Abstract
A high sensitivity wavelength modulated reinjection off-axis integrated cavity output spectroscopy (WM-RE-OA-ICOS) experimental setup was built at a 2 μm band. On the basis of an off-axis integrated output spectroscopy (OA-ICOS), combined with an optical reinjection (RE) approach to improve signal intensity, and wavelength modulation spectroscopy (WMS) to improve the signal-to-noise ratio (SNR) of the system, the experimental study of trace CO2 with high sensitivity was carried out using the setup. The performance was compared and evaluated, and the results show that: Compared with the OA-ICOS, the wavelength modulated reinjection OA-ICOS enhanced the signal intensity by 6.3 times, and the SNR increased 7.2 times from 179 to 1288. The Allan variance results showed that the detection limit of the system is 0.35 ppm when the average system time is 230 s. The setup was used to measure the indoor CO2 concentration for a long time (22 h), and the measured results were in line with the actual concentration change. The proposed method shows good performance enhancement for the OA-ICOS system in trace gas measurements.
Highlights
Accepted: 22 September 2021Laser absorption spectroscopy in the aspect of trace gases measurement, with high sensitivity and high selectivity, has caught the attention of relevant researchers [1,2,3], especially cavity-enhanced absorption spectroscopy (CEAS), and cavity ring-down spectroscopy (CRDS)
Centeno et al [12,13] improved the structure of reinjection off-axis integrated cavity output spectroscopy (RE-OA-ICOS)
CO standard gas with a concentration of 400 ppm was selected as improvement of signal-to-noise ratio (SNR)
Summary
Laser absorption spectroscopy in the aspect of trace gases measurement, with high sensitivity and high selectivity, has caught the attention of relevant researchers [1,2,3], especially cavity-enhanced absorption spectroscopy (CEAS), and cavity ring-down spectroscopy (CRDS). By establishing a high-finesse cavity and coupling the narrow-band laser into the cavity with mode matching, the effective absorption path length can be increased to thousands of meters and the detection sensitivity can be greatly improved. Centeno et al [12,13] improved the structure of reinjection off-axis integrated cavity output spectroscopy (RE-OA-ICOS) They optimized the distance between the reinjection mirror and the front cavity mirror and the SNR increased up to 10 times. Nadeem et al [14,15] obtained the maximum enhancement through a grid search and the genetic algorithm optimization of the reinjection method It achieved a gain of 1000 times the signal intensity on a short cavity (only 3 cm long), which further developed the miniaturization and integration of the optical gas sensor. Based on OA-ICOS, the device combines the reinjection method and wavelength modulation spectroscopy and, provides a new solution for high sensitivity spectral measurement
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