Imaging thermometry in a spray flame synthesis process via single shot two-line atomic fluorescence using pulsed narrow-band optical parametric oscillators

The present work demonstrates shot-to-shot calibrated imaging thermometry in a spray-flame-synthesis (SFS) process by utilizing two-line atomic fluorescence (TLAF) of indium atoms. Measurements in this highly turbulent, particle-laden flame were accomplished by implementing two optical parametric oscillators (OPOs) designed for pulsed operation in order to generate radiation with the required excitation wavelengths. The OPOs were optimized towards a narrow-band spectral output while maintaining high tunability. A linear OPO design was realized, which is capable to produce laser radiation with a bandwidth as narrow as 148 pm. These laser sources were employed to excite the \({5}^{2}{P}_{1/2}\to {6}^{2}{S}_{1/2}\) transition at 410.13 nm and the \({5}^{2}{P}_{3/2}\to {6}^{2}{S}_{1/2}\) transition at 451.13 nm using to two coplanar light sheets formed, one from each OPO. As this TLAF-method requires calibration, the light sheets were split by a 50:50 beam splitter and guided through two burners simultaneously, one being a calibration flame with a known temperature profile and the other one being the flame of a spray-flame-synthesis burner. By recording the fluorescence signals from both flames in the same camera frame, calibration could be conducted for each single shot, which allows to compensate for shot-to-shot spatial irradiance fluctuations of the OPOs. The shown TLAF-measurement set-up thus enables highly reliable and accurate single-shot measurements of flame temperatures in SFS.