Reducing N₂O induced cross-talk in a NDIR CO₂ gas sensor for breath analysis using multilayer thin film optical interference coatings

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 and is an under-used bio-marker for underlying respiratory conditions. Current detection methods do not adequately meet these requirements and suffer from considerable cross-talk associated with the commonly used anaesthetic gas nitrous oxide (N₂O). In this work, we report how cross-talk can be reduced in a commercially available, low power (35 mW) non-dispersive infrared (NDIR) CO₂ gas sensor using thin film multilayer optical filters. Current sensor spectral response, spans 2500 nm–5000 nm via use of a pentanary alloy LED/photodiode optopair 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, a multilayer thin film optical interference bandpass filter has been designed and deposited directly onto the diode epi-structures using microwave plasma assisted DC magnetron sputtering. Three different coating configurations have been explored; LED-only coated, photodiode only coated and both coated. Gas sensor response to N₂O for each coating configuration has been explored. It was found that application of an optical bandpass filter onto both the sensor LED and photodiode only was the most effective method of reducing sensor response to N₂O, however no signal was observed in one of the two “LED and PD coated”, therefore optimal coating configuration for cross-talk reduction is subject to further investigation.