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Conference on Lasers and Electro-Optics

Observation of Raman-Gain Lasing and Four-Wave Mixing in Sodium Vapor

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

Published Apr 25, 1988• 0 citations
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Abstract

The Doppler-broadened Raman gain of sodium vapor with a strong cw pump lases within a ring cavity. This lasing four-wave mixes with the pump to produce a third, complementary beam. Observations agree with theory only if atomic motion is included.

Related papers

GAIN/FEEDBACK APPROACH TO OPTICAL INSTABILITIES IN SODIUM VAPOR

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•Jan 1, 1989•0 citations

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We have observed oscillation in the wings of the Doppler-broadened sodium D lines when they are pumped by a cw dye laser and enclosed in an optical cavity. The oscillation is blue (red) detuned from the pump laser by the ground-state hyperfine splitting of sodium when the pump-laser frequency is in the blue (red) wing. The gain mechanism is explained on the basis of optical pumping and stimulated Raman scattering.

Optics Letters•May 1, 1985•58 citations
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 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.
We have used a two-wave mixing gain process in sodium vapor to construct a self-pumped phase conjugator. This was done by forming a resonator around a quartz sodium cell equipped with Brewster angle windows. A pump beam is then directed into the sodium cell at a small angle to the longitudinal axis of the resonator. At sufficiently high cw pump intensities, the ac-Stark effect pro- vides sufficient gain to achieve oscillation. Although this gain is unidirectional it occurs on the wings of the sodium resonance so that the backward traveling optical field in the resonator does not experience loss due to sodium absorption. The counterpropagating optical fields in the resonator interact with the pump in an internal four- wave mixing process to produce a fourth optical field (phase conjugate to the pump field). Our experiment was conducted at the sodium D2 resonance. The pump laser was a 699 autoscan single frequency ring dye laser providing pump intensities up to several hundred watts per square centimeter.