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AIP conference proceedings

Parametric processes and gain saturation in resonantly enhanced optical phase conjugation in Na vapor near a two-photon resonance

Reiner K. Wunderlich, W. R. Garrett, M. G. Payne

Published Jan 1, 1987• 0 citations
Publisher

Abstract

Conjugate wave generation has been studied in Na vapor near two‐photon resonance with the 4D state. Measurements of reflection gain at the laser frequency, stimulated Raman scattering, amplified spontaneous emission, and parametric four‐wave mixing intensities are used to delineate the role of parasitic processes in determining conjugate ‘‘reflection’’ gain profiles.

DOI
10.1063/1.36726

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The propagation of a nearly resonant strong laser in Na vapor results in a number of non-linear optical processes such as parametric four-wave mixing (FWM), three-photon scattering (TPS), and Raman scattering.1 The Raman and TPS processes involve the emission of radiation at specific and unique frequencies determined by the detuning of the laser from the atomic resonance, δ, and the Rabi frequency, Ω. Parametric FWM, however, need only satisfy the condition that 2ωL = ω3+ ω4, and can occur over a broad range of frequencies. The frequency range is restricted only by the available gain for the FWM process. The gain depends strongly on the possibility of phase matching at the given frequencies involved in the interaction, as well as on nearly all of the experimental parameters such as relaxation rates (collisional and otherwise), Rabi frequencies, laser detuning, the length of the Na interaction region and the Na vapor pressure.

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AIP conference proceedings•Jan 1, 1989•0 citations
Conjugate wave generation has been studied in Na vapor near two‐photon resonance with the 4D state. Measurements of reflection gain at the laser frequency, stimulated Raman scattering, amplified spontaneous emission, and parametric four‐wave mixing intensities are used to delineate the role of parasitic processes in determining conjugate ‘‘reflection’’ gain profiles.
Experimental studies have been made of conjugate wave generation in a Na metal vapor heat pipe. Detailed measurements of gain profiles, pump beam absorption, and parametric wave generation at infrared frequencies for phase conjugation near the two-photon resonance with the Na 4d2D5/2 state, yield details of the effects of parasitic processes on degenerate four-wave mixing. Studies are conducted with an excimer-pumped dye laser system with ~4 ns pulses of < 10mJ/pulse, traversing Na vapor heat pipe. Included in the measurements are pump beam depletion, probe and conjugate beam intensities, and infrared light which is generated by the strong pump beams through stimulated Raman (SR) and amplified spontaneous emission processes. The role of ac Stark shifting in degenerate four-wave mixing near the 4d level is studied. We find a new effect involving a large contribution to the ac Stark effect from the internally generated SR photons. We observe a rapid onset with pressure of stimulated Raman emission accompanied by a dramatic increase in the ac Stark profile of the 4d level, and the sharp onset of gain saturation for the phase conjugate signal, due to the competition of these two processes. This effect results in an ac Stark coefficient which can be very nonlinear in laser power and Na pressure.
The nonlinear response of sodium vapor with a laser tuned near the 4D two-photon resonance is investigated over a large range of sodium concentrations and pump intensities with emphasis on the influence of coherent cancellation effects on the induced polarizations and the primary mechanism for the generation of the new frequencies. Over a wide range of circumstances, the response of the medium is dominated by parametric four-wave mixing as compared to amplified spontaneous emission or stimulated electronic hyper-Raman scattering. Secondary four-wave-mixing fields are generated by stimulated Raman scattering of one parametric four-wave-mixing field and a coupling of these fields with the driving laser field. A sharp onset of saturation of the parametric four-wave-mixing signals with sodium number density and a transition from a quadratic to a linear power dependence is observed. Both effects are explained in terms of the two-photon cancellation effect.
Two types of parametric four-wave mixing are observed when a single intense laser beam propagates through Na vapor near the D(1) and D(2) resonance lines. The first is related to three-photon scattering, while the second results from the coherent parametric interaction of the laser with excited-state Raman scattering. Identification of the two processes was greatly facilitated by the use of an optical multichannel detector system to observe the total emission spectrum for each individual laser pulse. The experimental conditions that determine whether stimulated emission or one or both of the four-wave mixing processes dominates the emission spectrum are described.
Experimental investigations of two-photon enhanced stimulated hyper-Raman and parametric four-wave-mixing emissions are described for tuning unfocused laser beams near two-photon resonance with 3D and 4D states in sodium vapor. Over wide ranges of vapor pressures, path lengths, and laser power densities, the gains for stimulated hyper-Raman (SHR) scattering and for parametric four-wave mixing (PFWM) can become very greatly reduced from values expected from simple considerations. The suppression of gain can arise from interferences produced at two-photon and three-photon resonances, and from ac Stark shifts produced by internally generated near-resonant fields. Quantitative comparisons are made between observed and theoretically predicted values for SHR and PFWM production. Of 12 predicted consequences of the interferences and Stark shifting on the nonlinear behavior, ten are demonstrated through the experiments.
The propagation of a nearly resonant strong laser in Na vapor results in a number of non-linear optical processes such as parametric four-wave mixing (FWM), three-photon scattering (TPS), and Raman scattering.1 The Raman and TPS processes involve the emission of radiation at specific and unique frequencies determined by the detuning of the laser from the atomic resonance, δ, and the Rabi frequency, Ω. Parametric FWM, however, need only satisfy the condition that 2ωL = ω3+ ω4, and can occur over a broad range of frequencies. The frequency range is restricted only by the available gain for the FWM process. The gain depends strongly on the possibility of phase matching at the given frequencies involved in the interaction, as well as on nearly all of the experimental parameters such as relaxation rates (collisional and otherwise), Rabi frequencies, laser detuning, the length of the Na interaction region and the Na vapor pressure.