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Physical Review A

Optical pumping of Rb vapor using high-power Ga1−x Alx As diode laser arrays

William J. Cummings, O. Häusser, W. Lorenzon, D. Swenson, B. Larson

Published Jun 1, 1995• 40 citations
Publisher

Abstract

The use of high-power ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$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 $^{3}\mathrm{He}$ targets. The average Rb polarization was deduced at three different Rb vapor pressures using the polarization induced in the $^{3}\mathrm{He}$ buffer gas. The largest $^{3}\mathrm{He}$ 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 $^{3}\mathrm{He}$. This result compares favorably with other light sources for this target. The measured average Rb polarizations are compared to theoretical calculations.

DOI
10.1103/physreva.51.4842

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The use of high-power {\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}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 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.
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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.
A method is described for producing several liters of nuclear spin polarized 129Xe gas via spin exchange with an optically pumped Rb vapor. We use a 140 W AlGaAs laser diode array whose broad spectral output is efficiently absorbed by employing ∼10 atm of 4He to pressure broaden the Rb D1 absorption profile. 129Xe is polarized in a continuous gas flow and is then cryogenically accumulated and stored. Extensions of this technique should enable the production of tens of liters of 129Xe with a nuclear spin polarization of order 50%. Production of laser-polarized 129Xe in liter quantities is important for the continued development of magnetic resonance imaging using spin-polarized 129Xe.