High-Speed OH PLIF Imaging in Flames Using Third Harmonic of an Amplified Femtosecond Laser

Optical parametric amplifiers (OPAs) are traditionally used for frequency conversion of ultrashort, femtosecond-duration laser pulses from near-infrared (NIR) to ultraviolet (UV) wavelengths for electronic excitation of flame species such as OH. However, the implementation of OPA outputs for efficient planar laser-induced fluorescence (PLIF) imaging is limited because of the low wavelength conversion efficiency. Typical UV outputs generated from OPAs do not have sufficient pulse energy for single-laser-shot imaging of minor intermediate flame species. Therefore, as an alternative to the OPA, direct frequency conversion schemes using nonlinear β-barium-borate (BBO) crystals have been implemented. In this study, the fundamental laser output wavelength near 850-nm and 1-kHz was used for the third harmonic generation (THG) of fs pulse to obtain approximately 283.3-nm radiation. An average pulse energy in excess of 180 μJ/pulse is reported when pumped by 2mJ/pulse of 850-nm input. The THG output was initially used to excite OH radical in a premixed H2/air flame established over a near-adiabatic Hencken burner. Subsequently, a laser sheet was used to study the 2D flame development and dynamic flame structure via OH detection in CH4/H2 (1:1) diffusion flame established over a turbulent jet burner. Overall, the per-pulse conversion efficiency of ~ 9% (~ 6× more than conventional OPA systems) around 283-nm and at 1 kHz was achieved. The present THG system is not only more efficient but also compact, robust, and cheaper than commercial OPA systems; hence it can be a significant step towards kHz-rate e OH PLIF diagnostics of various flames of practical relevance.