High-Sensitivity High-Resolution Diode Laser Spectroscopy in the Near-Infrared Region

Abstract: High-resolution spectroscopy measurements have been performed using near-infrared diode lasers. Special emphasis was placed upon high accuracy absorption lineshape measurements, which provide the basis for quantitative gas analysis. For this purpose, relevant spectral characteristics of the radiation emitted by diode lasers were studied. High-resolution measurements using newly developed long-wavelength GalnAsSb diode lasers were demonstrated. The experimental technique utilised constant-current fast wavelength scanning, which minimises effects of variations in the diode laser output characteristics during a scan. The technique was combined with two-tone frequency modulation spectroscopy (TIFMS), which greatly improved measurement sensitivity. An accurate theory of TIFMS lineshapes accounting for dispersion and nonlinear distortion of the diode laser modulation-response was developed. The applicability of TIFMS for accurate concentration and linewidth measurements was demonstrated. A least-squares fitting procedure for retrieval of the spectral information from direct detection and TIFMS lineshapes using the Voigt and collisionally narrowed profiles was developed. Comparison of measurements using TIFMS and direct detection was performed, showing the two to be in excellent agreement. The high accuracy achieved in spectral recordings, and the rigorous treatment of TIFMS lineshapes, allowed quantitative measurements of the collisional narrowing effect using weak absorptions. Pressure-induced broadening and shift coefficients, and line strengths of molecular oxygen A-band transitions at 760 nm and water vapor transitions at 1830 nm have been determined. The experimental and retrieval techniques developed allow sensitive and accurate measurements of gaseous species in a broad range of environments.