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Nonlinear Optics

Feasibility Study of a Bichromatic Guidestar - Coherent 2-Photon Bichromatic Excitation of Mesospheric Sodium

Jens Biegert, Jean‐Claude Diels

Published Jan 1, 2000• 0 citations
Publisher

Abstract

Theoretical results from a full hyperfine structure density matrix computation indicate that a bichromatic sodium guidestar based on two-photon bichromatic coherent excitation should require laser powers only on the order of 10 W.
DOI
10.1364/nlo.2000.mb5

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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.
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).
Calculations of backscatter emission of mesospheric sodium atoms in a laser guide star that is excited by pulses ranging from 30-ns to 0.9-μs duration are described. The efficient use of such pulses at saturating irradiance values is shown to require ~3 GHz of spectral broadening to provide access to the full absorption spectrum of the D2 line. The broadening is provided by frequency modulation. A set of density matrices was used to account for all 24 hyperfine states and inhomogeneous Doppler broadening. At the broadband (3-GHz) saturation irradiance of 4 W/cm2, both linearly and circularly polarized laser beams are shown to produce emission rates exceeding 60% of the maximum possible rate-equation rate for the 0.9-μs pulses. As expected, circular polarization produced more backscatter than did linear polarization, but the enhancement never exceeded 1/3 in the calculations that are reported. A brief estimate of state precession in the Earth’s magnetic field suggests that achieving even this enhancement will require that the time scale for optical pumping be held to less than 1 μs, which will require the use of irradiances greater than 0.7 W/cm2 and spectral coverage of the full 3-GHz hyperfine-plus-Doppler absorption profile, at least until most of the population is pumped out of the F = 1 ground states.