Abstract
A compact widely-tunable fiber-coupled sensor for trace gas detection of hydrogen sulfide (H₂S) in the mid infrared is reported. The sensor is based on an external-cavity quantum cascade laser (EC-QCL) tunable between 7.6 and 8.3 μm wavelengths coupled into a single-mode hollow-core waveguide. Quartz-enhanced photoacoustic spectroscopy has been selected as detecting technique. The fiber coupling system converts the astigmatic beam exiting the laser into a TEM(00) mode. During a full laser scan, we observed no misalignment between the optical beam and the tuning fork, thus making our system applicable for multi-gas or broad absorber detections. The sensor has been tested on N₂:H₂S gas mixtures. The minimum detectable H₂S concentration is 450 ppb in ~3 s integration time, which is the best value till now reported in literature for H₂S optical sensors.
Highlights
Environmental monitoring is among the topics of larger interest for both scientific and civil purposes
The sensor is based on an external-cavity quantum cascade laser (EC-quantum cascade lasers (QCLs)) tunable between 7.6 and 8.3 μm wavelengths coupled into a single-mode hollow-core waveguide
The use of mid-IR external cavity (EC)-QCLs, able to cover with a single-device a tuning range larger than 100 cm 1, enables the quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor to monitor a large number of different molecules by proper tuning of the laser frequency [17]
Summary
Environmental monitoring is among the topics of larger interest for both scientific and civil purposes. The use of mid-IR external cavity (EC)-QCLs, able to cover with a single-device a tuning range larger than 100 cm 1, enables the QEPAS sensor to monitor a large number of different molecules by proper tuning of the laser frequency [17]. The accuracy of this multi-gas sensing depends on the capability of keeping the laser focused between the prongs of the ADM during the spectral scan. With our mid-IR QEPAS system we reached a H2S detection limit in both dry and wet N2 down to 450 ppb in 3 seconds integration time (330 ppb for 30s integration time), which represents the best achieved value with optical techniques
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