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Optica Publishing Group
  • International Quantum Electronics Conference
  • 1996 OSA Technical Digest Series (Optica Publishing Group, 1996),
  • paper ThJ4

Laser beam switching, splitting and bending in sodium vapor

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Abstract

A circularly polarized laser beam propagating through sodium vapor can optically pump sodium atoms into a nonabsorbing groundstate. This causes an intensity dependent refractive index gradient along as well as transverse to the laser beam propagation direction which may give rise to a number of non-linear spatio-temporal intensity and polarization pattern creating processes. Examples [1,2] are self-focusing and defocusing of laser beams, beam splitting, beam bouncing or processes which transform incident Gaussian beam intensity profiles into ring profiles. Here we report on such proceses which result in switching, splitting and bending of laser beams. If two co-propagating laser beams of right and left hand circular polarization partly overlap, mutual attraction or deflection of the beams can be observed depending on their frequency detunign with regard to the atomic D1 transition. Similar, but not quite the same behavior has first been observed in sodium vapor without buffergas [3]. In our experiment, argon buffergas is used to homogeneously broaden the D1 transition and to reduce the atomic diffusion rate out of the beam in order to enable efficient optical pumping. In contrast to the case without buffergas, this allows for a description in terms of a semiclassical model [4] based on the Maxwell-Bloch equations where the atom is treated in the J=1/2 to J=1/2 approximation. In fig. 1 experimental results and numerical simulations are summarized in a phase diagram for the case where a left and a right hand circularly polarized beam overlap. The beam center separation at cell entrance is about 10μm for beam waists of about 120μm. Several regions in the power versus wavelength parameter space are distinguished where beam switching, large beam splitting, small beam splitting, and no measurable splitting at the cell exit window occur. Beam switching [5] takes place at low beam intensities and at small frequency detunings. One beam alone can optically pump the medium and penetrate through the cell. As soon as the second beam is turned on, the first beam is switched off and no light at all penetrates the cell. At larger detunings, especially towards shorter wavelengths, and larger beam intensities the initially almost overlapping beams leave the cell clearly separated by more than a beam waist. In fig.2 this beam splitting behavior is shown as the laser frequency is swept through the atomic D1 resonance. The final beam separation as a function of initial beam overlap is displayed in fig.3. It is quite remarkable that even for a small initial separation of beam centers a large final separation occurs. This clearly indicates that a strongly nonlinear interaction mechanism is involved and that the complete overlap represents a spatially unstable state. The final beam separation for an initial beam separation is 100μm

© 1996 Optical Society of America

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