Degenerate four-wave mixing experiment of C₂ molecules in a forward geometry for high temperature measurement

Laser-based spectroscopic techniques such as laser induced fluorescence (LIF), coherent anti-stokes Raman scattering (CARS), and degenerate four-wave mixing (DFWM) have been demonstrated as useful diagnostic tools for the detection of molecular species or atoms in hostile environments such as combustion and plasma processes. They do not significantly perturb the probing system, and can provide high spatial and temporal resolution. Recently resonant DFWM has received much attention because it enhances the poor aspect of CARS and LIF on the measurement of minor species in luminous fields. DFWM in forward geometry has several advantages in the view point of practical applications. It is affected less by turbulence because the three beams more or less traverse the same region of the flame, and is less sensitive to beam steering at high pressure flame. In addition, both the signal detection system and excitation laser system can be spatially separated well, which is the main requirement of remote sensing by using the coherent properties of the DFWM signal. Moreover, the background noise coming from stray light can be better eliminated because scattered or reflected light is involved less in the signal propagation in the spatially separated phase matching direction. However, the DFWM spectrum analysis in the forward geometry needs much complicated study on lineshape and saturation behavior because the spectrum includes significant Doppler broadening in high temperature flames.