Planar Laser-Induced Fluorescence of the OH Radical using a Tunable MHz-Rate UV Source

Abstract

We report on a source for MHz-rate, pulse-burst, tunable high-energy UV radiation and its application to high-speed imaging and temperature measurements of combustion based on planar laser-induced fluorescence (PLIF) spectroscopy of the OH radical. The system, comprised of a high-speed injection-seeded optical-parametric oscillator (OPO) pumped by a frequency-doubled MHz-rate pulse-burst Nd:YAG laser, was characterized in a laminar hydrogen-air diffusion flame stabilized over a near-adiabatic burner. High-speed OH PLIF was demonstrated at rates up to 50 kHz in a sonic hydrogen-air diffusion flame, showing the capability to capture turbulent fluid-flame interactions. In addition to high-speed OH PLIF at a fixed excitation wavelength, methods for planar spectroscopy are discussed, including wavelength scanning during a pulse burst as well as dual-wavelength injection seeding. Excitation scans of the UV radiation during a pulse burst can be achieved using various cavity stabilization techniques, such as discrete linearization of the OPO cavity modes. Preliminary work on an alternative two-line approach is also presented, with wavelength switching times as low as 20 ns. The OPO in this case is pumped by a double-pulsed burstmode pump laser with inter-pulse separation as low as 25 ns and pulse energies of 30mJ at 532nm. Feasibility studies of high-speed planar OH scanning and two-line spectroscopy are planned in laminar and turbulent flames.