Abstract
By combining frequency-division multiplexing and normalized wavelength modulation spectroscopy, a robust remote multi-species sensor was developed and demonstrated for practical hydrocarbon monitoring. Independently modulated laser beams are combined to simultaneously interrogate different gas samples using an open-ended centimeter-size multipass cell. Gas species of interest are demodulated with the second harmonics to enhance sensitivity, and high immunity to laser power variation is achieved by normalizing to the corresponding first harmonics. Performance of the optical sensor was experimentally evaluated using methane (CH4) and acetylene (C2H2) samples, which were separated by a 3-km fiber cable from the laser source. Sub-ppm sensitivity with 1-s time resolution was achieved for both gas species. Moreover, even with large laser intensity fluctuations ranging from 0 to 6 dB, the noise can be kept within 1.38 times as much as that of a stable intensity case. The reported spectroscopic technique would provide a promising optical sensor for remote monitoring of multi hazardous gases with high robustness.
Highlights
Gas monitoring is crucial for safe operation at chemical plants, coal mines, and gas stations, where leaked toxic, flammable, or explosive gases may cause serious accidents [1,2].Besides, the ability of simultaneous multiple components detection is attracting rising attention in human breath diagnosis [3]
Various typical multi-species sensing techniques were afterwards developed based on photoacoustic spectroscopy (PAS), dual-frequency comb (DFC), and laser heterodyne radiometer (LHR) [8,9,10,11]
Wu et al reported a dual-gas quartz enhanced photoacoustic spectroscopy (QEPAS) sensor for H2 O and C2 H2 with two near-infrared lasers, and the excited acoustic signals were separated by two different demodulation frequencies, i.e., the fundamental and the first overtone vibrations of the same QTF [8]
Summary
Gas monitoring is crucial for safe operation at chemical plants, coal mines, and gas stations, where leaked toxic, flammable, or explosive gases may cause serious accidents [1,2]. Various typical multi-species sensing techniques were afterwards developed based on photoacoustic spectroscopy (PAS), dual-frequency comb (DFC), and laser heterodyne radiometer (LHR) [8,9,10,11]. Influence of laser fluctuation on the spectrum recovery can be effectively suppressed by a dual-beam regime with balanced detection [14] or a division process [15] For both single-beam and dual-beam regimes, it is still challenging to stabilize the light power coupled on photodetectors, especially for remote sensing after long-distance transmission with inevitable and irregular scattering loss, inconstant transmission loss, even devices’ unstable operation. Frequency-division multiplexing (FDM) and normalized wavelength modulation spectroscopy are harnessed towards real-time remote multi-species sensing with simpler analysis. Only the non-electrified probe serves in the work area while all the others could operate in a much gentler monitoring area, guaranteeing intrinsic safety and stable operation for hazardous gas sensing
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