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Physical Review Letters

Gain-feedback approach to optical instabilities in sodium vapor

G. Khitrova, John F. Valley, H. M. Gibbs

Published Mar 21, 1988• 120 citations
Publisher

Abstract

The gain-feedback approach to lasing and optical instabilities has been applied to sodium vapor driven by a nearly resonant intense field. The observed lasing frequencies agree with the two-beam-coupling gain curve calculated for a Doppler-broadened two-level medium. Rayleigh-gain lasing is seen for no external cavity with use of counterpropagating beams, and Raman-gain lasing is seen in a ring cavity.

DOI
10.1103/physrevlett.60.1126

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The gain-feedback approach to lasing and optical instabilities has been applied to sodium vapor driven by a nearly resonant intense field. The observed lasing frequencies agree with the two-beam-coupling gain curve calculated for a Doppler-broadened two-level medium. Rayleigh-gain lasing is seen for no external cavity with use of counterpropagating beams, and Raman-gain lasing is seen in a ring cavity.
We use the gain-feedback approach1 to explain a number of instabilities observed in sodium vapor with the laser frequency vL tuned to the defocusing side of line center at v0. The gain curve, which includes motion of the atoms, predicts two regions of possible gain: Rayleigh gain for frequencies v just above vL and Raman gain for v ≅vL - vR, where vR is the on-resonance Rabi frequency; see Fig. 1.2 Lasing based on these two gain mechanisms has been seen in three different feedback configurations. The new observations are: Rayleigh-gain lasing with no external mirrors using the distributed feedback of counterpropagating pump beams, Rayleigh-gain lasing without a foreign gas, Raman-gain lasing in a ring cavity (see Fig. 2), and both Rayleigh and Raman lasing using a single feedback mirror.
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.
We observe amplification of a linearly polarized laser beam propagating through a Doppler-broadened atomic potassium vapor driven by an intense, linear and orthogonally polarized counterpropagating laser beam. The observed gain spectra are well explained by a model that incoporates only the effects of the coherent driving of the atomic dipole moment and not atomic recoil. Our results suggest that the recently reported observations of amplification and lasing using a laser-driven sodium vapor may not arise from the effects of collective atomic recoil.
We present here instabilities observed in the transverse profile of a continuous-wave light beam that crosses a sample of sodium vapor twice by means of a feedback mirror. The wave front and the intensity of the beam are spatially modulated by the intensity-dependent dispersive and absorptive action of the vapor. Experimental evidence indicates that the vapor acts as a phase-conjugate mirror, thus providing an active cavity when it is coupled with the feedback mirror. The instabilities develop as a consequence of sideband generation at the resonant modes of this cavity. The transverse-mode profiles of these sidebands are analyzed experimentally.