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Conference on Lasers and Electro-Optics

Low-power polychromatic guidestar through coherent excitation of the 4d state in atomic sodium

Jens Biegert, Jean‐Claude Diels

Published Jan 1, 2001• 0 citations
Publisher

Abstract

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.

DOI
10.1109/cleo.2001.947943

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Annual Meeting Optical Society of America•Jan 1, 1991•0 citations
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.
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.
Sodium laser guide star (LGS) becomes an essential part in modern astronomical adaptive optics system, especially for the next generation extremely large ground based telescope. The LGS technology has experienced the developmental stages as natural guide star, Rayleigh LGS, sodium LGS and constellation of LGS. The sky coverage is still limited in that the LGS cannot not be used to detect the tip/tilt aberrations. While the polychromatic laser guide star (PLGS) is one of the most effective ways to enlarge the sky coverage to 100%. Previous simulation models are insufficient for the accurate calculation of the return flux, especially for the simulation model of PLGS which is generated by one-photon excitation of mesospheric sodium atoms. The simulation model based on Bloch equations proposed in this paper can be used to compute the return flux of one-photon excited PLGS precisely. Doppler broadening, beam atom exchanging, collisions and recoil are taken into account in the model. The return flux is validated by the return efficiency. The simulation results indicate that with one-photon excitation of sodium atoms, a return efficiency of 330 nm is minimum compared with those of other wavelengths; the saturation power density will decrease with recoil increasing and increase with collision rate increasing; an optimal line-width exists up to maximum the photon return efficiency. In the best case, when the power density is 10 W/m2 at the sodium layer, the maximum return efficiency at 330 nm is 0.907 photons/s/sr/atom/(W/m2) with an optimal laser line-width of 18 MHz.
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.
Some of the adaptive optics systems contemplated to ameliorate the effects of the atmospheric turbulence utilize guide stars. Guide stars are artificial beacons formed in the atmosphere by the interaction of a laser with matter in the atmosphere. Sodium guide stars are formed as a result of laser interaction with the sodium layer in the upper atmosphere. A small rf FEL is one of the candidates considered as a potential source capable of creating a sodium beacon. The pulse format of such a laser consists of trains of short optical pulses. The duration of an individual pulse is typically shorter than that of relevant relaxation times in the medium. The separation of the pulses may be longer or of the order of the relevant atomic memory time scale. To properly describe interactions of the atmospheric sodium layer with a laser characterized by such a pulse format one must utilize the optical Bloch equations (OBE). The OBE have been solved numerically using a simplified two-level model, and the resulting sodium layer response has been characterized. Additionally, an analytic solution has been obtained for the case in which both the population and coherence relaxation times are equal. The model is capable of providing useful insights.