Two-tone frequency-modulation spectroscopy for quantitative measurements of gaseous species: theoretical, numerical, and experimental investigation of line shapes

High-resolution absorption measurements by use of two-tone frequency-modulation spectroscopy with diode lasers

“…In the present study we extend our previous investigation of TTFMS for high-accuracy quantitative measurements 15 to experimental data recorded over a broad range of pressures. An extensive test of the reliability of the line parameters derived by the leastsquares fitting procedure is performed by comparison of the result with line parameters derived from spectra recorded by use of direct absorption.…”

“…Lineshape analysis by least-squares fitting of an assumed line profile to experimental data, yielding accurate line-shape information, was demonstrated only in a few papers with FMS 13 and TTFMS. 14,15 A leastsquares fitting procedure, with an appropriate theory of the detected signal and well-known line parameters, is used to increase not only the accuracy of quantitative concentration measurements but also the accuracy of measurements of gas parameters such as temperature, velocity, and pressure. 16,17 The ability to perform high-sensitivity measurements with high accuracy in a varying environment is important in areas such as combustion diagnostics and chemical analysis.…”

“…15 Also, the weak dispersion component, which appears in the heterodynedetected signal when the detection phase is adjusted for maximum signal amplitude, was investigated for various experimental situations. Moreover, nonlinear distortion in the modulation response of the diode laser 21 was considered in the theory, which provided a better description of data.…”

“…For better measurement selectivity, the FM index ␤ was chosen to be approximately 0.7, although ␤ Ϸ 1 provides optimum signal amplitude. 15 At three different pressures ͑30, 200, and 400 Torr͒ additional spectra were recorded with ␤ changed to approximately 0.5 and 1, respectively. These additional spectra were used to determine the FM index more accurately, as we describe in Subsection 4.C.…”

Simulation of Dicke-narrowed molecular spectra recorded by off-axis high-finesse optical cavities

“…In this frame, for example, the capability of retrieving precise spectral information from Dicke-narrowed lineshapes recorded by two-tone frequency modulation spectroscopy has been discussed [21].…”