Absorption measurements of water-vapor concentration, temperature, and line-shape parameters using a tunable InGaAsP diode laser

TLDR

The system employs a distributed feedback InGaAsP diode laser at ~1.38 µm, tuned by current modulation for 1‑cm⁻¹ tuning at 80‑Hz, enabling spectrally resolved absorption measurements. The technique accurately measured water‑vapor concentration, temperature, and collision‑broadening parameters, matching calculations and independent measurements, and provided line‑strength and self‑broadening coefficients for several spectral lines in a static cell as well as in laboratory air and postflame gases.

Abstract

A tunable diode laser diagnostic based on spectrally resolved laser absorption has been developed to detect water vapor. The system uses a distributed feedback InGaAsP diode laser, emitting at ~ 1.38 µm. The diode laser is tuned in wavelength by modulation of the current, resulting in 1-cm(-1) tuning at 80-Hz repetition rate. The directly measured absorption spectra yield values for water-vapor concentration and temperature, as well as a collision-broadening line shape. To our knowledge, we accurately determined required data for H(2)O line strengths and self-broadening coefficients for several spectral lines in a static cell filled with pure water vapor. The temperature and concentration of the water vapor present in laboratory room air and in the postflame gases above a methane-air flat flame burner have also been measured. These results agree well with calculated values and independent measurements.