Wide Range Si₃N₄ Optical Sensor Using Cascaded Mach-Zehnder Interferometer Coupled Ring Resonator

A cascaded MZI-coupled ring resonator (CMCR) optical sensor based on silicon nitride waveguide is proposed. The sensor is composed of a large size microring and a bus waveguide coupled with the microring for four times. The three-segment bus and the microring waveguide between the four coupling regions form three MZI of the same size, forming a three-stage cascaded MZI. The resonant effect of the cascaded MZI transforms the smooth comb spectrum of the microring into a V-shaped comb spectrum, expands the effective free spectral range of the ring resonator, and realizes the cascaded MZI-coupled microring sensor with a large measurement range. For the cascade MZI-coupled microring with a radius of 100μm, the effective free spectrum range is 53.7nm, which is more than 24 times larger than that of the all-pass microring of the same size. The refractive index sensitivity is 339.49 nm RIU/and the detection limit is 2.95 × 10⁻⁵ RIU. these excellent properties prove the feasibility of cascaded MZI-coupled microring structure for large measurement range sensing, and are expected to be used in large refractive index range sensing applications in the future.