Measurement of water vapor line strengths in the 1.4-2.7 µm range by tunable diode laser absorption spectroscopy

- Accurate line strength measurement of nine NIR water vapor lines.
- dTDLAS set-up using a single pass cell and different diode lasers (DFB and VCSEL).
- Detailed uncertainty assessment, combined expanded uncertainty of 1.1-2.5% (95% conf. interval).
- Results compare well to literature values, but show lower uncertainty.

Line strengths of nine water vapor absorption lines in the wavelength range between 1.37 and 2.71 µm with line strengths of 10-23-10-20 cm/molecule have been measured using direct tunable diode laser absorption spectroscopy (dTDLAS). Four different light sources were used: three distributed feedback (DFB) diode lasers with wavelengths of 1.37 µm, 2.55 µm and 2.71 µm for measuring one application-specifically selected absorption line with each laser, and a vertical-cavity surface-emitting laser (VCSEL) radiating around 1.39 µm for the measurement of six further absorption lines. Despite the different light sources and line strengths, a uniform measurement and data evaluation method was developed and applied to all lines, and the experimental set-up was kept as similar as possible. This allows a thorough and uniform uncertainty analysis and evaluation of the contributions of the individual experimental parameters to the uncertainty of the derived line strengths. A comprehensive and transparent uncertainty analysis is given for the measurements. Uncertainties of our measured line strengths are in the 1.1-2.5% range (k=2, 95% confidence level). Our measured line strength values agree well with line strengths in the HITRAN 2012 database and other literature sources, we realized lower uncertainties up to a factor of 5-10.