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Physical Review A

Optical pumping of high-density Rb with a broadband dye laser and GaAlAs diode laser arrays: Application to He3 polarization

Mark E. Wagshul, T. E. Chupp

Published Oct 1, 1989• 70 citations
Publisher

Abstract

We have studied laser optical pumping of Rb at densities up to 4\ifmmode\times\else\texttimes\fi{}${10}^{14}$ atoms/${\mathrm{cm}}^{3}$ and produced polarized $^{3}\mathrm{He}$ by spin exchange with Rb. Numerical calculations show that for a high-density Rb vapor GaAlAs diode laser arrays can provide results similar to those obtained with a broadband ${\mathrm{Kr}}^{+}$-ion--dye-laser system. We have confirmed these calculations with measurements using a 40-GHz full width at half maximum bandwidth dye laser and 100- and 500-mW GaAlAs diode laser arrays (Spectra Diode Labs Array Nos. SDL-2410C and SDL-2450C). We have optically pumped Rb of density [Rb]=4\ifmmode\times\else\texttimes\fi{}${10}^{14}$ atoms/${\mathrm{cm}}^{3}$, and we have measured up to 70% $^{3}\mathrm{He}$ polarization in a 4-${\mathrm{cm}}^{3}$ volume with 600 mW from the dye laser. With the diode laser arrays, we have pumped with 30 and 300 mW with results comparable to those obtained with the dye laser. With a 1-W diode laser array, we anticipate 70--80 % $^{3}\mathrm{He}$ polarization in a 4-${\mathrm{cm}}^{3}$ volume.

