The Miniaturization of an Optical Absorption Spectrometer for Smart Sensing of Natural Gas

Natural gas is the primary energy resource in both households and industry. Due to the molecular similarity of the main constituents, i.e., hydrocarbons, composition analysis of natural gas requires a highly selective and sensitive measurement technique. Smart sensing of natural gas in a large energy grid favors the use of methods that provide low unit cost in high-volume production, such as on-chip devices that can be micromachined at the wafer level, while maintaining the performance of complex benchtop instruments. Optical absorption spectroscopy with wideband optical filters offers a comparable performance in a smaller footprint. However, the gas cell, where the absorption takes place, challenges the miniaturization of the spectrometer. This paper presents two approaches with analysis and experimental validation to integrate the gas cell and the linear variable optical filter (LVOF) at the wafer level. The first approach uses 45$^{\circ}$ inclined mirrors to steer the light beam through the sample gas, while the second approach, the gas-filled LVOF, functionally integrates the gas cell into the resonator cavity of the filter. Both devices are self-referenced and compatible with fabrication in a CMOS process, and therefore highly suitable for smart gas sensing.