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AIAA Plasmadynamics and Lasers Conference

Measurement of Rubidium Number Density Under Optically Thick Conditions

Matthew D. Rotondaro, Charles F. Wisniewski, Martiqua L. Post, Gordon D. Hager

Published Jun 28, 2010• 3 citations
Publisher

Abstract

A measurement of rubidium number density under optically thick conditions has been demonstrated by measuring the wings of the D1 absorption spectra using a laser with a 0.16 nm (75 GHz) fine tuning range. This technique can measure the absolute concentration in rubidium under conditions where the absorption coefficient and path length product yield conditions where the central region of the line is opaque. The laser was tuned to a region sufficiently far into the short wavelength wing of the absorption where transmission through the cell was possible. The laser was then scanned through the central opaque region of the line to the adjacent long wavelength wing. The wavelength of the scan was calibrated by using a 1.5 GHz etalon and a cell containing only naturally occurring rubidium as a frequency reference. The measured absorption spectra for various cell conditions of temperature and pressure were then fit to a pressure broadened Voigt profile thereby allowing the determination of the rubidium number density.

DOI
10.2514/6.2010-4881

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Study of atomic transition self-broadening and shift with the help of a nano-cell

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Journal of Physics B Atomic Molecular and Optical Physics•Sep 3, 2008•14 citations
A measurement of rubidium number density under optically thick conditions has been demonstrated by measuring the wings of the D1 absorption spectra using a laser with a 0.16 nm (75 GHz) fine tuning range. This technique can measure the absolute concentration in rubidium under conditions where the absorption coefficient and path length product yield conditions where the central region of the line is opaque. The laser was tuned to a region sufficiently far into the short wavelength wing of the absorption where transmission through the cell was possible. The laser was then scanned through the central opaque region of the line to the adjacent long wavelength wing. The wavelength of the scan was calibrated by using a 1.5 GHz etalon and a cell containing only naturally occurring rubidium as a frequency reference. The measured absorption spectra for various cell conditions of temperature and pressure were then fit to a pressure broadened Voigt profile thereby allowing the determination of the rubidium number density.
We have made systematic studies of rubidium (Rb) number density and polarization with the purpose of comparing a variety of techniques used to make these measurements. We have used five different temperatures, 40°C to 100°C in 15° increments, to compare number density measurements made using optical absorption with those made using Faraday rotation. A diode laser operating in the region near the Rb D1 transition was used in the case of the former, while for the latter two different lasers were used, one each operating in the vicinity of the Rb D1 and D2 transitions. These number density results are compared with those obtained from vapor temperature formulae (e.g., Killian, CRC, etc.). We have also made a systematic study of Rb optical pumping using an amplified tunable diode laser which has been incrementally broadened in order to determine the effect of linewidth broadening on Rb polarization. We vary the frequency of the circularly-polarized pump laser across the D1 transition absorption profile for each of four linewidths (0.01, 0.5, 2, and 8 GHz) and measure the Rb polarization using Faraday rotation. The Rb number density for these measurements is on the order of 1012 cm−3, with 1 Torr of N2 as the buffer gas. These results are then compared with polarization measurements taken using a re-pump laser operating near the Rb D2 transition, and with theoretical calculations of the rubidium polarization.
Abstract : An absorption spectrum of a rubidium alkali metal dispenser (AMD) cell was obtained in order to determine the system s suitability for use in a diode pumped alkali laser (DPAL) and use in high-temperature spectroscopic studies. The AMD produced a concentration of 3.65 0.16 1010 cm-3, which is in the ideal range for Beer s Law region absorption spectroscopy, but too low to make a high-power DPAL in a 10 cm cell with a poor Q resonator. Before AMDs can be used to determine pressure broadening and shifting coefficients, issues concerning contamination and producing rubidium vapor at pressure must be resolved. A separate portion of this study focused on a sealed rubidium Pyrex cell with neon buffer gas. A continuous wave titanium sapphire beam was focused into the cell with various lenses to maximize pump intensity through the cell. Images were taken of the side fluorescence of the cell to determine the bleached regions. The total horizontal propagation of the bleached region for each image was proportional to the pump laser power and the inverse of the rubidium vapor density. Disparity between the bleached volume estimated from the beam parameters and the theoretical bleached volume suggested the beam intensity had a strong radial dependence. When the pump wavelength was changed, visible side fluorescence occurred in regions where the laser intensity was at least 0.125 times the saturation intensity and therefore not in the regions where the decay was determined by Beer s Law.
Abstract A cell with a submicron thickness was used to measure the broadening and shift of the D1 and D2 lines of Rb atoms by neon. The resolution of hypefine components is achieved by a combination of two techniques. First, the Rb vapor column thickness in the direction of propagation of the laser radiation was chosen equal to half of its wavelength λ under conditions of resonance with the atomic transition frequency. For rubidium atoms λ/2 ~ 400 nm. At a nanocell thickness L ~ λ/2, in the transmission spectrum A(ν), the spectral lines of atomic transitions are narrowed due to the exclusion of Doppler broadening. Second, further narrowing of the detected signals was achieved by the double differentiation of the transmission spectrum, A''(ν). The transmission spectra of pure rubidium vapor and rubidium vapor with the addition of neon have been measured at different pressures. The measured values of the shift coefficients of the D1 and D2 lines of rubidium in the presence of Ne were –1.1 ± 0.2 MHz/Torr and –2.1 ± 0.2 MHz/Torr, respectively. The broadening coefficients of the D1 and D2 lines coincide and are equal to 10 ± 1 MHz/Torr. Due to its high spectral resolution, this technique allows separate measurements for each individual transition.
The alkali atom concentration plays an important role in the performance of a diode pumped alkali vapor laser (DPAL). At the rubidium DPAL operational region, the alkali concentration is as high as 1 0 13 − 1 0 14 c m − 3 , which is “optically thick,” or opaque, for the 780 nm or 795 nm doublet D lines when the traditional scanning absorption spectrum method is used for concentration measurement. To solve this problem, we propose the use of a probe laser of 420 nm, which corresponds to the 5 2 S 1 / 2 to 6 2 P 3 / 2 transition and has a lower absorption cross section compared to the D-line doublet. Due to the moderate absorption strength at a fixed 420 nm probe wavelength, we realized fast, online measurement of the Rb concentration in a real-world DPAL. By combining it with the quasi-two-level model, we further provided the population distribution in the lower three energy levels. This fast, online diagnostic method could be well applied in DPAL concentration measurement, and could show the dynamics of a laser’s performance.
A new method and its principle are presented for measuring the each component gas pressures in Rubidium (Rb) by the analysis of absorption spectral profile. The experiment system is set up to obtain Rb absorption spectra. And then each component gas pressures in atom vapor cell is estimated. First, the relationships between transmittance of probe light, atom density and absorption cross section are introduced, and the factors which influence the absorption spectral profile and methods to measure gas pressures are given. Second, the frequency-dependence curves of transmittance and the absorption spectra are obtained through tuning the laser frequency through the Rb D1 transition. Finally, the gas pressures of Rb, N2 and He are achieved, through fitting absorption spectral profile referring to half-width and minimum transmittance value of absorption spectra. The experiment results show that gas pressures in Rb atom vapor cell can be accurately measured by absorption spectrometric methods, which will be helpful for the following study of atom vapor cell. The gas pressures of N2 and He measured by the experiments are well matched with design values. The Rb gas pressure is 30%~50% less than the saturated vapor pressure and the suppression may be due to the adsorption of the cell surfaces coated with octadecyltrichlorosilane (OTS) film.