Fabrication and analysis of low-loss silicon high-mesa waveguides

Breath sensing is an effective tool for health monitoring. Previously, high-mesa waveguide structures have been proposed by our group for realizing a compact breath-sensing photonic circuit. By using the doped SiO₂ as the waveguide core, 50% concentration CO₂ has been detected. One issue of preventing parts per million (ppm)-order detection is the low portion of evanescent light (Γ=2.2%) in the doped SiO₂ waveguides. In order to realize low propagation loss α and high Γ simultaneously, thin silicon (Si) waveguides with a Γ as high as 37.6% have been proposed and fabricated in this work. A thermal oxidation technique was applied to further reduce α, so that α was decreased from 1.45 to 0.84 and 0.29 to 0.2 dB/cm for the 0.5 and 3-µm-wide waveguide, respectively. According to our analysis, the significantly decreased α is attributed to recovering the damaged Si core and smoothing the waveguide sidewalls. The high Γ and effective loss reduction show a promising potential of applying Si high-mesa waveguides to realize ppm-order sensing.