Large-modulation-depth 2f spectroscopy with diode lasers for rapid temperature and species measurements in gases with blended and broadened spectra.

A method that uses tunable diode lasers is developed for rapid temperature and concentration measurements of gases with highly broadened and congested spectra. Wavelength modulation absorption spectroscopy with 2f detection is utilized, because this derivative method offers benefits in dealing with blended spectral features. The 2f signal depends critically on the modulation depth of the laser alpha, which is increased to values above those typically achieved when wavelength modulation spectroscopy with diode lasers is performed. The 2f method with large modulation depths is validated by using near-IR diode lasers to probe pressure-broadened water-vapor features in the 1.4-microm region over a range of temperatures from 296 to 800 K and at pressures as high as 20 atm. Modulation depths as high as alpha = 0.8 cm(-1) are attained at modulation frequencies of 50 kHz and measurement bandwidths of 15 kHz. Comparisons of experimental results with 2f simulations, based on the HITRAN spectral database, provide confirmation of the capability of this method for rapid measurements of gas temperature and species concentration.