Optical sensor systems for biological and medical applications have been widely developed in order to satisfy the current
requirements such as a miniaturization, cost reduction, label-free detection and fast response. Here, we demonstrate a
highly sensitive optical sensor based on two cascaded microring resonators (MRRs) exploiting the Vernier effect. The
architecture consists of a filter MRR connected to a sensor MRR via a common waveguide. The external medium of the
filter MRR is isolated with a top cladding layer, while the sensor MRR interacts with the analyte sample via an opening.
The sensor chip, that includes an array of five cascaded MRRs, was designed and fabricated on a silicon nitride platform.
A first test has been performed with sodium chloride (NaCl) concentrations in deionized (DI) water providing a
sensitivity of 1.03 nm/% (6317 nm/RIU). A limit of detection of 3.16 x 10-6 RIU was demonstrated for the current
sensor, respectively. Several concentrations of isopropanol in ethanol ranging from 0% to 10% were also investigated.
These preliminary measurements show a sensitivity as high as 0.95 nm/% at ~1535 nm compared to 0.02 nm/% from a
single sensor MRR. For a moderated alignment between the chip and cleaved optical fibers, tapered grating couplers are
included at the ends of waveguides. Hence, by combining the Vernier effect and the silicon nitride material, cascaded
MRRs will be a powerful optical configuration for biosensing applications in a wide operating wavelength range.