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Applied Optics

Fast online rubidium DPAL atomic concentration measurement by 420 nm probe laser

Hui-Zi Zhao, Hong-Yan Wang, Hao Tang, Liang Li, Zi-Ning Yang, Wei-Qiang Yang, Kai Han, Xiao-Jun Xu

Published Oct 14, 2021• 1 citation
Publisher

Abstract

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.

DOI
10.1364/ao.440435

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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.
The diode-pumped alkali laser (DPAL) is a new type of high-powered laser sources which has been paid much attention in recent years. The fluorescent spectra can be used to investigate how the collisions between atomic rubidium and various buffer gases are affected when a sealed rubidium vapor cell is pumped by a LD. In this study, the cross sections between the fine-structure levels of atomic rubidium in a vapor cell were first theoretically deduced by using a gas kinetic procedure. And then, the sensitized fluorescence was experimentally obtained by means of a series of spectral measurements. Finally, the influence of the temperature on the cross section between the fine -structure levels of atomic rubidium was studied with the systematical analyses. The results are thought to be helpful for deeply understanding the theoretical characteristics of a DPAL at the atomic physics level.
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 : The Doppler-broadened two-photon absorption (TPA) cross-section for the 52S1/2- 52D5/2 transition in Rb is measured using direct absorption methods. A detailed model of the intensity profile was developed to account for a focused Gaussian beam propagating through a two-photon absorption medium. A peak absorbance of 24% was observed for an intensity of 6.28 kW/cm2 at the focus, a Rb density of 4.6 x 1015 cm-3 concentrations from 1.61-8.52 x 10(15) cm-3 line. Extracting the hyperfine-broadened TPA cross-section from 87 test configurations, while varying the pump power, alkali concentration and focal length, yielded an error weighted average of 6.75x10-21 cm4/W. This cross-section is sufficient for a pulsed dye laser to bleach the pump transition in the Two-Photon Pumped Alkali Laser (TPAL) that lases at 420 nm and 5.2 mum. Additionally, optically pumped atomic rubidium lasers pumped in the blue satellite of the D2 line from the ground Rb-Ar or Rb-Kr collision pair to the dissociative B2 sum 1/2 produce laser emission at 780.2 nm. Lasing is achieved for the Rb-Ar system and 757.1 - 760.4 nm for the Rb-Kr system. Slope efficiencies exceed 0.25% using a heat pipe configuration. The gain is very high with photon build-up times of 1-3.7 ns.
Laser spectroscopy experiments are often conducted with a probe that does not significantly alter the medium’s properties. For a two-level atom a clear measure of the strength of a probe beam in terms of the saturation intensity is known. We show that for a multilevel atom the situation is very different, and the effects of optical pumping are crucial to understanding the opacity of the medium. We present a simple theoretical analysis for Doppler-broadened spectroscopy of alkali metals on the D2 line that emphasizes the importance of the transient nature of the population dynamics, and the crucial role of the distribution of the times of flight of atoms through the probe beam. Experimental results are obtained with room temperature rubidium vapor probed by an extended-cavity diode laser and confirm our theoretical prediction.
Diode-pumped alkali laser (DPAL) technology offers a means of achieving high-energy gas laser output through optical pumping of the D-lines of Cs, Rb, and K. The exciplex effect, based on weak attractive forces between alkali atoms and polarizable rare gas atoms (Ar, Kr, Xe), provides an alternative approach via broadband excitation of exciplex precursors (XPAL). In XPAL configurations, we have observed multi-quantum excitation within the alkali manifolds which result in infrared emission lines between 1 and 4 μm. The observed excited states include the 42FJ states of both Cs and Rb, which are well above the two-photon energy of the excitation laser in each case. We have observed fluorescence from multi-quantum states for excitation wavelengths throughout the exciplex absorption bands of Cs-Ar, Cs-Kr, and Cs-Xe. The intensity scaling is roughly first-order or less in both pump power and alkali concentration, suggesting a collisional energy pooling excitation mechanism. Collisional up-pumping appears to present a parasitic loss term for optically pumped atomic systems at high intensities, however there may also be excitation of other lasing transitions at infrared wavelengths.
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.