Ultrafast diagnostics of reacting flows and plasmas

Optical measurement techniques have become powerful tools for the detailed study of the chemistry and physics of reacting flows, and plasmas. Traditional combustion diagnostics based on continuous-wave and low-repetition-rate ns- pulsed lasers continue to dominate fundamental studies and applications; however, revolutionary advances in the science and engineering of both ultrashort-pulse (femtosecond) lasers and high-repetition-rate (burst-mode) lasers are driving the advancement of existing diagnostic techniques and enabling the development of new measurement approaches. The ultrashort pulses afforded by femtosecond laser systems provide tremendous peak powers—allowing nonlinear signal generation with broad spectral coverage—and unprecedented temporal resolution for studying chemical kinetics and dynamics. The high pulse-repetition rates of ultrashort-pulse amplifiers as well as ns- and ps-pulse burst-mode lasers allow previously unachievable data-acquisition bandwidths for the study of turbulent time series and combustion instabilities. More importantly, the high pulse energies emanating from these advanced laser systems afford the ability to extend measurement capabilities beyond point-wise measurements to multi-dimensional (line [1D], planar [2D], or even volumetric [3D]) imaging. The rapid growth of ultrafast laser-based spectroscopic measurements has been fueled by the need to achieve the following: 1) time-resolved single-shot measurements 2) simultaneous detection of multiple species, 3) spatially resolved measurements, 4) interference-free measurements (collisional broadening, photolytic dissociation, etc.), and 5) higher dimensionality (line, planar, or volumetric). Several state-of-art ultrafast-laser–based spectroscopic techniques and their remarkable developments will be reviewed in meeting one or all of the above five needs for measurements of temperature and key chemical species concentrations in reacting flows and plasmas.Optical measurement techniques have become powerful tools for the detailed study of the chemistry and physics of reacting flows, and plasmas. Traditional combustion diagnostics based on continuous-wave and low-repetition-rate ns- pulsed lasers continue to dominate fundamental studies and applications; however, revolutionary advances in the science and engineering of both ultrashort-pulse (femtosecond) lasers and high-repetition-rate (burst-mode) lasers are driving the advancement of existing diagnostic techniques and enabling the development of new measurement approaches. The ultrashort pulses afforded by femtosecond laser systems provide tremendous peak powers—allowing nonlinear signal generation with broad spectral coverage—and unprecedented temporal resolution for studying chemical kinetics and dynamics. The high pulse-repetition rates of ultrashort-pulse amplifiers as well as ns- and ps-pulse burst-mode lasers allow previously unachievable data-acquisition bandwidths for the study of turbulent time se...