Optical Waveguides for Infrared Spectroscopic Detection of Molecular Gases

Abstract

Fields like medical diagnostics, urban and industrial environmental monitoring or basic microbiological research greatly benefit from advances in chemical and biological sensing. These applications require rapid sample analysis, reduced needs for sample handling, or good sensor network. Such demands can be met with miniaturised sensors utilising methods which secure sufficient sensitivity and selectivity such laser absorption spectroscopy. However, such instruments are nowadays bulky, expensive, and require large sample volumes. Optical waveguides, a cornerstone of integrated opto-chemical sensors, are aiming at replacing current bulky and costly instrumentation based on free-space optics. They can realize large optical interaction pathlengths as well as provide simple functions of beam splitting or combining on a compact photonic chip, thus substituting e.g., multi-pass cells, Fabry-Perot cavities, or free-space interferometers such as those in FTIR instruments. To achieve competitive sensitivities, however, the waveguide device needs to meet two criteria: Low loss to allow long interaction paths, and large light–analyte interaction per unit length. This thesis presents the analysis, fabrication methods, and characterisation of optical waveguides for infrared tuneable diode laser absorption spectroscopy. A free standing waveguide for use in the mid-infrared spectral domain was developed to tackle the challenges above. Moreover, the waveguide features negligible etalon fringes in transmission, which otherwise interfere with measured spectral signatures. Compared to a free space beam, an outstanding 7 % stronger light-analyte interaction strength was measured with the waveguide.