Gas traces measurement by photoacoustic spectroscopy using Helmholtz resonator-based sensors

Photoacoustic spectroscopy is a well-established gas traces optical detection technique, which consists in the generation of an acoustic wave in the investigated gas compound excited by a modulated laser beam, and in the detection of this sound wave with a sensitive microphone. The sensitivity of this technique can be greatly enhanced through the use of acoustic resonators. A wide range of resonant configurations has been developed and reported in the literature in the last decades. Among these, Helmholtz resonators are known for their simplicity of implementation, though with a reduced efficiency. The main goal of this work is to demonstrate that Helmholtz resonators can be successfully applied to photoacoustic spectroscopy, delivering sensitivities in the same order of magnitude as other more common configurations, as well as offering a powerful tool to perform differential measurements. Two Helmholtz-based sensors are presented within this thesis. The first sensor has been developed for ammonia sensing, taking benefit from the properties of antimonide-based lasers; the final design has been the result of a carefully optimised compromise between high sensitivity, noise reduction, technical constraints, compact size and straightforward use. After several implemented improvements, an ultimate sub-ppm concentration detection limit has been achieved. The second sensor exploits the intrinsic phase shift existing between the two volumes of a Helmholtz resonator to perform differential measurements. Each of the two volumes of the resonator is filled with a different concentration of the same gas, whereas the exciting laser beam is split in two arms that separately illuminate the volumes, the resulting photoacoustic signal being proportional to the difference between the respective concentrations of the probed gas in the volumes. A particular detection scheme has been implemented to guarantee a linear measurement. The achieved sensitivity is not as high as obtained with the first sensor; nonetheless, the developed sensor offers the possibility to perform measurements that would otherwise require two different sensing devices, resulting in a clear gain in terms of cost and of detection complexity.

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