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Journal of the Optical Society of America B

Optical processes for middle atmospheric Doppler lidars: Cabannes scattering and laser-induced resonance fluorescence

C. Y. She, Huailin Chen, David A. Krueger

Published Jul 10, 2015• 17 citations

Abstract

The problem of light scattering from a monochromatic laser beam by an ensemble of atmospheric atoms or molecules in thermal equilibrium under ambient wind and temperature is revisited with two emphases. First, the essence of and difference between laser Cabannes scattering (CS) and laser-induced resonance fluorescence (LIF), which permit atmospheric temperature and wind measurements, are simply presented. A classical model is shown to yield the same angular distribution in scattered power for both, a factor of two larger Doppler shift for the former (CS), in the backward direction with the same line-of-sight (LOS) wind, and a dramatic 17 order larger backscattering cross section for the latter (LIF). Second, the LIF process is developed from first principles, and we present the relations between absorption cross section, differential scattering cross sections, and LIF emission spectrum for atoms with hyperfine structure. By doing this, we (1) equate the decoherence rate among the mixed excited substates to half of the Einstein coefficient to comply with energy conservation, and (2) present a clear physical insight into the origin of the Hanle effect, whose quantitative evaluation for these atoms requires quantum mechanical treatment. We then derive LIF emission rates for each of the eight and 10 pathways associated with the D1 and D2 transitions, and deduce the temperature and LOS wind dependence in differential backscattering cross sections of atmospheric sodium and potassium atoms. Using these cross sections allows retrieval of atmospheric parameters in the mesopause region (80–110 km in altitude) by a metal resonance lidar.

DOI: 10.1364/josab.32.001575

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The problem of light scattering from a monochromatic laser beam by an ensemble of atmospheric atoms or molecules in thermal equilibrium under ambient wind and temperature is revisited with two emphases. First, the essence of and difference between laser Cabannes scattering (CS) and laser-induced resonance fluorescence (LIF), which permit atmospheric temperature and wind measurements, are simply presented. A classical model is shown to yield the same angular distribution in scattered power for both, a factor of two larger Doppler shift for the former (CS), in the backward direction with the same line-of-sight (LOS) wind, and a dramatic 17 order larger backscattering cross section for the latter (LIF). Second, the LIF process is developed from first principles, and we present the relations between absorption cross section, differential scattering cross sections, and LIF emission spectrum for atoms with hyperfine structure. By doing this, we (1) equate the decoherence rate among the mixed excited substates to half of the Einstein coefficient to comply with energy conservation, and (2) present a clear physical insight into the origin of the Hanle effect, whose quantitative evaluation for these atoms requires quantum mechanical treatment. We then derive LIF emission rates for each of the eight and 10 pathways associated with the D1 and D2 transitions, and deduce the temperature and LOS wind dependence in differential backscattering cross sections of atmospheric sodium and potassium atoms. Using these cross sections allows retrieval of atmospheric parameters in the mesopause region (80–110 km in altitude) by a metal resonance lidar.

In Chapter 3, we move to a semiclassical treatment (quantum theory) of light absorption and scattering, specifically from atoms. We start with a description of how lidar measures Doppler shift, and the fundamental difference between the measurement when the laser is in resonance with an atomic transition (resonant) and when it is not (nonresonant). We follow with a treatment of quantum polarizability and the resulting absorption cross section, leading to the differential resonance scattering cross section and its contrast with the classical result. After quantum polarizability, we demonstrate the radiation pattern of coherently excited atoms. This takes us to an interpretation of the Hanle Effect. Following these descriptions of the phenomena that impact resonance lidar, we extend our understanding by closing the chapter with an overview of the rudimentary physics of sodium laser guide stars.

A laser resonant scattering process involves two steps, excitation and emission. That emission occurs spontaneously is well accepted. That the atoms involved in the emission are excited coherently by a laser beam leading to a non-isotropic angular distribution of emission (an antenna pattern) is not well known. The difference between coherent and incoherent excitation leads to the Hanle effect. In this paper, I discuss the physics of Hanle effect, and its influences on the backward scattering intensity of Na, K, and Fe atomic transitions and the associated Na and Fe resonant fluorescence lidar systems.

