Influence of Light Coupling Configuration and Alignment on the Stability of HWG-Based Gas Sensor System for Real-Time Detection of Exhaled Carbon Dioxide

Abstract

A mid-infrared tunable diode laser absorption 2 spectroscopy (TDLAS) gas sensor based on hollow waveguide 3 (HWG) gas cell for real-time exhaled carbon dioxide (eCO2) 4 detection is reported. A 2.73 μm distributed feedback (DFB) laser 5 was used to target a strong CO2 absorption line, and wavelength 6 modulation spectroscopy (WMS) with the second harmonic 7 (WMS-2ƒ) was used to retrieve the CO2 concentration with high 8 sensitivity. The influence of different parameters, including 9 coupling configuration of HWG, laser-to-HWG and 10 HWG-to-detector coupling alignment on the stability of the 11 HWG sensor is systematically studied. The HWG eCO2 sensor 12 showed a fast response time of 2.7s, detection limit of 17 ppmv, 13 and measurement precision of 20.9 ppmv with a 0.54 s temporal 14 resolution. The eCO2 concentrations changed in breath cycles 15 were measured in real time. The Allan variance indicated that 16 the detection limit can reach 1.7 ppmv, corresponding to a 17 detection sensitivity of 1.3×10-8 cm-1 Hz-1/2 , as the integration time 18 increases to 26 s. This work demonstrates the performance 19 characteristics and merits of HWG eCO2 sensor for exhaled 20 breath analysis and potential detection for other exhaled gases.

Compact Hollow Waveguide Mid-Infrared Gas Sensor For Simultaneous Measurements of Ambient CO2 and Water Vapor

“…Hence, the aperture of diaphragm is set to 1 mm. Apart from that, the influences of in-coupling (position of the laser related to the HWG) and out-coupling (position of the detector related to the HWG) on system stability are also investigated [24]. The laser was placed at the central position for long stability time.…”

TDLAS second harmonic demodulation based on Hilbert transform

“…Characterized by high sensitivity, high resolution, and fast response, tunable diode laser absorption spectroscopy (TDLAS) is extensively employed in industrial process gas analysis [1,2], environmental monitoring [3,4], and biomedical science [5][6][7]. TDLAS technology can be roughly divided into direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS).…”

Simultaneous measurement of ammonia and ethylene using hollow waveguide‐based wavelength modulation spectroscopy with a 9.56 μm quantum cascade laser

“…Hollow waveguide (HWG) can miniaturize TLAS gas sensors with very small sampling volume and fast response time, which has been widely used as a novel gas cell recently. 10,19 In our previous work, we have quantitatively investigated the adsorption effect of ammonia gas molecules on the inner surface of Ag/AgI coated HWG, 20 the of transmission loss property of semiconductor laser beam propagating in the HWG, 21 and the effects of gas flow regimes on the complicated spectral line broadening in HWGs. 22 Based on the above research, HWG enhanced TLAS gas sensors have been developed and demonstrated for atmospheric environment monitoring 23 and exhaled breath analysis.…”