The recent development of interband cascade lasers (ICLs) and quantum cascade lasers (QCLs) based trace gas
sensors enables the targeting of strong fundamental rotational-vibrational transitions in the mid-infrared, which are
one to two orders of magnitude more intense than transitions in the near-infrared. This has led to the development of
mid-infrared compact, field deployable sensors based on two sensor system platforms, laser absorption and
quartz enhanced spectroscopy. These sensor platforms are applicable for environmental monitoring, atmospheric
chemistry and for use in the petrochemical industry. The spectroscopic detection and monitoring of three molecular
species, methane (CH4), ethane (C2H6) [1], formaldehyde (H2CO) [2] and hydrogen sulphide (H2S) [3] will be
described.
KEYWORDS: Photoacoustic spectroscopy, Quartz, Acoustics, Resonators, Gas lasers, Quantum cascade lasers, Microresonators, Sensors, Interference (communication), Signal to noise ratio
We report here on the realization of a single-tube on-beam quartz-enhanced photoacoustic (QEPAS) spectroscopy sensor employing a custom-made quartz tuning fork (QTF) with a large prong spacing. The prongs of the QTF have been designed in order to provide a quality factor twice higher when the QTF operates in the first overtone flexural mode than in the fundamental mode. The influence of the microresonator tube on the main parameters characterizing the sensing performance of the QEPAS spectrophone, including the quality factor, the magnitude of the QEPAS signal and the associated background noise was investigated in detail.
We report here an analysis of the performance of a quartz-enhanced photoacoustic (QEPAS) system operating in a pulsed mode by employing a quantum cascade laser (QCL). The QEPAS system is based on a quartz tuning fork (QTF) having fundamental resonance frequency of 4.2 kHz and a first overtone resonance of 25.4 KHz. Water vapor was used as a target gas by selecting its absorption line falling at 1296.5 cm-1 with a line strength of 1.69⋅10-22 cm/molecule. The QEPAS signal was investigated, while varying the QCL duty-cycle from continuous wave operation, down to 5%, which corresponds to a laser power consumption of 0.17 mW and a pulse-width of 4 μs.
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