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Annual Meeting Optical Society of America

Self-bending of a cw beam in sodium vapor

Grover A. Swartzlander, Huimin Yin, A. E. Kaplan

Published Jan 1, 1988• 0 citations
Publisher

Abstract

We report the first observations of the cw self-bending effect1 using an asymmetrical laser beam propagating in sodium vapor and discuss some concerns of practical and physical importance. The laser frequency was detuned below the D2 resonance until the maximum self-bending angle was observed. At ~200°C the nonlinear refractive index was found to be predominantly Kerrlike (Δn ≃ n 2 I) at experimental peak intensities up to ~220 W/cm2. This result is inconsistent with our numerical calculations of the saturation of the anomalous dispersion that includes power-broadened hole-burning of an inhomogeneously broadened two-level system,2 which predicts strong saturation of Δn. By analyzing either the self-deflection angle or the intensity at a single point in the far-field region as the input power was varied, we were able to determine that n2 ≃ − 10 − 7 cm2/W. Self-deflection angles as large as 5.9 mrad or eight times the diffraction angle were observed, which is more than enough for some useful applications such as resonatorless optical bistability, optical switching, and interconnecting, nonlinear optical coupling, and radiation protection, the latter of which we have demonstrated.

DOI
10.1364/oam.1988.wz2

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Nonlinear optical experiments in sodium vapor and comparison with Doppler-broadened two-level-atom theory.

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Two spectral regions of gain exist for a weak probe beam propagating through a medium of two-level-atoms pumped by a strong near-resonance field. Experimentally a cw ring-dye laser is used to explore this gain at the Na D₂ resonance in a vapor. Plane-wave calculations of probe-gain spectra which include the Doppler broadening inherent in a vapor agree well with experimental spectra obtained with a Fabry-Perot interferometer. Such two-beam-coupling gain might have applications as optical pre- or power amplifiers. The gain is also the primary step in four-wave-mixing. Mixing of the pump and sideband which experiences gain produces the medium polarization from which the fourth-wave arises. For phase-matched propagation the fourth-wave, which is at a frequency that experiences little or negative probe-gain (i.e., absorption), grows at nearly the same rate as the primary sideband. Together the two sidebands extract far more than twice as much energy from the pump than does the primary sideband acting alone. Experimentally four-wave-mixing which arises from noise at the gain-sideband-frequency is sometimes accompanied by conical emission at the fourth-wave sideband. Since this sideband is also seen on axis the explanation cannot be simply phase-matching. Simulations which include the full transverse nature of the experiment are currently running on a CRAY supercomputer. These simulations indicate that the radial variation of the medium index of refraction is responsible for conical emission.

