Study and Characterization of Silicon Nitride Optical Waveguide Coupling with a Quartz Tuning Fork for the Development of Integrated Sensing Platforms

Highlights: What are the main findings? We successfully coupled a silicon nitride waveguide with a custom-designed, low-frequency, and T-shaped QTF, enabling both Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) and Light-Induced Thermoelastic Spectroscopy (LITES) techniques for sensing. We achieved comparable signal-to-noise ratios with QEPAS and LITES when detecting 1.6% water vapor concentration, with performance limited by the output power illuminating the QTF. What is the implication of the main finding? Demonstrated the feasibility of integrating photonic components with piezoelectric resonators for portable gas-sensing applications. Identified on-chip laser-waveguide integration as a key route to compact sensing platforms. This work demonstrates an ultra-compact optical gas-sensing system, consisting of a pigtailed laser diode emitting at 1392.5 nm for water vapor (H2O) detection, a silicon nitride (Si3N4) optical waveguide to guide the laser light, and a custom-designed, low-frequency, and T-shaped Quartz Tuning Fork (QTF) as the sensitive element. The system employs both Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) and Light-Induced Thermoelastic Spectroscopy (LITES) techniques for trace gas sensing. A 3.8 mm-wide, S-shaped waveguide path was designed to prevent scattered laser light from directly illuminating the QTF. Both QEPAS and LITES demonstrated comparably low signal-to-noise ratios (SNRs), ranging from 1.6 to 3.2 for a 1.6% indoor H2O concentration, primarily owing to the reduced optical power (~300 μW) delivered to the QTF excitation point. These results demonstrate the feasibility of integrating photonic devices and piezoelectric components into portable gas-sensing systems for challenging environments.