Optically Pumped Barium Filtered LIDAR

Investigator: Madison Hetlage

Atomic and molecular filters are a proven tool in laser based diagnostics. Their use as notch filters has greatly expanded the usefulness of scattering phenomena in both ground testing and remote sensing applications. By tuning a laser to the resonance frequency of a strong transition, intense Mie scattering signals from particles and dust as well as diffuse scattering from windows and walls can be suppressed by several orders of magnitude [1]. The transmitted signal can then be used to measure a variety of parameters. However, current filtering technology limits researchers to the use of the frequency

Atomic Vapor Notch Filter Capabilities, taken from [1]

doubled Nd:YAG signal (532 nm) or more rare and complicated lasers such as Ti:Sapphire and Dye lasers [1]. The visible spectrum presents eye safety issues and lacks the molecular scattering signal strength found in the UV. This work aims to develop a vapor filter functioning at the near UV wavelength of the Nd:YAG third harmonic. The frequency required for this filter, which utilizes an excited state transition in atomic barium vapor, falls between the ozone absorption region and the retinal hazard region, provides a stronger backscattered signal than visible light, and can be easily attained with the robust and commonly used high-power Nd:YAG laser [2]. These benefits have significant implications for remote sensing and ground testing technology, including LIDAR (LIght Detection And Ranging), Thomson and filtered Rayleigh scattering, and flow imaging. This project looks to specifically apply this novel filter to atmospheric LIDAR measurements.

The transition of interest for this absorption occurs between the excited, metastable 3D2 state in barium vapor and the 3F02 state at 28179.367 1/cm, which corresponds to a wavelength of approximately 354.8 nm. The figure to the right introduces a simplified scheme for generating this absorption feature (which is shown by the purple arrow on the right) by optically pumping barium. In this scheme, a low power continuous wave laser is used to induce the spin forbidden transition at 791.3 nm demonstrated by the red arrow. Population in the excited state decays through collisional quenching and spontaneous emission to the 3DJ metastable manifold, where energy pooling is observed due to the forbidden transition back to the ground state [2]. This pumping scheme, along with pumping at 553.7

Simplified Barium Optical Pumping Diagram, taken from [2]

nm (as shown by the green arrow), has been used extensively to study the collisional dynamics and kinetics of the low-lying levels of the barium atom [3-9]. These studies have demonstrated a 60s lifetime of the 3D2 state [9] and a 75-80\% ground state depletion through this pumping scheme with less than 1mW of pump power [3, 8]. While other atomic species within the tuning range of the Nd:YAG third harmonic were considered, barium was chosen for this work due to its comparatively high vapor pressure and prior demonstration of efficient pumping to its metastable state.

Preliminary barium vapor cell design

The initial work on this project has been toward the development of a consistent and dense barium vapor source with optical access. Barium’s relatively low vapor pressure requires high temperatures (~800K) to achieve sufficient optical density at 355 nm. The atom’s high reactivity necessitates a low-pressure vacuum environment to avoid oxidation. To satisfy these conditions, the stainless steel vacuum cell shown to the left was fabricated. Commercial heat tape provides even heating, while stainless steel endcaps prevent condensation on the viewport windows and high temperature rated insulation limits heat loss. The side arm allows for pressure control and vacuum line access. Absorption spectroscopy measurements in this system at 553.7 nm (shown by the green arrow above) have demonstrated a consistent and stable barium vapor source. With the application of the pump beam at 791.3 nm, the ground state depletion can be measured and ultimately, absorption spectroscopy of the desired transition can be performed. A multi-level radiative and collisional kinetics model is under development to compliment this experimental work. The combination of experimental and modelling will result in a complete understanding of this novel filter.

Future work on this project will look to apply this filter to atmospheric LIDAR measurements. A ground based LIDAR system is in development at the LDPDL for this purpose. Ultimately, we hope to to demonstrate the true potential of this filter through implement into current Nd:YAG based airborne systems. This work was undertaken as a part of a NASA University Leadership Initiative (ULI) grant investigating shape shifting aircraft for sonic boom reduction in supersonic flight. More information on the project as a whole can be found here.

