Sponsored Student: Optimising Breath Analysis Using Thin Film Optical Filters

Carbon dioxide (CO₂) gas sensing is an important aspect in the biomedical field of Capnography, where cheap, fast and accurate measurement of exhaled CO₂ vs. time is crucial in the evaluation of lung and tracheal function during surgical anaesthesia. Current detection methods do not adequately meet these requirements and suffer from considerable cross-talk due to the commonly used anaesthetic gas nitrous oxide (N2O). Here, N₂O cross-talk has been eliminated in a world leading, low power (3.5 mW) nondispersive infrared detector (NDIR) CO₂ gas sensor using thin film multilayer optical filtering. Current sensor spectral response, spans 2500 nm-5000 nm via use of a pentanary alloy LED/photodiode pair grown by molecular beam epitaxy (MBE), resulting in sensor sensitivity to gases with absorption bands in this region, including N₂O. To reduce the effective spectral response of the sensor, capturing only CO₂, multilayer thin film optical filters have been characterised and deposited directly onto the photodiode epi-structure using reactive DC magnetron sputtering. Various multilayer designs and deposition techniques have been explored. Optical filtering efficacy in cross-talk reduction has been tested in novel breath emulation apparatus to evaluate its effectiveness in emulated CO₂ vs. time waveforms containing typical percentages of N₂O used during surgical anaesthesia.