•Jan 1, 1989•0 citations
We report the first observations of the cw self-bending effect1 using an asymmetrical laser beam propagating in sodium vapor and discuss some concerns of practical and physical importance. The laser frequency was detuned below the D2 resonance until the maximum self-bending angle was observed. At ~200°C the nonlinear refractive index was found to be predominantly Kerrlike (Δn ≃ n 2 I) at experimental peak intensities up to ~220 W/cm2. This result is inconsistent with our numerical calculations of the saturation of the anomalous dispersion that includes power-broadened hole-burning of an inhomogeneously broadened two-level system,2 which predicts strong saturation of Δn. By analyzing either the self-deflection angle or the intensity at a single point in the far-field region as the input power was varied, we were able to determine that n2 ≃ − 10 − 7 cm2/W. Self-deflection angles as large as 5.9 mrad or eight times the diffraction angle were observed, which is more than enough for some useful applications such as resonatorless optical bistability, optical switching, and interconnecting, nonlinear optical coupling, and radiation protection, the latter of which we have demonstrated.
Observations of the self-deflection effect for an asymmetrical continuous-wave laser beam in sodium vapor are described, and comparisons based on theoretical calculations are made. A self-bending angle as large as eight diffraction widths was recorded, and strong attenuation of the on-axis radiation due to self-bending was measured. At ~200°C the self-deflection angle increased linearly with beam power, and we determined that (n2)max ≃ −10−7 cm2/W for intensities below 220 W/cm2. While numerical calculations, based on an inhomogeneously broadened two-level system, predict strong saturation of the self-bending effect, we observed only moderate saturation.
Two intersecting laser beams of orthogonal circular polarization near resonant to the homogeneously broadened D1 transition of Na vapor were observed to deflect each other upon contact. The experimental results are in good agreement with calculations based on a model which includes diffraction and a local nonlinear atomic susceptibility wihch fully takes into account saturation and optical pumping effects. The contact deflection (or ``bouncing'') was also observed when both beams were tuned to the atomic resonance frequency.
Nonlinear optical transverse effects, which arise from the interaction of a Gaussian beam with a nonlinear medium, are discussed. They are cw on-resonance enhancement (CORE), cw off-resonance rings, crosstalk, intracavity phase switching, self-defocusing of a bistable etalon, and thin-sample-encoding (TSE) self-lensing (i.e., self-focusing or self-defocusing) bistability and instabilities. CORE is the enhancement of the on-axis intensity of an on-resonance laser beam in a two-level medium. We report the first observation of CORE using sodium vapor. Cw off-resonance rings, resulting from a pure phase encoding or from simultaneous phase and amplitude encodings, are observed using sodium vapor. These rings have been observed many time before, but we have made the first careful comparison with numerical calculations. Parallel operation and external switching are two crucial themes for the application of bistable optical devices (BODs). For parallel operation, crosstalk between nearby BODs on the same etalon due to diffraction coupling may need to be avoided. Minimum separations needed to ensure independent operation are computed. For external switching, intracavity phase switching using a plane-wave input can switch on (or off) a BOD using an external pulse. The validity of this concept for a Gaussian input is numerically simulated. Self-defocusing effects, resulting in some interesting effects in the transverse bistable loops, are observed using a GaAs-GaAlAs bistable etalon. The results are confirmed by numerical simulations. TSE self-lensing bistability is observed using a short sodium cell and a single mirror. TSE self-lensing bistability results from an enhancement in the feedback via self-lensing of the beam. The lack of an optical resonator and the use of a thin medium make TSE setup a good candidate to study the Ikeda instability. We report the first observation of the Ikeda instability in this all-optical passive system using a cw input. Numerical simulations show a rich bifurcation sequence not yet fully studied experimentally.
A circularly polarized laser beam propagating through sodium vapor is shown to be deflected by the inhomogeneous magnetic field from a nearby electric current-carrying thin wire aligned parallel to the beam. The effect arises from the refractive index gradient induced by magnetic-field-modified optical pumping. The experimentally observed beam deflection is in good agreement with predictions from a $J\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2\ensuremath{\leftrightarrow}J\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2$ model for the full four-dimensional spatialtemporal evolution of the interaction between radiation field and atoms. The time of order 100 ns needed for the beam to switch between undeflected and deflected positions is determined by the average Larmor precession period and is 2 orders of magnitude shorter than the optical pumping time.
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.
Two spectral regions of gain exist for a weak probe beam propagating through a medium of two-level-atoms pumped by a strong near-resonance field. Experimentally a cw ring-dye laser is used to explore this gain at the Na D₂ resonance in a vapor. Plane-wave calculations of probe-gain spectra which include the Doppler broadening inherent in a vapor agree well with experimental spectra obtained with a Fabry-Perot interferometer. Such two-beam-coupling gain might have applications as optical pre- or power amplifiers. The gain is also the primary step in four-wave-mixing. Mixing of the pump and sideband which experiences gain produces the medium polarization from which the fourth-wave arises. For phase-matched propagation the fourth-wave, which is at a frequency that experiences little or negative probe-gain (i.e., absorption), grows at nearly the same rate as the primary sideband. Together the two sidebands extract far more than twice as much energy from the pump than does the primary sideband acting alone. Experimentally four-wave-mixing which arises from noise at the gain-sideband-frequency is sometimes accompanied by conical emission at the fourth-wave sideband. Since this sideband is also seen on axis the explanation cannot be simply phase-matching. Simulations which include the full transverse nature of the experiment are currently running on a CRAY supercomputer. These simulations indicate that the radial variation of the medium index of refraction is responsible for conical emission.