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EFFICIENT ISOTOPE SEPARATION USING SEMICONDUCTOR LASERS

J. P. Woerdman, Alan D. Streater, F. Wittgrefe, E. R. Eliel

Published Jul 18, 1988• 0 citations
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Abstract

Recently separation of the two naturally occurring Rb isotopes (28% 87Rb and 72% 85Rb) has been achieved1 using the light-induced drift (LID) effect;2 enrichment up to 95% 87Rb has been observed. In those experiments a high-power (~ 150mW) single-frequency dye laser was used. In the case of single-frequency excitation such power is required since most of the Rb atoms are trapped in the non- resonant hyperfine (hf) ground level due to optical hf pumping.3 Here we demonstrate that the Rb isotopes can be separated equally well using a low-power (3.3 mW) AlGaAs diode laser which has strong relaxation oscillation sidebands. The presence of these sidebands greatly increases LID by eliminating optical hf pumping.

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Experiments were carried out to evaluate the performances of a semiconductor laser pumped rubidium (87Rb) atomic clock. Two kinds of Rb gas cells were used and their performances were compared [gas cell A (natural rubidium (87Rb/85Rb =\frac{3}{7}) and buffer gases) and gas cell B (87Rb and buffer gases)]. The highest microwave frequency stabilities were estimated as3.4 \times 10^{-12} \tau^{-1/2}and2.7 \times 10^{-12} \tau^{-1/2}at the optimal gas cell temperatures of 60°C and 48°C for the gas cellsAandB, respectively (τ: integration time). The light shift, i.e., microwave frequency shift induced by laser light, was measured as -0.50 Hz/MHz and -0.11 Hz/MHz for the gas cellsAandBat their optimal operating conditions given above. As an improved experiment by utilizing high temporal coherence of the laser, a novel double resonance spectral line shape with a drastically narrower linewidth was demonstrated. A technique, similar to FM laser spectroscopy, was employed for this purpose by utilizing laser FM sidebands which are induced by microwave frequency modulation and nonlinear susceptibility of three-level87Rb atoms. The minimum linewidth obtained was 20 Hz, which can be used as a sensitive frequency discriminator for an improved87Rb atomic clock.

IEEE Journal of Quantum Electronics•Apr 1, 1987•61 citations
Recently separation of the two naturally occurring Rb isotopes (28% 87Rb and 72% 85Rb) has been achieved1 using the light-induced drift (LID) effect;2 enrichment up to 95% 87Rb has been observed. In those experiments a high-power (~ 150mW) single-frequency dye laser was used. In the case of single-frequency excitation such power is required since most of the Rb atoms are trapped in the non- resonant hyperfine (hf) ground level due to optical hf pumping.3 Here we demonstrate that the Rb isotopes can be separated equally well using a low-power (3.3 mW) AlGaAs diode laser which has strong relaxation oscillation sidebands. The presence of these sidebands greatly increases LID by eliminating optical hf pumping.
The use of paraffin-coated cells has led to a breakthrough in the experimental study of light-induced drift (LID) of alkalis in noble gases. In recent experiments isotope separation of the two naturally occurring Rb isotopes through the LID effect has been achieved (enrichment of the rare isotope up to 95%), 1,2 and direct measurements of the drift velocity of Na in noble gases as a function of various experimental parameters have been made.3 In these experiments a single single-mode dye laser, tuned within the Doppler profile of the resonance line, was used to excite a specific velocity group of atoms (isotope selective in the case of Rb). Optical pumping between the ground-state hyperfine levels F = I+½ and F=I−½ plays a dominant (limiting) role in determining the magnitude of the LID effects by strongly depleting the resonantly excited hyperfine substate (of a specific isotope).
Light-induced drift (LID) is a transport phenomenon in a gas mixture with dramatic manifestations such as the optical piston1 and applications in atomic isotope separation.2 The induced drift velocity is proportional to the excited state fraction, and for Na in Xe high drift velocities have been achieved using two lasers.3 Here one laser excites atoms from one hyperfine ground state and the second laser atoms from the other, avoiding hyperfine pumping. Excited in this way, the Na atom represents a Λ type three-level system, where coherent effects can be important.
Experiments were carried out to evaluate the performances of a semiconductor laser pumped rubidium (87Rb) atomic clock. Two kinds of Rb gas cells were used and their performances were compared [gas cell A (natural rubidium (87Rb/85Rb =\frac{3}{7}) and buffer gases) and gas cell B (87Rb and buffer gases)]. The highest microwave frequency stabilities were estimated as3.4 \times 10^{-12} \tau^{-1/2}and2.7 \times 10^{-12} \tau^{-1/2}at the optimal gas cell temperatures of 60°C and 48°C for the gas cellsAandB, respectively (τ: integration time). The light shift, i.e., microwave frequency shift induced by laser light, was measured as -0.50 Hz/MHz and -0.11 Hz/MHz for the gas cellsAandBat their optimal operating conditions given above. As an improved experiment by utilizing high temporal coherence of the laser, a novel double resonance spectral line shape with a drastically narrower linewidth was demonstrated. A technique, similar to FM laser spectroscopy, was employed for this purpose by utilizing laser FM sidebands which are induced by microwave frequency modulation and nonlinear susceptibility of three-level87Rb atoms. The minimum linewidth obtained was 20 Hz, which can be used as a sensitive frequency discriminator for an improved87Rb atomic clock.