Stimulated Raman scattering in molecular hydrogen pumped by a tunable Alexandrite laser
Creators: name: Guerra, David Victor
Publisher: name: American University
Year: 2023

description: An application of lasers which demands knowledge of the temporal and spectral characteristics of a pulse is light detection and ranging (LIDAR) measurements of specific atmospheric constituents. To employ a laser in this task, the operating wavelength must overlap one of the strong absorption lines of the atom or molecule under investigation. Since many of the atmospheric gases of interest, such as CO and CO$\sb2$ have strong absorption lines in the infrared, a tunable source of coherent radiation in this frequency band is needed. Considering there are no tunable solid state lasers which operate in the infrared, a shifting of the visible radiation, from a tunable alexandrite laser, into the infrared by Stimulated Raman Scattering (SRS) in various molecules is a possible solid state based source of the necessary radiation. The hydrogen molecule was chosen as the frequency shifting mechanism because it provides the largest (0-1) vibrational energy shift of 4155 cm$\sp{-1}$. Coupled with the tuning range of the Alexandrite laser, 720-780 nm, radiation of 1-1.15 $\mu$m, 1.8-2.2 $\mu$m, and 7.0-28.0 $\mu$m is energetically possible for the $\rm 1\sp{st},\ 2\sp{nd},$ and $3\sp{\rm rd}$ Stokes shifts respectively. To better understand the processes involved in this frequency shifting, theoretical modeling was coupled with the analysis of experimental bread-board measurements. Plane wave and gaussian amplitude distribution models of the multiple Stokes generation by SRS and four-wave mixing have given insight into the interplay between these third order nonlinear parametric processes. The predicted thresholds for $1\sp{\rm st}$ Stokes components is 0.9 MW for both models with and without four-wave mixing terms. In contrast, the predicted thresholds for $2\sp{\rm nd}$ Stokes is 1.5 MW and 3.5 MW for models with and without four-wave mixing terms respectively. Experimentally, the thresholds were measured at 0.7 MW and 1.5 MW for $1\sp{\rm st}$ and $2\sp{\rm nd}$ Stokes respectively. It is clear that the inclusion of four-wave mixing terms significantly improves the agreement between the predicted and measured thresholds of $2\sp{\rm nd}$ Stokes conversion. The linewidth of the $1\sp{\rm st}$ Stokes component was measured to a consistent value broader than that of the pump laser. This broadening may also be attributed to the presence of other nonlinear processes involved in the frequency conversion. From these studies, I conclude that, at the high peak powers which the pump laser was operated, other nonlinear processes, such as four-wave mixing, contributed significantly to the conversion of radiation into the multiple Stokes orders and affect the linewidth of the resulting radiation. Therefore, these processes and their effects must be considered when designing a system which employs frequency shifting by SRS in molecular hydrogen gas. The theoretical and experimental investigation of the energy, wavelength, and linewidth of the Stokes shifted pulses are presented in this dissertation.