Towards a Fully Integrated Refractometric Sensor in Silicon Nitride: Integrated Spectrometer Design

This paper presents the first steps toward a fully integrated refractometer featuring a plasmonic hybrid-waveguide sensor (PHWS) with its dedicated integrated spectrometer. Unlike conventional silicon-based spectrometers, our approach utilizes silicon nitride for its broad optical transparency from visible to mid-infrared wavelengths, enabling versatile applications. Our primary focus is on the spectrometer architecture that integrates a tunable cascaded microring resonator (CMRR) with an arrayed waveguide grating (AWG), exploiting the Vernier effect to achieve high-resolution spectral scanning over an extended wavelength range. As a demonstrator, we implement a CMRR with a 0.22 nm full-width at half-maximum in combination with a 5-channel AWG, enabling spectral scanning across each 10 nm bandwidth channel. As an illustration of its potential, the architecture is found to achieve a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2.8 \times 10^{-4}$</tex> RIU refractive index resolution when combined with a PHWS offering a 780 nm/RIU sensitivity. Our approach offers key advantages over standalone spectrometers by enabling compactness, full integration, and tunability within a single chip, paving the way for advanced on-chip sensing applications.