Effect of Spontaneous Emission in Hollow Waveguide on Gas Absorbance Measurement by Absorption Spectroscopy

Hollow waveguide (HWG) as an absorption cell for gas spectral sensing has the advantages of low transmission loss, fast response speed, and high path length to volume ratio. However, due to the tubular structure of the HWG, the spontaneous emission generated in the HWG will be transmitted together with the incident laser, which reduces the measured absorbance. We measured and verified the spontaneous emission at the outlet of the HWG by basing on the laser gas absorption spectroscopy and combining with the distinguished polarization characteristics of laser and spontaneous emission. Then, we quantitatively determined the intensity distribution of the spontaneous emission output from the HWG based on the infrared (IR) imaging using an IR camera. We also investigated the effect of the spontaneous emission on the measured absorbance as a function of the HWG length by comparative experiments. The effect exponentially decreases as the HWG length increases since the increased loss. The method could be used for designing or optimizing of HWG-based sensors.