Infrared integrated optical evanescent field sensor for gas analysis Part I:: System design

A concept for an integrated optical gas measuring system using a free-standing single crystal silicon integrated optical waveguide as an integrated optical gas analysis sensor working in the middle infrared region is presented and the attainable resolution is estimated for practical applications. The cross-section of the waveguide is designed with respect to a pronounced evanescent optical field, which interacts with the surrounding gas atmosphere. The silicon waveguide has an optical transparency from about 1.3 to 6.3 mu m thus allowing to monitor pollutants like CO, CO2, NO, N2O, NO2, e.g. emitted in combustion processes as well as CH4 and H2O. The light at the waveguide output contains qualitative and quantitative information about the gases present. The high step in refractive index from the silicon core to gas allows very small bending radii so that a sufficiently long waveguide can be realised on a small chip area (about 1m on 1cm(2)) by arranging it as a spiral or meander in the wafer plane. (c) 2004 Elsevier B.V. All rights reserved.