Design of a tunable Fabry–Perot filter based on a silicon wafer for gas sensing applications in the infrared

Abstract

Tunable optical Fabry–Perot Interferometers (FPIs) have been designed as filters on sensors for gas detection in the mid infrared range where their radiation absorption is at its maximum. FPIs are commonly designed using two properly aligned parallel mirrors, which makes them susceptible to vibrations and misalignment, and often require to be assembled using sophisticated mechanical systems to reduce these issues. In this work, the design of a tunable (FPI) filter based on a silicon wafer for gas sensing applications in the infrared range is presented. The thickness of the wafer was calculated considering the absorption spectrums of the target gas and the optical components in the sensor arrangement. Therefore, this filter has the transmission spectrum of an ideal FPI that matches the absorption peaks of the target gases. The detection of CH${}_4$ is presented as a case study but this filter can be applied to gases with well-defined ro-vibrational lines like CO and CO${}_2$. Experimental results show that the proposed filter can be used for the design of a highly selective gas sensor. Finally, the proposed filter has the advantage of being simple, easy to integrate, and low-cost, since the two parallel mirrors used in conventional FPIs are no longer needed and the associated alignment and the montage of mechanical components are reduced.