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1 February 2019 Extending the linear concentration range of a multi-gas-analyzer
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Abstract
Commercial gas sensors based on quantum cascade laser (QCL) spectroscopy usually rely on a single measurement technique, such as the direct absorption approach, the photoacoustic method, or measuring the dispersion. While each method has its advantages/disadvantages, it is obvious to combine different techniques in a sensor to improve its overall performance which might include its precision, limit of detection, speed but also the concentration range accessible by a given sensor set-up. In this regards a very promising combination of 2f wavelength modulation spectroscopy (2f-WMS) with heterodyne phase sensitive dispersion spectroscopy (HPSDS) is presented. WMS is a direct absorption based technique - within the constraints of the Lambert-Beer law - utilizing laser modulation in order to generate higher harmonics of the absorption signal, thus shifting the measured signal bandwidth to higher frequencies and reducing 1/f noise. HPSDS in contrast relies on quantifying the dispersion induced by the anomalous dispersion close to absorption lines of the target analyte. In contrast to 2f-WMS, HPSDS is independent of the light intensity reaching the detector, which allows to cover a significantly larger concentration range. The simplicity and the benefits of combining these two techniques are demonstrated for carbon monoxide (CO). Furthermore, the sensor is equipped with three additional QCLs for quantifying the analytes nitric oxide (NO), nitrous oxide (NO2) and sulfur dioxide (SO2) in the ppbv range using 2fWMS, making it perfectly suited for ambient air measurements.
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Andreas Genner, Pedro Martín-Mateos, Harald Moser, Johannes P. Waclawek, and Bernhard Lendl "Extending the linear concentration range of a multi-gas-analyzer", Proc. SPIE 10926, Quantum Sensing and Nano Electronics and Photonics XVI, 109262P (1 February 2019); https://doi.org/10.1117/12.2509331
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