Recent advances of sensor systems, based on mid-infrared interband cascade lasers (ICLs) and quantum cascade lasers (QCLs) for the detection of trace gas species and their applications in environmental monitoring, atmospheric chemistry, life sciences, and the petrochemical industry, will be reported. The development of compact ICL and QCL based trace gas sensors will permit the targeting of strong fundamental rotational-vibrational transitions in the mid-infrared, which are one to two orders of magnitude more intense than transitions involving overtone and combination bands in the near-infrared. One of most robust detection technique is based on the PhotoAcoustic Spectroscopy (PAS). It takes advantage of the thermal expansion of the target gas when it absorbs radiation from the excitation laser. These molecules, once excited by the absorbed optical radiation, subsequently relax to their ground state via non-radiative processes which produce localized heating within the sample and, hence, a localized pressure wave. In the standard PAS, the pressure wave is detected by a microphone. Quartz-enhanced PhotoAcoustic Spectroscopy (QEPAS) is an alternative approach to PAS detection of trace gases, utilizing a quartz tuning fork (QTF) as an ultra-compact resonant acoustic transducer with a high quality factor to detect photoacoustic excitations. . In QEPAS, quartz crystals with a resonant frequency of 32,768 Hz (215) are used due to their low cost, since they are used in timing applications such as watches, clocks and cell phones. Furthermore, a frequency of 32.768 kHz is slow enough for relaxation processes within the target gas of interest not to reduce the acquired QEPAS signal and fast enough that the signal build up time is <1 s . An acoustic micro-resonator (mR) tube for sound wave enhancement is an important component that is acoustically coupled with the QTF to strongly enhance the acoustic wave field. In a standard QEPAS configuration, a stainless steel tube is cut into two pieces and the QTF is inserted between them.
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