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Journal of the Optical Society of America B

Observation of continuous-wave on-resonance enhancement (A)

Kuochou Tai, Hyatt M. Gibbs, Michael C. Rushford, Nasser N. Peyghambarian, Martine Le Berre, Elisabeth Ressayre, Andree A. Tallet

Published Jan 1, 1984• 0 citations

Abstract

Recently, Boshier and Sandle1 predicted an unexpected and interesting phenomenon of the propagation of intense cw light through a strongly absorptive two-level medium: an enhancement of the on-axis intensity occurs with the laser frequency tuned to exact resonance with the atomic transition. Previously, cw self-focusing was thought to occur only on the high-frequency side of the resonance where nonlinear dispersion gives rise to an effective positive lens that focuses the entire beam,2,3 And indeed this cw on-resonance enhancement (CORE) arises from a different mechanism: the nonlinear absorption strips away the outer portions of the beam while diffraction of the light in the outer edge of the remaining core directs some of the beam toward the axis, resulting in near-field Fresnel fringes and CORE.

Related papers

Resonant self-focusing of a cw intense light beam

Martine Le Berre, Elisabeth Ressayre, Andree A. Tallet

A single pass of an intense cw light beam through a strongly absorbing medium can be characterized by two parameters, the input on-axis intensity normalized to the saturation value ${I}_{0,0}$, and the Fresnel number $F$ for an absorption length ${\ensuremath{\alpha}}^{\ensuremath{-}1}$. For both large ${I}_{0,0}$ and $F$, propagation can be divided in two parts: At the very beginning, the transverse profile of the beam experiences an encoding. For a beam tuned exactly on resonance, encoding reduces to a stripping of the profile from the wings to the center, progressively, as the light beam goes further into the cell. For $F>>1$ and ${I}_{0,0}>>1$, and both of the same order of magnitude, this stripping can be modeled by a circular aperture placed at the end of the encoding region. Afterwards, the beam is assumed to propagate in free space, its profile exhibits Fresnel interferences and on-axis enhancement. Using a perturbative treatment, the width and the location of the aperture are analytically estimated, and the model is successfully compared with numerical predictions of on-resonance self-focusing. For ${I}_{0,0}\ensuremath{\lesssim}F$, the stripping and free-space propagation cannot be separated because near-axis stripping coexists with large diffraction effects on the wings. This case, the best for observing cw on-resonance enhancement, would gain in a more sophisticated model, taking into account diffraction by successive apertures.

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Recently, Boshier and Sandle1 predicted an unexpected and interesting phenomenon of the propagation of intense cw light through a strongly absorptive two-level medium: an enhancement of the on-axis intensity occurs with the laser frequency tuned to exact resonance with the atomic transition. Previously, cw self-focusing was thought to occur only on the high-frequency side of the resonance where nonlinear dispersion gives rise to an effective positive lens that focuses the entire beam,2,3 And indeed this cw on-resonance enhancement (CORE) arises from a different mechanism: the nonlinear absorption strips away the outer portions of the beam while diffraction of the light in the outer edge of the remaining core directs some of the beam toward the axis, resulting in near-field Fresnel fringes and CORE.

A single pass of an intense cw light beam through a strongly absorbing medium can be characterized by two parameters, the input on-axis intensity normalized to the saturation value

{I}_{0,0}

, and the Fresnel number

F

for an absorption length ${\ensuremath{\alpha}}^{\ensuremath{-}1}$. For both large

{I}_{0,0}

and

F

, propagation can be divided in two parts: At the very beginning, the transverse profile of the beam experiences an encoding. For a beam tuned exactly on resonance, encoding reduces to a stripping of the profile from the wings to the center, progressively, as the light beam goes further into the cell. For

F>>1

and

{I}_{0,0}>>1

, and both of the same order of magnitude, this stripping can be modeled by a circular aperture placed at the end of the encoding region. Afterwards, the beam is assumed to propagate in free space, its profile exhibits Fresnel interferences and on-axis enhancement. Using a perturbative treatment, the width and the location of the aperture are analytically estimated, and the model is successfully compared with numerical predictions of on-resonance self-focusing. For ${I}_{0,0}\ensuremath{\lesssim}F$, the stripping and free-space propagation cannot be separated because near-axis stripping coexists with large diffraction effects on the wings. This case, the best for observing cw on-resonance enhancement, would gain in a more sophisticated model, taking into account diffraction by successive apertures.

There has been recent interest in the phenomenon of on-axis enhancement of an intense cw laser beam in a strongly absorbing, nonrefractive, nonlinear medium.1,2 The purpose of this paper is to present experimental observations of this cw on-resonant enhancement (CORE) and the associated Fresnel fringe structure and narrowing (self-focusing) of the spatial profile. We use the collisionally broadened D1 line of atomic sodium. We also illustrate the relatively close agreement of predictions1 based on a simple model of homogeneously broadened two-state absorbers.

Continuous-wave (cw) off-resonance rings and cw on-resonance enhancement (CORE) are observed as a result of an optical beam propagating through sodium vapor. They are interpreted in terms of diffraction-free encoding (DFE) followed by free-space propagation. They are discussed with respect to the three regimes of DFE, namely, pure phase encoding, on-resonance pure amplitude encoding leading to stripping, and the intermediate case involving both phase and amplitude encodings. First there are far-field interference rings displaying pure phase encoding. They are observed on both sides of resonance, as expected by theory. Second, there is CORE, which is a Fresnel diffraction manifestation resulting from the stripping of the beam profile by the nonlinear medium, and in the far field, the corresponding Fraunhofer patterns. CORE is observed in an inhomogeneously broadened system, and the role of Doppler broadening is theoretically studied. Finally, there are off-resonance rings involving both phase and amplitude encodings. They are numerically simulated and discussed with respect to DFE predictions.

We present a simple, analytically tractable, generalized aperture model for treating the stripping of a laser beam in a medium with saturable absorption. The model departs from previous treatments in allowing a soft transition from transparency to opacity, which yields a physically realistic diffraction-free amplitude encoding for the laser beam and thereby reduces unphysical fringes in subsequent beam evolution. The model is applied to the problem of cw exactly resonant self-focusing in a vapor of two-state atoms. It is confirmed that for strong beam saturation and absorption that is not too strong, beam propagation can be well modeled as a single stripping–diffraction sequence. A simple interpretation of the on-axis enhancement features is given.