A cw-terahertz gas analysis system with ppm detection limits

We present a demonstration system for gas analysis based on cw-terahertz absorption spectroscopy. The setup combines tunable 1.5 mm distributed-feedback (DFB) lasers, InGaAs photomixers and a multipass absorption cell into a high-resolution spectrometer. At gas pressures of 1 mbar, we achieved detection limits in the range of approx. 20 ppm for ammonia (NH3) and 100 ppm for sulfur dioxide (SO2). Absorption profiles with halfwidths of 7 MHz (2×10-4 cm-1) were measured.