DOI
10.1103/physreva.40.4447

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We have studied laser optical pumping of Rb at densities up to 4\ifmmode\times\else\texttimes\fi{}1014{10}^{14}1014 atoms/cm3{\mathrm{cm}}^{3}cm3 and produced polarized 3He^{3}\mathrm{He}3He by spin exchange with Rb. Numerical calculations show that for a high-density Rb vapor GaAlAs diode laser arrays can provide results similar to those obtained with a broadband Kr+{\mathrm{Kr}}^{+}Kr+-ion--dye-laser system. We have confirmed these calculations with measurements using a 40-GHz full width at half maximum bandwidth dye laser and 100- and 500-mW GaAlAs diode laser arrays (Spectra Diode Labs Array Nos. SDL-2410C and SDL-2450C). We have optically pumped Rb of density [Rb]=4\ifmmode\times\else\texttimes\fi{}1014{10}^{14}1014 atoms/cm3{\mathrm{cm}}^{3}cm3, and we have measured up to 70% 3He^{3}\mathrm{He}3He polarization in a 4-cm3{\mathrm{cm}}^{3}cm3 volume with 600 mW from the dye laser. With the diode laser arrays, we have pumped with 30 and 300 mW with results comparable to those obtained with the dye laser. With a 1-W diode laser array, we anticipate 70--80 % 3He^{3}\mathrm{He}3He polarization in a 4-cm3{\mathrm{cm}}^{3}cm3 volume.
The use of high-power ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$Alx{\mathrm{Al}}_{\mathit{x}}Alx​As diode laser arrays for optical pumping of Rb vapor has been examined both theoretically and experimentally. Our studies show that large atomic polarizations can be obtained using the broad spectral output of a diode laser array if the Rb resonance line is collisionally broadened by several atmospheres of a buffer gas. The objective was to study the effectiveness of diode laser arrays as light sources for optically pumped spin-exchange polarized 3He^{3}\mathrm{He}3He targets. The average Rb polarization was deduced at three different Rb vapor pressures using the polarization induced in the 3He^{3}\mathrm{He}3He buffer gas. The largest 3He^{3}\mathrm{He}3He polarization observed was 53% using a 20-W diode array with 1.9-nm full width at half maximum spectral width in 10.5 amagat of 3He^{3}\mathrm{He}3He. This result compares favorably with other light sources for this target. The measured average Rb polarizations are compared to theoretical calculations.
We have studied the efficiency of spin-exchange optical pumping (SEOP) in cells that contain either pure rubidium, mixtures of rubidium and potassium, or pure potassium. Optical pumping was performed with spectrally narrowed diode array lasers operating at either the wavelength of the Rb $5{s}_{1∕2}\ensuremath{-}5{p}_{1∕2}$ transition (795nm)(795\phantom{\rule{0.3em}{0ex}}\mathrm{nm})(795nm) or the K $4{s}_{1∕2}\ensuremath{-}4{p}_{1∕2}$ transition (770nm)(770\phantom{\rule{0.3em}{0ex}}\mathrm{nm})(770nm). Calculations of the expected efficiency are shown for practical conditions and compared to measurements. We have measured the vapor density ratio in Rb-K mixture cells using absorption of white light. Our focus is on the conditions that are relevant for neutron spin filters, but the results are relevant to all applications of SEOP. We find that both Rb-K and pure K cells yield improvements in the spin-exchange efficiency, and each approach has its own advantages. Overall, the best results for high polarizing rate with high polarization were obtained with mixture cells with K-Rb vapor density ratios between 2 and 6.
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.
Recently the demand for hyperpolarized noble gases is arising to improve NMR spectroscopy resolution for imaging. Hyperpolarization of noble gases is achieved by spin- exchange with alkali metal. atoms that undergo optical pumping by circularly polarized radiation. Optical pumping is usually achieved by high power semiconductor lasers. These sources have a bandwidth very large when compared with the transition bandwidth of the optically active species. This has two serious consequences: (i) the request of high power lasers, (ii) the losses that the unemployed energy delivered to the Rb vapor can provide in the efficiency of the energy transfer. Object of our investigation will be the comparison of two experimental set ups for optical pumping of Rb vapor where pumping is given in one case by a dye laser pumped by a UV nitrogen laser built in our laboratory according to the Blumlein scheme and in the other case by a semiconductor laser. Major elements that will be discussed are (1) the role of stimulated emission of Rb vapor to increase the pumping efficiency, (2) the fate of resonant photons that are emitted during Rb decays, (3) the role of applied magnetic fields in the efficiency of pumping processes.
A cryogenic technique has been used to produce polarized 3He^{3}\mathrm{He}3He targets of up to relative density p=12 atm (\ensuremath{\approxeq}3\ifmmode\times\else\texttimes\fi{}1020{10}^{20}1020 3He^{3}\mathrm{He}3He atoms/cm3{\mathrm{cm}}^{3}cm3; p=1 atm corresponds to 760 Torr or 101.3 kPa pressure at 273 K). In these targets 3He^{3}\mathrm{He}3He nuclei are polarized by spin-exchange collisions with optically pumped rubidium atoms. From transmission measurements at wavelengths of 790--800 nm, we have determined pressure shifts, linewidths, and line-shape asymmetries for the Rb 5S1/2{\mathit{S}}_{1/2}S1/2​\ensuremath{\rightarrow}5P1/2{\mathit{P}}_{1/2}P1/2​ D1 transition. The Rb spin-destruction rate was found to exhibit a quadratic increase versus 3He^{3}\mathrm{He}3He pressure, which indicates the importance of Rb${\mathrm{\ensuremath{-}}}^{3}$He${\mathrm{\ensuremath{-}}}^{3}$He collision processes. The transmission results for circularly polarized light are well described by a model that predicts the dependence of the average Rb polarization on Rb density, 3He^{3}\mathrm{He}3He pressure, light intensity, and cell geometry. The Rb${\mathrm{\ensuremath{-}}}^{3}$He spin-exchange cross section, 〈${\mathrm{\ensuremath{\sigma}}}_{\mathrm{SE}}$v〉=6.1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}20}$ cm3{\mathrm{cm}}^{3}cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, was found to be independent of 3He^{3}\mathrm{He}3He pressure up to p=12.1 atm. Maximum 3He^{3}\mathrm{He}3He polarizations of 72--79% were observed with cells of 17 cm3{\mathrm{cm}}^{3}cm3 volume that contained 3He^{3}\mathrm{He}3He at p=6--9 atm.
We investigate 129Xe^{129}\mathrm{Xe}129Xe-Cs (D1,D2{D}_{1},{D}_{2}D1​,D2​) spin exchange optical pumping (SEOP) at high Xe densities ($~0.12−−2.44amagat)usingnewlyavailablehigh−power(0.12--2.44 amagat) using newly available high-power (0.12−−2.44amagat)usingnewlyavailablehigh−power(>$40 W) laser diode arrays and compare with 129Xe^{129}\mathrm{Xe}129Xe-Rb D1{D}_{1}D1​ SEOP under similar conditions. At elevated Xe densities, the spin-exchange rate (per alkali-metal atom, ${\ensuremath{\gamma}}^{\ensuremath{'}}$) for Cs-129Xe^{129}\mathrm{Xe}129Xe is $~1.5−foldgreaterthanthatforRb−1.5-fold greater than that for Rb-1.5−foldgreaterthanthatforRb−^{129}\mathrm{Xe}$. Higher spin-exchange rates and lower 129Xe^{129}\mathrm{Xe}129Xe spin-destruction rates for Cs-129Xe^{129}\mathrm{Xe}129Xe versus Rb-129Xe^{129}\mathrm{Xe}129Xe contribute to  ~ twofold improvement in 129Xe^{129}\mathrm{Xe}129Xe nuclear spin polarization measured at 9.4 T---with the largest gains observed at the highest Xe densities.