimal impact on the performance of the three ring system.