One-Dimensional Hybrid fs/ps Coherent Anti-Stokes Raman Scattering System for Simultaneous Rotational and Rovibrational Spectroscopy in Reactive Flows

This work reports on the development of a novel three-beam, one-dimensional hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (CARS) system capable of concurrent rotational and vibrational spectroscopy for high-fidelity measurements of temperature and species concentrations in transient reactive flows at extreme conditions, including elevated pressures and near-extinction or otherwise unstable flames. In this first-stage work, the system is implemented and tested in a high-pressure chambered counterflow burner for measurements of counterflow diffusion flames. The three-beam geometry confines the CARS probe volume to the flame core, reducing contamination from cooler surrounding gas while maintaining sufficient signal for quantitative profiling. Early imaging tests indicate an imaging resolution of ~70 µm, with potential to reach ~37 µm through further optical optimization. Characterization of white-light generation in the optical access windows establishes an operating regime in which the CARS signal remains free from nonlinear distortions. Supporting 1D simulations of CH4-air and C2H4-air flames highlight sensitivities to pressure, strain rate, and fuel type, particularly near extinction, guiding ongoing system development. This work establishes a foundation for extended one-dimensional CARS measurements in challenging reactive flows, providing a pathway toward high-fidelity diagnostics under extreme and transient conditions.