Characterization of Evanescent Field Gas Sensor Structures Based on Silicon Photonics

Photonic sensors that operate in the mid-infrared spectral range are an emerging field for photonic microsystems. In this paper, we present a photonic gas sensor concept based on silicon waveguides using infrared evanescent field absorption. The waveguides were specifically designed for CO<inline-formula><tex-math notation="LaTeX">$_{2}$ </tex-math></inline-formula> sensing at a wavelength of <inline-formula><tex-math notation="LaTeX">$\lambda ={\text{4.26}}\,\mathrm{\mu m}$</tex-math></inline-formula> as possible application for the proposed sensor platform. The waveguide cross section as well as the substructure were investigated using finite-element simulations and the devised structures were fabricated using mass fabrication processes exclusively. In order to evaluate the potential for long interaction path lengths using polysilicon strip waveguides, a study on the intrinsic losses of polysilicon waveguides was conducted. The lowest intrinsic damping that was obtained for polysilicon strip waveguides was 3.98 dB/cm. Furthermore, the sensing capability of the devised waveguides was tested with quantitative CO<inline-formula> <tex-math notation="LaTeX">$_{2}$</tex-math></inline-formula> measurements down to a concentration of 500 ppm CO <inline-formula><tex-math notation="LaTeX">$_{2}$</tex-math></inline-formula>. From the quantitative measurements, the evanescent field ratio was estimated and was in the range between <inline-formula><tex-math notation="LaTeX">$\eta ={\text{14}{\%}}\hbox{--}{\text{16}{\%}}$</tex-math></inline-formula>.