Compact diode-laser spectrometer ISOWAT for highly sensitive airborne measurements of water-isotope ratios

The tunable diode-laser absorption spectrometer ISOWAT for airborne measurements of the water-isotope ratios ¹⁸O/¹⁶O and D/H is described. The spectrometer uses a distributed feedback (DFB) diode laser to probe fundamental rovibrational water-absorption lines at around 2.66 μm. Very-low-noise system components along with signal averaging allow for a detection limit of 1.2 and 4.5‰ for measurements of ¹⁸O/¹⁶O and D/H, respectively, for a water-vapour mixing ratio of 100 ppmv and an averaging time of 60 s. This corresponds to a minimum detectable absorbance of ～5×10^?6 or ～6.6×10^?10 cm^?1 when normalized to pathlength. In addition to its high sensitivity, the spectrometer is highly compact (19-inch rack at a height of 35 cm, excluding pump and calibration unit) and light weight (<40 kg total). The total power consumption is around 350 W, and the instrument is fully automated. ISOWAT will be calibrated during flight with known water-isotope ratios using a compact calibration-gas source.