Development of an Optically Pumped Barium Vapor Filter for an Ultraviolet High Spectral Resolution Lidar

Current laser based remote sensing techniques rely heavily on bandpass and notch filters to manipulate the collected, backscattered photons. High spectral resolution lidars (HSRL), which have been used to study aerosol properties and temperature profiles, often utilize molecular iodine vapor notch filters to separate the aerosol and molecular scattering components. Iodine is a convenient filter candidate due to its high vapor pressure and many electronic transitions occurring near the second harmonic of the high power and robust Nd:YAG laser (532 nm). However, the visible spectrum presents eye safety issues, increased sunlight background, and decreased scattering strength when compared to lower wavelengths in the ultraviolet region, where filtering is currently limited to etalon approaches. This work presents a novel vapor notch filter functioning near the ultraviolet Nd:YAG third harmonic (355 nm). Filtering is achieved by optically pumping a high temperature barium vapor to an excited and metastable electronic state, 6s5d 3D2, from which light at 354.8 nm can be absorbed. A detailed theoretical and experimental study has been performed to characterize the light transmission of this filter and understand the complex kinetics involved in the optical pumping scheme. These results demonstrate a unique, highly tunable, cusped, non-Maxwellian absorption feature and have informed current work to manipulate the barium kinetics to further optimize the optical depth, spectral width, and center frequency of the filter transmission. While this filtering technology could augment several established diagnostic techniques for ground-based testing, the rapid tunability, ease of alignment when compared to etalons, and near ultraviolet frequency are particularly advantageous for remote sensing systems. Preliminary lidar simulations employing this notch filter show promise for the enabling of higher precision and safer atmospheric profiling when compared to typical visible light based systems. To fully demonstrate the effectiveness of this filter, a ground based HSRL system has also been developed at Texas A&M University. The simulated and preliminary experimental lidar results will be presented in detail.