Towards high-precision hollow waveguide-based gas sensors adapting nonuniform pressure and immune to flow fluctuation

Hollow waveguide (HWG) based gas sensors boast fast response time, high-sensitivity, compactness, but lessprecision for their intrinsic very small bore-diameter and sampling volume. We investigated the effects of gas flow regimes on the complicated spectral line broadening in HWGs, and analyzed quantitatively by taking atmospheric water vapor spectrum as an example. We derived an equivalent pressure to simplify the calculation of spectral line broadening in nonuniform pressure of HWGs, and proposed to enhance the performance of such kinds of sensors with wavelength modulation spectroscopy by laminar flow regime, pump down mode, and the optimized flow rate. Experimental results showed dramatic improvements with the scheme: sensitivity increase by 18.6%, lower uncertainty, and immune to flow fluctuation. The method is efficient, convenient and with no extra cost. It can promote both HWG and substrate-integrated HWG (iHWG) based gas sensors.