Mid-infrared absorption gas sensing using a silicon strip waveguide

Optical sensing is an emerging field for photonic microsystems that operate in the mid-infrared spectral range. In this work we present a photonic gas sensor based on infrared evanescent field absorption, designed for CO2 sensing. The sensing structure comprises a strip waveguide on a silicon nitride layer. A modal analysis was performed using the finite element and the finite difference time domain method. The fabricated waveguides were characterized and the concept was validated with quantitative CO2 measurements. The measured transmittance at various CO2 concentrations was fitted using the Beer-Lambert law, and the results proved that the presented concept is feasible for CO2 gas sensing. The devised demonstrator device allowed to detect CO2 concentrations down to 5000 ppm, which is the workplace exposure limit in most jurisdictions.