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

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