Common oscillator multi-leg burst-mode laser systems for interference-free multimodal imaging of reacting flows

Understanding gas-phase combustion reactions in turbulent flows and energetic materials requires high-speed multiparameter imaging at kHz-MHz frame rates. Such measurements require energies per pulse in the range of 10s to 100s mJ at MHz rates, which led to development of burst-mode nanosecond lasers with burst durations up to 100 ms and energies per burst of up to 400 J. Burst-mode laser technology have enabled significant advances in measurement capabilities, for accessing multiple species, temperature, and velocity. However, the number of simultaneously measured parameters are limited by number of lasers and interference between modalities having similar output spectral characteristics. Here we report the laser system with three outputs with up to 1 J/pulse at 10 kHz and variable time delays between outputs to avoid interference between modalities. The laser is based on the common all-fiber oscillator and free-space Nd:YAG flashlamppumped preamplifier to reduce the cost and size of the overall system. The oscillator/preamplifier produces up to four pulses with user-selected time delays between pulses. This four-pulse sequence is repeated at user selected repetition rate up to 500 kHz. The pulse sequence is split between three free-space Nd:YAG amplifiers by two Pockels cells with time resolution of 10 ns. Special precaution is taken to minimize the crosstalk between amplifiers with less than 5% prepulse/afterpulse in the fundamental output and less than 1% in the harmonics. The system is designed for measurements of species (via planar laser induced fluorescence, PLIF) and velocity (via particle image velocimetry, PIV) at rates up to 500 kHz and total burst duration of 10 ms, with the feasibility of achieving MHz rates with further developments. The simultaneous measurements of velocity using PIV and PLIF imaging of hydroxyl and formaldehyde in a turbulent jet flame is demonstrated.