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The development of highly sensitive and selective optical sensor systems using tunable semiconductor-laser-based spectroscopic trace-gas detection techniques is reported in this chapter. The quantitative detection and monitoring of tracegas molecules in real-world applications such as atmospheric chemistry, pollution monitoring, and industrial process control in most cases require the targeting of fundamental vibrational-rotational (V-T) molecular absorption bands located between the 3- and 24-μm wavelengths. The mid-infrared fundamental absorption bands of several small molecules of potential interest for trace-gas monitoring are shown in this chapter. within two mid-infrared atmospheric transmission windows. The upper panel shows absorption spectra in the atmospheric window between the bending fundamental of water centered at around 1600 cm-1 and the water OH stretches starting above 3200 cm-1. The lower panel shows absorption spectra in the atmospheric window below the water bending fundamental. The logarithmic ordinate scales are the integrated intensities of the lines on a per-molecule basis. These spectral regions can be covered by narrow-linewidth and high-performance semiconductor lasers, in particular quantum cascade lasers (QCLs) and interband cascade lasers (ICLs). Therefore, trace-gas optical spectroscopic sensors using a QCL or ICL as an excitation source are responsible for improving the spectral resolution of the measurements and achieving real time, continuous ultrasensitive detection of trace-gas molecular species at the concentration levels from the percent level down to parts per trillion (ppt). In this chapter the spectroscopic detection and monitoring of various specific molecular species, such as ethane (C2H6), methane (CH4), nitrous oxide (N2O), ammonia (NH3), nitric oxide (NO), carbon monoxide (CO), and sulfur dioxide (SO2) are described. All of these molecules were detected based on three different detection techniques: tunable diode laser absorption spectroscopy (TDLAS), conventional photoacoustic spectroscopy (CPAS), and quartz-enhanced photoacoustic spectroscopy (QEPAS). Other ultrasensitive and highly selective spectroscopic techniques that are employed by research groups for trace-gas detection include: balanced detection, laser-induced breakdown spectroscopy (LIBS), noise immune cavity enhanced optical heterodyne molecular spectroscopy (NICE-OHMS), Faraday rotation spectroscopy (FRS), and frequency comb spectroscopy.These spectroscopic techniques can achieve minimum detectable absorption losses in the range from 10-8 to 10-11 cm-1/Hz. The choice of an optimum detection technique depends on the requirements of the specific application and the characteristic features of the single-mode-operated infrared laser source, such as available optical power, tunable wavelength, or beam quality. Moreover, to perform gas detection measurements, various parameters such as gas pressure and modulation depth also need to be optimized.
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