Miniaturization of hollow waveguide cell for spectroscopic gas sensing

Gas sensing system has found applications in industrial process and environmental monitoring. Infrared hollow waveguide is one of the best choices for gas cell in spectroscopic sensing due to the advantages of low loss, flexibility, and fast response. In order to miniaturize the gas sensing system while maintain optical path length, long hollow waveguide can be coiled into a small box to reduce physical dimensions. Bending causes additional loss for hollow waveguide, therefore it is necessary to optimize the parameters for bent hollow waveguide cell. In this paper, a calculation method was established to analyze the performance of a gas sensing system with a bent hollow waveguide as the absorption cell. Simulation results show the relationship between gas absorption intensity and system parameters such as waveguide length, bore diameter, bending radius, system noise, and divergence angle of the light source. Optimized parameters for the waveguide cell were given based on the simulation results. An experimental system was set up by using a Fourier transform infrared (FTIR) spectrometer and flexible hollow waveguides. Preliminary experiments on waveguide length, bending radius, and system signal-to-noise ratio (SNR) were conducted and the measured data agree well with simulation results.