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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Coherent pumping of mesospheric sodium to create a bichromatic guidestar

Jens Biegert, J.-C. Diels, Peter W. Milonni

Published Jan 1, 2000• 0 citations

Abstract

Summary form only given. We exploit coherent interaction properties to efficiently pump sodium from the 3s/sub 1/2/ ground state to the 4s/sub 1/2/ upper state via the 3p/sub 3/2/ state. Even though the excitation is nonlinear, the required intensities are reduced to levels required to resonantly pump single photon transitions in sodium, because of the particular choice of laser frequencies which are near resonance with the intermediate levels. Furthermore, the intermediate state is left unpopulated after the excitation sequence. A computer simulation solves the nonlinear Schrodinger equation for this system, comprised of 32 hyperfine levels connected by 112 possible transitions. The levels are summed over the velocity distribution for mesospheric sodium to account for Doppler broadening which is the dominant dephasing process.

DOI: 10.1109/cleoe.2000.910235

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Summary form only given. We exploit coherent interaction properties to efficiently pump sodium from the 3s/sub 1/2/ ground state to the 4s/sub 1/2/ upper state via the 3p/sub 3/2/ state. Even though the excitation is nonlinear, the required intensities are reduced to levels required to resonantly pump single photon transitions in sodium, because of the particular choice of laser frequencies which are near resonance with the intermediate levels. Furthermore, the intermediate state is left unpopulated after the excitation sequence. A computer simulation solves the nonlinear Schrodinger equation for this system, comprised of 32 hyperfine levels connected by 112 possible transitions. The levels are summed over the velocity distribution for mesospheric sodium to account for Doppler broadening which is the dominant dephasing process.

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Summary form only given. Adaptive optics systems, guided by artificial sodium beacons have reached fundamental limits, because of (i) the low densities of atoms in the mesosphere; (ii) the low level of excitation dictated by the requirement to avoid saturation, and (iii) the monochromatic return which makes it impossible to correct for differential tilt (the largest error-about 90%-in image formation). Different attempts are being made to taylor laser pulse profiles to more efficiently access the Doppler groups of the atomic lines, while avoiding saturation (e.g. only 6.4 mW/cm/sup 2/ for the D2 line), and to assist in the correction for differential tilt by adding Rayleigh or natural beacons. Both problems severely limit the further advancement in this field due to the increased complication and cost. We have presented an alternative, which circumvents these problems by using coherent excitation of sodium with short pulses. The saturation limitation is avoided in this approach in which each pulse totally inverts (3s /spl rarr/ 4s) the interaction volume. Subsequent cascade resonance fluorescence at two wavelengths allows for correction of the tilt.

A new scheme for coherently pumping two-photon transitions is demonstrated theoretically-It is shown that a total inversion can be achieved by using a pair of bichromatic ultrashort pulses tuned close to an intermediate resonance. The proximity of the intermediate resonance greatly enhances the efficiency of the process, making it possible to reach total inversion with energy densities of the order of microjoules per square centimeter. At the same time, the resonance conditions are controlled in such a way that at no time a dipole moment is created (the off-diagonal elements corresponding to dipole transitions remain <0.1).