Gas detection is important for industrial safety, environmental protection, and environmental
monitoring. Absorption spectroscopy is a widely used technique for gas detection. By applying laser
remote sensing techniques, gas sensing with large standoff distances becomes possible. The industry
standard method uses the double frequency technique, in which the laser wavelength is modulated about
a center wavelength corresponding to the absorption peak of the target gas species, and the optical
signal is detected at the second harmonic of the modulation frequency. Alternatively, by selecting two
wavelengths which correspond to strong and weak absorption of the target gas species, two-dimensional
imaging or visualization of gas leaks becomes possible. These techniques have been applied to
detection and imaging of natural gas (methane gas) leaks. Laser sensing is also used for in situ gas
analysis in power plants and incineration plants. Recent progress in quantum cascade lasers has enabled
detection of minor species in harsh environments such as flue gas.
Keywords: Absorption spectroscopy, 2f method, differential absorption method, methane gas,
visualization, flue gas, stack exhaust gas, differential absorption lidar, atomic laser absorption spectroscopy,
quantum cascade laser.
