Leonov, Boris S. (2023-05). Development of Compact Optical Parametric Oscillators to Enable New Stand-off Detection Capabilities in High-Enthalpy Environments. Doctoral Dissertation.
Thesis
In complex environments, such as plasmas, hypersonic flows, and combustion, rapid high-resolution spectroscopy of multiple species is of great significance, however, this objective is hin-dered by the lack of availability of tunable radiation sources that can access the wide spectral range of optical transitions. Most diagnostic methods utilized for measurements in gases require specific wavelengths of electromagnetic radiation. Additionally, sources of rapidly pulsed, high-energy, narrow-linewidth, wavelength-tunable laser radiation are limited. The Optical Parametric Oscillator (OPO) is a well-known solution for this laser tunability problem, utilizing a solid-state non-linear optical medium. The OPO was first developed in the late 1960s and has recently received a new wave of interest from researchers across a multitude of disciplines. This new wave of interest was triggered by the need for high-repetition-rate diagnostics which typically require solid-state devices to create tunable laser radiation due to their high nonlinear coefficients and robustness. High-repetition-rate diagnostics are strongly preferred for hypersonics because of the short timescales of phenomena of interest, the dynamic nature of the testing environment, and the short test duration associated with impulse facilities. This Ph.D. dissertation presents results related to developing novel, high-resolution, and high-speed laser-based diagnostic capabilities using OPOs. The research efforts are broken down into two major components: narrow-linewidth OPOs and their applications for high-resolution spectroscopy, and high-repetition-rate OPOs utilized for diagnostics of high-enthalpy environments. The first milestone of this Ph.D. work was achieved through the development of a custom injection-seeded and active-cavity-locked OPO. This OPO was designed with a broad tunability range (720 nm - 860 nm) for the first experimental demonstration of novel Slow Light Imaging Spectroscopy (SLIS) of inelastic scattering through thermometry of liquid water. A direct correlation between the time delay of the scattered light pulses passing through a rubidium vapor cell and the water temperature was established and quantified. The second milestone was the development and demonstration of a high-speed laser diagnostic capability using OPOs integrated with Pulse-burst Lasers. The demonstration was done through 250 kHz repetition rate nitric oxide Planar Laser-Induced Fluorescence (PLIF) synchronized with 250 kHz nitric oxide luminescence photography measurements in the vicinity of hypersonic Mach Stems at the Hypervelocity Expansion Tunnel (HXT) at Texas A&M University. Such quasi-simultaneous measurements allowed for effective separation of PLIF signal and otherwise overwhelming natural emission from electronically excited nitric oxide. Subsequently, CFD validation and identification of dominant shear layer structures were performed. The validation work was done by comparing calculated nitric oxide fluorescence from the CFD results against experimentally acquired data. The maximum deviation of the signal level under 1.5 % in a quasi-equilibrium region was achieved across a wide range of experimental conditions. Additionally, shear layer frequencies related to the Kelvin-Helmhotz instability (ranging from 55 kHz to 35 kHz) were found to be a growing function of the Reynolds number. The modeling efforts and areas of theoretical interest of this dissertation include nonlinear interactions of electromagnetic radiation and media with a particular focus on parametric amplification in solid-state optical crystals, optical dispersion near absorption features of atomic vapors, and quantitative laser-induced fluorescence.
