Abstract
Multi-component and multi-point trace gas sensing in the wavelength modulation spectroscopy is demonstrated based on the frequency-division multiplexing and time-division multiplexing technology. A reference photodetector is connected in series with a reference gas cell with the constant concentration to measure the second-harmonics peak of the components for wavelength stabilization in real time. The central wavelengths of the distributed feedback lasers are locked to the target gas absorption centers by the reference second-harmonics signal using a digital proportional-integral-derivative controller. The distributed feedback lasers with different wavelengths and modulation frequencies are injected into the gas cell to achieve multi-components gas measurement by the frequency-division multiplexing technology. In addition, multi-point trace gas sensing is achieved by the time-division multiplexing technology using a photoswitch and a relay unit. We use this scheme to detect methane (CH4) at 1650.9 nm and water vapor (H2O) at 1368.597 nm as a proof of principle with the gas cell path length of 10 cm. The minimum detection limits achieved for H2O and CH4 are 1.13 ppm and 11.85 ppm respectively, with three-point gas cell measurement; thus 10.5-fold and 10.1-fold improvements are achieved in comparison with the traditional wavelength modulation spectroscopy. Meanwhile, their excellent R-square values reach 0.9983 and 0.99564 for the concentration ranges of 500 ppm to 2000 ppm and 800 ppm to 2700 ppm, respectively.
Highlights
Over the last few decades, tunable diode laser absorption spectroscopy (TDLAS) has been widely used in the trace gas detection in various fields, such as industrial production control [1], mine safety monitoring [2, 3], combustion processes [4, 5], Article type: RegularZongliang WANG et al.: Multi-Component and Multi-Point Trace Gas Sensing in Wavelength Modulation SpectroscopyBased on Wavelength Stabilization environmental monitoring [6, 7], and explosive analysis
These results demonstrate that multi-component and multi-point detection can be achieved by the proposed scheme using FDM and time-division multiplexing (TDM)
Multi-component and multi-point trace gas sensing in wavelength modulation spectroscopy (WMS) is proposed based on FDM and TDM; the minimum detection limit (MDL) is significantly improved by the application of the wavelength stabilization scheme
Summary
Over the last few decades, tunable diode laser absorption spectroscopy (TDLAS) has been widely used in the trace gas detection in various fields, such as industrial production control [1], mine safety monitoring [2, 3], combustion processes [4, 5], Article type: RegularZongliang WANG et al.: Multi-Component and Multi-Point Trace Gas Sensing in Wavelength Modulation SpectroscopyBased on Wavelength Stabilization environmental monitoring [6, 7], and explosive analysis. Over the last few decades, tunable diode laser absorption spectroscopy (TDLAS) has been widely used in the trace gas detection in various fields, such as industrial production control [1], mine safety monitoring [2, 3], combustion processes [4, 5], Article type: Regular. Zongliang WANG et al.: Multi-Component and Multi-Point Trace Gas Sensing in Wavelength Modulation Spectroscopy. Among TDLAS techniques, the method that is often referred to as wavelength modulation spectroscopy (WMS) [10,11,12] is the most representative because of its high minimum detection limit (MDL) and sensitivity. WMS is typically used to detect trace gas because of its higher detection sensitivity in comparison with direct absorption spectroscopy
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