References:

[1] Miles, R. B., Lempert, W. R., and Forkey, J. N., “Laser Rayleigh scattering,” Measurement Science and Technology, Vol. 12, No. 5, 2001, pp. R33–R51. doi:10.1088/0957-0233/12/5/201, URL http://stacks.iop.org/0957-0233/12/i=5/a=201?key=crossref.ffc46fd8c1de22e9dcf9ee6fa4b097f7.

[2] Hetlage, M. E., Wu, Y., and Limbach, C., “Feasibility Analysis of Optically Pumped Barium Vapor for Filtered Rayleigh Scattering at the Nd:YAG Third Harmonic,” American Institute of Aeronautics and Astronautics (AIAA), 2019. doi:10.2514/6.2019-3382.

[3] Vadla, C., Niemax, K., Horvatic, V., and Beuc, R., “Population and deactivation of lowest lying barium levels by collisions with He, Ar, Xe and Ba ground state atoms,” Zeitschrift for Physik D Atoms, Molecules and Clusters, Vol. 34, No. 3, 1995, pp. 171–184. doi:10.1007/BF01437686, URL http://link.springer.com/10.1007/BF01437686.

[4] Kallenbach, A., and Kock, M., “Kinetics of a laser-pumped barium vapour. II. Experiments and calculations,” Journal of Physics B: Atomic, Molecular and Optical Physics, Vol. 22, No. 10, 1989, pp. 1705–1720. doi:10.1088/0953-4075/22/10/022, URL http://stacks.iop.org/0953-4075/22/i=10/a=022?key=crossref.cf9eefa69107ea9d208f56baf9b8240a.

[5] Filippo, G. D., R, S. G.-K., Milosevic, S., and Pedersen, J. O. P., “Energy pooling in barium with state excitation,” Journal of Physics B: Atomic, Molecular and Optical Physics, Vol. 29, No. 10,1996, pp. 2033–2048. doi:10.1088/0953-4075/29/10/013, URL http://stacks.iop.org/0953-4075/29/i=10/a=013?key=crossref.6661979fbbdeade78be30491842ca09a.

[6] Namiotka, Ehrlacher, Sagle, Brewer, Namiotka, Hickman, Streater, and Huennekens, “Diffusion of barium atoms in the 6s5d 3DJ metastable levels and the 6s2 1S0 ground state through noble-gas perturbers.” Physical review. A, Atomic, molecular, and optical physics, Vol. 54, No. 1, 1996, pp. 449–461. URL http://www.ncbi.nlm.nih.gov/pubmed/9913497.

[7] De Filippo, G., Romstad, D., Guldberg-Kjær, S., Milošević, S., and Pedersen, J., “Depopulation cross sections for low lying states of barium,” Zeitschrift for Physik D Atoms, Molecules and Clusters, Vol. 39, No. 1, 1997, pp. 21–28. doi:10.1007/s004600050105, URL http://www.springerlink.com/openurl.asp?genre=article{&}id=doi:10.1007/s004600050105.

[8] Carlsten, J. L., Mcilrath, T. J., Parkinson, W. H., Carlstents, J. L., and Mcilrathfil, T. J., “Absorption spectrum of the laser-populated 3D metastable levels in barium,” Tech. Rep. 1, 1975. URL https://iopscience.iop.org/article/10.1088/0022-3700/8/1/009/pdf.

[9] Migdalek, J., and Baylis, W. E., “Multiconfiguration Dirac-Fock calculations of two electric quadrupole transitions in neutral barium,” PHYSICAL REVIEW A, Vol. 42, No. 11, 1990, pp. 6897–6899. URL https://journals.aps.org/pra/pdf/10.1103/PhysRevA.42.6897.