Counter-propagating fs/ps CARS for sub-100  µm resolution gas-phase thermometry

A novel, to our knowledge, counter-propagating (CoP) phase-matching configuration of hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) for ultra-high-resolution measurements of gas temperature is demonstrated. The resulting spatial resolution is on the order of 10s of µm—more than an order of magnitude improvement over traditional phase-matching geometries—which enables spatially resolved gas-phase measurements of steep temperature gradients and minimizes effects of spatial averaging. Two additional advantages of this measurement system are the ability to simply control the location of the measurement volume by adjusting relative pulse timing and the size of the probe volume by adjusting pulse duration. The spatial resolution of the counter-propagating CARS (CoPCARS) system was quantified, and the gas temperature across a high-velocity microscale jet emanating from a hypodermic needle was measured using CoPCARS.