Cascaded ring resonator and Mach-Zehnder interferometer with a Sagnac loop for Vernier-effect refractive index sensing

•A new silicon photonic integrated sensor based on the Vernier effect is proposed.•Double performance can be achieved compared to state-of-the-art Vernier sensors.•Half the operating bandwidth of state-of-the-art Vernier sensors can be employed.•A rigorous and detailed modelling of the new Vernier devices is presented.•<30000ppm of NH3 in water and less than 5000 ppm of CO2 in air can be detected.