250-kHz UV/IR sodium planar laser-induced fluorescence and application to 3D imaging of a flame.

In this article, we introduce high-repetition-rate planar laser-induced fluorescence of sodium using a UV/IR scheme in which the 3s-4p transition at 330 nm is excited, followed by detecting fluorescence from the 3d-3p intermediate transition at 819 nm for qualitative imaging of flame structure. This approach avoids background interference from strong sodium emission from the flame at 589 nm by using strong long-pass filters. A flame was rendered in three dimensions (3D) by sweeping the laser sheet across it, highlighting the utility of the UV/IR excitation/detection scheme in resolving complex structures. The ethanol-based flame was mixed with water, and NaCl salt was dissolved to provide the atomic sodium necessary for the flame fto be excited by the laser. The success of filtering for background removal assisted the production of high-resolution 3D renderings of a "frozen" flame, which were possible due to the 250 kHz repetition rate of the laser.