Optical MEMS filter for infrared gas detection

Abstract

An optical filter is proposed, which can be adapted to infrared detection of various gases. The filter is based on a diffraction grating structured on a micro-electro-mechanical system (MEMS) which can be switched between two states by applying an external actuation voltage of 0-5V. The optical grating is etched into silicon, and has a period and a depth adapted to the gas to be detected. The grating presented in this poster is a two-level binary grating designed to give a reflection spectrum centred at a 2μm wavelength. It has 500nm deep grooves and has a period of 5- 10μm. The grating is structured on top of an array of silicon beams, where every second beam can be moved vertically. The filter switches between two states by applying an actuation voltage of 0V and 5V. The optical spectrum of the diffracted light changes according to actuation voltage. By deflecting every second grating element 500nm (λ/4) vertically, the filter can be switched between a function with a single band which peaks at a certain centre wavelength, and a function with two sidebands positioned on either side of the centre wavelength. The single band can be designed to coincide with the absorption lines of the gas to be detected, while the sidebands are positioned outside of the absorption region of the gas. By illuminating the gas with light diffracted off this filter and switching between the two filter states, an intensity modulation of the light is achieved which is dependent upon the amount of gas present in the transmission path of the light. The proposed filter provides a robust, flexible, and potentially low cost solution for infrared gas detection of a wide range of gases.

Optical MEMS filter for infrared gas detection