Optical pumping of the ground states of sodium can radically alter the shape of the laser-induced fluorescence excitation spectrum, complicating measurements of temperature, pressure, etc., which are based on these spectra. Modeling of the fluorescence using rate equations for the eight hyperfine states of the sodium D manifolds can be used to quantify the contribution to the ground state pumping of transitions among the hyperfine excited states induced by collisions with buffer gas atoms. This model is used here to determine, from the shape of experimental spectra, cross sections for $\ensuremath{\Delta}F$ transitions of the

{P}_{3/2}

state induced by collisions with helium and argon atoms, for a range of values assumed for the ${P}_{1/2}, \ensuremath{\Delta}F$ cross sections. The hyperfine cross sections measured using this method, which to our knowledge is novel, are compared with cross sections for transitions involving polarized magnetic substates

{m}_{F}

measured previously using polarization sensitive absorption. Also, fine-structure transition $(\ensuremath{\Delta}J)$ cross sections were measured in the pumped vapor, giving agreement with previous measurements made in the absence of pumping.

The resonant excitation of the sodium D2 transition by laser radiation was investigated experimentally in this thesis. Line polarization measurement of the resonance fluorescence was employed to investigate the excitation process. The sodium atoms, which were collimated in a very narrow beam, were excited by a monochromatic laser beam with the frequency tuned to resonance with the 32S 112(F=2) - 32P312 hyperfine transition. The direction of the laser beam was perpendicular to the atomic beam and the fluorescence photons were detected along a direction perpendicular to both the laser and atomic beams. The experimental measurements were performed to determine the effect due to the variation of the laser intensity and frequency on the line polarization. A theoretical expression of the line polarization as a function of density matrix elements of the upper excited hyperfine sub -states was derived. A full quantum electrodynamic theory was used to derive an atomic operator equation of motion which was needed to calculate atomic operator elements. These elements then yielded the corresponding density matrix elements. The experimental results and the theoretical modelling were in a good agreement. The first experimental results for the line polarization of the highly excited states, 42D512 and 42D312, are also presented. The use of two lasers enabled these highly excited states to be excited. The line polarization of the fluorescence for transitions from the 4D level down to lower states is derived in terms of the density matrix elements of the upper excited states. Two limiting cases for the theoretical predictions of the line polarization are considered.

Saturation effects may significantly reduce the backscattered signal expected for resonance fluorescence lidar systems. Saturation arises when the laser energy density within the layer is large enough to significantly alter the population densities of the atomic states within the layer. These altered state populations lead to nonlinear absorption of the laser energy resulting in reduced rate of fluorescence and increased rate of stimulated emission. The level of saturation is determined by the laser pulse length, pulse energy, beam-width, and linewidth. Analysis reveals that the saturation effects can be characterized by two parameters: the saturation time tau(s) and the rms laser linewidth Deltaomega(rms).The saturation time is the characteristic time of stimulated emission and is an indication of the level of saturation. The saturation time depends on pulse energy, pulse length, and beamwidth. The laser linewidth is important since it determines the effective absorption cross section of the atoms and also the percentage of atoms illuminated for an inhomogeneously broadened absorption line. The impact of saturation on lidar design is also examined. Design examples, including lidar systems for atmospheric research and laser guide stars for adaptive imaging applications in astronomy are studied in detail.

The effect of collisions on the resonance fluorescence spectrum of two-level atoms excited by monochromatic resonance radiation is studied. Analysis is performed for systems where the Doppler broadening is small compared to the collision frequency (high buffer gas pressures) and for the general case where collisions arbitrarily change the phase of the radiation-induced dipole moment (from completely interrupted to completely unaffected phase of the dipole moment). Both at a relatively low and at a high excitation radiation intensity, the resonance fluorescence spectrum is shown to depend on whether the two-level system is closed or open. This is especially true for the narrow unshifted Rayleigh scattering line. It is shown that, although the absorption line is homogeneously broadened, the resonance fluorescence spectrum exhibits a clearly pronounced anisotropy. In a direction close to the direction of propagation of the excitation radiation, the Rayleigh scattering line is maximally narrowed. Under certain conditions (that can easily be created in experiments), the width of this line is proportional to the diffusion coefficient of atoms interacting with the radiation.