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Japanese Journal of Applied Physics

Theoretical Study on Optical-Pumping Self-Focusing

T. Yabuzaki, Hirokazu Hori, Masao Kitano

Published Mar 1, 1982• 4 citations
Publisher

Abstract

A theoretical study is made on the steady-state self-focusing and self-defocusing of a circularly-polarized laser beam, occurring in an atomic vapor mixed with a buffer gas at a relatively high pressure. The presence of the buffer gas removes the theoretical difficulties arising from hole-burning, the transit time, and hyperfine pumping. It also localizes the optically-pumped atoms in the vicinity of the laser beam, so that self-focusing occurs even at extremely weak light intensities. A preliminary experiment is also descirbed, the results of which show that optical-pumping self-focusing can be observed at laser intensities down to 30 µW.

DOI
10.1143/jjap.21.504

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A theoretical study is made on the steady-state self-focusing and self-defocusing of a circularly-polarized laser beam, occurring in an atomic vapor mixed with a buffer gas at a relatively high pressure. The presence of the buffer gas removes the theoretical difficulties arising from hole-burning, the transit time, and hyperfine pumping. It also localizes the optically-pumped atoms in the vicinity of the laser beam, so that self-focusing occurs even at extremely weak light intensities. A preliminary experiment is also descirbed, the results of which show that optical-pumping self-focusing can be observed at laser intensities down to 30 µW.
Circularly polarized laser beams propagating through sodium vapour and tuned to the buffer-gas-broadened atomic transition can optically pump sodium atoms into a non-absorbing ground state. This causes an intensity-dependent refractive index gradient along as well as transverse to the laser beam propagation direction, giving rise to a number of nonlinear spatio-temporal intensity and polarization pattern creating processes. In the case of a single circularly polarized laser beam we have observed self-focusing and defocusing, the transformation of the incident Gaussian beam intensity profiles into ring profiles, a large shift of about 5 GHz of the maximum of the absorption profile when suitable magnetic fields are applied and the deflection of a beam by the inhomogeneous transverse magnetic field of a current-carrying wire. When two beams of opposite circular polarization are superimposed, astonishing effects such as the mutual deflection of both beams (beam bouncing), the mutual extinction of both beams (beam switching), the separation of initially overlapping beams (beam splitting) and the mutual attraction of both beams (beam attraction) can be observed. While most of the effects can be well described for the stationary state by a to atomic transition model, the correct description of the dynamics requires the consideration of all hyperfine states.
Summary form only given. The interaction between an intense pump beam and a weak probe beam in an atomic vapor has been the subject of numerous investigations. Besides the well-known features in the frequency spectrum of the light, interesting spatial phenomena are observed. They are due to the inhomogeneous intensity distribution of a laser beam, coupled to propagation, and range from self-focusing or defocusing effects to the generation of localized distributions of light that propagate self-similarly (bright or dark solitons). We investigate the reshaping of a probe beam due to the interaction with a pump in sodium vapor, a problem that is important in many different contexts.
We report steady-state self-focusing, self-trapping, and self-defocusing of a cw dye laser beam in sodium vapor for frequencies within several Doppler widths of the DDD-line resonance transitions. We measured the variation of the beam profile as the light propagates through the vapor. We observed a 20-mW beam self-trapped in a 12-cm-long filament having a half-power diameter of 70 \ensuremath{\mu}m.
We report steady-state self-focusing, self-trapping, and self-defocusing of a cw dye laser beam in sodium vapor for frequencies within several Doppler widths of the DDD-line resonance transitions. We measured the variation of the beam profile as the light propagates through the vapor. We observed a 20-mW beam self-trapped in a 12-cm-long filament having a half-power diameter of 70 \ensuremath{\mu}m.
We have observed a collapse of the Na hyperfine doublet into a single peak due to laser-induced optical pumping when the product of laser power and buffer gas pressure exceeds \ensuremath{\sim}0.1 mW Torr. The complex line shapes observed under these conditions have been quantitatively explained by a model which describes the laser-induced change in the population of the ground-state hyperfine levels and also includes pressure broadening and diffusion of atoms out of the laser beam. The line-shape distortion can occur at laser intensities 103{10}^{3}103 below that required for radiative saturation, and must be considered in experiments involving the line shape or the population of the hyperfine levels.