DEVELOPMENT OF ULTRAFAST COHERENT ANTI-STOKES RAMAN SCATTERING (CARS) SPECTROSCOPY FOR HIGH PRESSURE SYSTEMS

Chirped-probe pulse femtosecond coherent anti-Stokes Raman scattering (CPP fs CARS) was used to study high pressure gas-phase thermometry. The experimental measurements were mostly performed in a static gas cell and in a canonical flat flame burner. The purpose of this study is to provide insights for the future rocket relevant combustion measurements.

The optical effects associated with ultrashort pulse propagation in the high-pressure system were investigated. For example, the femtosecond pulse can receive large amount of frequency chirp when transmitting through thick glass windows of the optical section in the high-pressure system. The effects of pump and Stokes frequency chirp were investigated both experimentally, by inserting disks of SF11 glass into the pump and Stokes beam paths to study the flame thermometry, and theoretically by incorporating pulse chirp into the TDDM simulations to calculate the Raman excitation efficiency. Meanwhile, the ultrashort pulses can experience self-phase modulation in the high-pressure gas medium. The effects of self-phase modulation (SPM) on the power spectra of femtosecond pulses will have significant impact on the fs CARS profile. On the other hand, the extend and the behavior of SPM reply on the laser intensity and are also species-specific. The optimal laser intensities in high-pressure gas mediums like N2, O2, CO2 and CH4 were investigated.

To prepare for future rocket relevant combustion studies, CPP fs CARS thermometry was developed for CO2, O2 and H2. Especially for CO2 and O2, they have close vibrational frequencies but very different coherence dephasing rates. Relative concentration between CO2 and O2 can then be extracted by using a short probe delay, and the temperature information can be determined by using long probe delays and the O2 transitions will not interfere with CO2 and nonresonant contribution of the CARS signal can be suppressed. CO2 CPP fs CARS measurements inside the high-pressure high-temperature gas cell were presented and discussed. Collisional narrowing effects for CO2 especially for high gas number density situation were discussed.