Water vapor detection based on three-line TDLAS with a continuous wave quantum cascade laser

Water vapor is an important gas species in atmosphere. In this work, we present a three-line tunable diode laser absorption spectroscopy (TDLAS) gas sensor for water vapor detection based on direct absorption spectroscopy. A continuous wave quantum cascade laser (QCL) is employed as the light source with a wavelength near 7.15 µm. The output wavelength of the QCL is tuned to scan across three absorption lines of water vapor simultaneously in the detection. A detection limit of 10.7 ppm v (parts per million by volume) is obtained in 30-s integration time with a 40 cm gas cell. This detection limit could be further improved if wavelength modulation spectroscopy (WMS) or a multi-pass gas cell is introduced to the system.