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Influence of stimulated Raman scattering on the conversion efficiency in four wave mixing

Richard Wunderlich, Moore, W. R. Garrett, M. G. Payne

Published Jan 1, 1988• 1 citation
OpenAlex

Abstract

Secondary nonlinear optical effects following parametric four wave mixing in sodium vapor are investigated. The generated ultraviolet radiation induces stimulated Raman scattering and other four wave mixing process. Population transfer due to Raman transitions strongly influences the phase matching conditions for the primary mixing process. Pulse shortening and a reduction in conversion efficiency are observed. 8 refs., 3 figs.

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Secondary nonlinear optical effects following parametric four wave mixing in sodium vapor are investigated. The generated ultraviolet radiation induces stimulated Raman scattering and other four wave mixing process. Population transfer due to Raman transitions strongly influences the phase matching conditions for the primary mixing process. Pulse shortening and a reduction in conversion efficiency are observed. 8 refs., 3 figs.
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.
We look at the effects of population transfer on parametric four-wave mixing (PFWM) in Na vapor. We show experimentally that, during the laser pulse, phase matching conditions necessary for the generation of axial PFWM1 are destroyed by population transfer. Shortening of the PFWM pulse with respect to the laser pulse length is observed. In addition, by use of counterpropagating laser beams wherein the degree of temporal and spatial overlap in the medium is controlled, we show how simple measurements can provide direct evidence of population transfer spoiling of PFWM production.
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.
A phosphate neodymium glass laser emitting picosecond pulses tunable in the 1.05–1.07 μ range was developed to study nonlinear optical processes in atomic barium vapor. Its temporal characteristics were investigated using a modified second harmonic beam method. Electron stimulated Raman scattering and four-photon parametric conversion in atomic barium vapor were used to obtain efficient conversion of the third harmonic of the picosecond phosphate laser (350–356 nm) to the Stokes (580–600 nm) and anti-Stokes (250–253 nm) waves. The highest conversion quantum efficiency for the Stokes wave was 15%. Electron stimulated Raman scattering in an (6 s 2 ) 1 S –(6 s 7 s ) 1 S –(6 s 6 p ) 1 P 0 system, having a strongly forbidden first transition, was recorded for the first time. In the same system, two-photon pumping generated four-photon parametric superradiance and it was found that the signal and idler waves had anomalously broad spectra (wider than 100 cm −1 ).