Diode-Laser Induced Fluorescence Spectroscopy of an Optically Thick Plasma in Combination with Laser Absorption Spectroscopy

Distortion of laser-induced fluorescence profiles attributable to optical absorption and saturation broadening was corrected in combination with laser absorption spectroscopy in argon plasma flow. At high probe-laser intensity, saturated absorption profiles were measured to correct probe-laser absorption. At low laser intensity, nonsaturated absorption profiles were measured to correct fluorescence reabsorption. Saturation broadening at the measurement point was corrected using a ratio of saturated to non-saturated broadening. Observed LIF broadening and corresponding translational temperature without correction were, respectively, ？GHz and ？K and corrected broadening and temperature were, respectively, ？GHz and ？K. Although this correction is applicable only at the center of symmetry, the deduced temperature agreed well with that obtained by LAS with Abel inversion. 1. Introduction Diode laser-induced fluorescence (DLIF) has a feature of high wavelength resolution on the order of picometers, which makes it useful to obtain translational temperature by measuring Doppler broadening of an atomic line of gases [1, 2] to the same degree as diode laser absorption spectroscopy (DLAS) [3, 4]. The advantage of DLIF over DLAS, which is a line-of-sight measurement, is the possibility of point measurements. However, in optically thick plasma, absorption of the excitation laser and reabsorption (or self-absorption) of fluorescence can broaden the fluorescence profile. The temperature deduced from observed fluorescence tends to be overestimated. Hertz-corrected DLIF profiles in flame consider the laser absorption effect by solving a nonsymmetric 1D distribution of the depleting laser using an iterative procedure [5], although fluorescence reabsorption was not corrected. Although intense lasers with substantial fluorescence that can achieve a high signal-to-noise ratio are preferred, intense lasers are known to cause additional broadening, termed as saturation broadening or power broadening [6]. Moreover, their saturation regime is usually avoided. Instead, the linear region is used in LIF temperature measurements [7]. In contrast, in low-pressure plasma, the saturation intensity is low and the fluorescence profile is easily saturated. The saturation effect in LIF was investigated in earlier studies [8–10], in which the laser spectral width was comparable to Doppler broadening, and in which the spectral wing of laser was able to induce substantial fluorescence, causing additional broadening. In DLIF, however, the laser spectral width is about 1？MHz and three others