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.