Abstract
In this review, methane absorption characteristics mainly in the near-infrared region and typical types of currently available semiconductor lasers are described. Wavelength modulation spectroscopy (WMS), frequency modulation spectroscopy (FMS), and two-tone frequency modulation spectroscopy (TTFMS), as major techniques in modulation spectroscopy, are presented in combination with the application of methane detection.
Highlights
Due to global warming and climate change, the monitoring and detection of atmospheric gas concentration has come to be of great value
The emission wavelength of the narrow-linewidth diode laser is scanned over the target gas absorption line, and tunable diode laser absorption spectroscopy (TDLAS) has become an effective technique for the rapid and online analysis of gas component concentration, due to the advantage of high spectrum resolution [13,14,15,16,17,18,19,20,21,22]
When modulation frequency is larger than the width of the absorption line of interest, the technique is named by frequency modulation (FM)
Summary
Due to global warming and climate change, the monitoring and detection of atmospheric gas concentration has come to be of great value. Optical spectroscopy for detecting gases is based on the Beer-Lambert law [10,11,12], in which the light attenuation is related to the effective length of the sample in an absorbing medium, and to the concentration of absorbing species, respectively. By this theory, the emission wavelength of the narrow-linewidth diode laser is scanned over the target gas absorption line, and tunable diode laser absorption spectroscopy (TDLAS) has become an effective technique for the rapid and online analysis of gas component concentration, due to the advantage of high spectrum resolution [13,14,15,16,17,18,19,20,21,22]. After introducing methane absorption characteristics and recent progress of tunable diode lasers (TDLs), recent advances in methane detection using modulation spectroscopy are presented
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