MZI-based sensor for liquid recognition on the SiO₂/TiO₂ platform

In addition to the telecom and datacom sectors, the photonic sensing sector is one of the fastest-growing markets for Photonic Integrated Circuits (PICs). The ability to combine complex and multifunctional passive and active elements on a single photonic chip, along with the adaptability of the technological platform to specific applications, opens up opportunities to construct various structures tailored to specific purposes. One of the most popular solutions for PIC-based sensors involves the implementation of ring resonators. Unfortunately, these structures are highly sensitive to production variations, often causing issues with the predictability and repeatability of the entire sensing system. Promising alternatives to these elements are Mach-Zehnder interferometers (MZIs), which offer lower manufacturing variability and greater design flexibility, however, at the cost of increasing the final size of the PIC chip. In this work, we present and discuss the results of modeling and designing sensors based on MZI structures on a sol-gel-derived HYPHa (SiO₂/TiO₂) technological platform. Sol-gel aqueous phase deposition and the possibility of direct nanoimprinting of waveguide structures enable low-cost fabrication of integrated devices with low propagation loss. The study considers the impact of changes in the refractive index resulting from partial and/or complete coverage of the sensing area with the tested substance. Selective coverage of the sensor with the tested substance, although requiring an additional manufacturing process, allows for significant improvement in sensitivity. We tested both variants in several sensing systems, designed and fabricated for three different wavelengths in the visible and near-infrared regions (633, 1310, and 1550 nm), with the maximal sensitivity of over 32000 nm/RIU for the sensor designed for 1550 nm.