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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Broadly tunable resonant Raman gain and cw lasing in an optically thick Doppler-broadened medium

A. K. Popov, Sergei A. Myslivets, U. Hinze, E. Tiemann, B. Wellegehausen, Gennady Tartakovsky

Published Sep 8, 1999• 0 citations
Publisher

Abstract

Interactive numerical simulator, based on MATLAB/SIMULINK platform, for virtual experimentation and optimization of frequency tunable optically pumped dimer laser has been created. Nonperturbative theory considering features of quantum coherence and interference effects at Doppler broadened transitions under two strong driving fields accounting for collisions and other kinetic processes in vapor-gas mixture as well as for propagation effects in optically thick medium is developed. The results are in good agreement with real experiments.

DOI
10.1117/12.361170

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Interactive numerical simulator, based on MATLAB/SIMULINK platform, for virtual experimentation and optimization of frequency tunable optically pumped dimer laser has been created. Nonperturbative theory considering features of quantum coherence and interference effects at Doppler broadened transitions under two strong driving fields accounting for collisions and other kinetic processes in vapor-gas mixture as well as for propagation effects in optically thick medium is developed. The results are in good agreement with real experiments.
Resonant nonlinear-optical interference processes in four-level Doppler-broadened media are studied. Specific features of amplification and optical switching of short-wavelength radiation in a strongly-absorbing resonant gas under coherent quantum control with two longer wavelength radiations, are investigated. The major outcomes are illustrated with virtual experiments aimed at inversionless short-wavelength amplification, which also address deficiencies in this regard in recent experiments. With numerical simulations related to the proposed experiment in optically-dense sodium dimer vapor, we show optimal condition for optical switching and the expected gain of the probe radiation, which is above the oscillation threshold.
Experimentally observed near-field and far-field spatial profiles, frequency spectra, and cone angles are in good agreement with numerical computations of a theory that includes Doppler-broadened Raman-gain amplification of Doppler-broadened resonance fluorescence, four-wave mixing, propagational coupling, self-trapping of the pump beam, and diffraction and pump-induced refraction of the new frequencies. The use of a cw laser beam detuned from resonance by only a few gigahertz makes the computational problem tractable, leading to a simple physical model of this spatiotemporal instability.
An influence of nonlinear interference processes at quantum transitions in strong resonance electromagnetic fields on absorption, amplification and refractive indices as well as on four-wave mixing processes is investigated. Doppler broadening of the coupled transitions, incoherent excitation, relaxation processes, as well as power saturation processes associated with the coupled levels are taken into account. Both closed (ground state is involved) and open (only excited states are involved) energy level configurations are considered. Common expressions are obtained which allow to analyze the optical characteristics (including gain without inversion and enhanced refractive index at vanishing absorption) for various V, (Lambda) and H configurations of interfering transitions by a simple substitution of parameters. Similar expressions for resonant four-wave mixing (FWM) in Raman configurations are derived too. Crucial role of Doppler broadening is shown. Theory is applied for numerical analysis of some recent and potential experiments.
The interaction of a strong nearly resonant cw pump laser and a weak cw probe laser in a medium consisting of two-level atoms was studied by numerical integration of the Maxwell-Bloch equations. With propagation into the optically thick medium, the numerically calculated probe gain spectrum deviates substantially from one calculated by a four-wave-mixing formulation of coupled-wave theory with an undepleted pump. For probe frequencies inside the modified gain region, the laser field, initially in an unstable steady-state condition, evolves to a stable solution of the sine-Gordon equation.
The influence of nonlinear interference processes and of changes in populations in strong electromagnetic fields, and also of the Doppler broadening on resonant four-wave frequency mixing is investigated for a system of Raman-type transitions. The case in which a quantum system interacts with one weak field and two strong fields is considered. Each of the energy levels is assumed to be perturbed by just one strong field. It is shown that the Doppler broadening alters drastically the contributions of the populations of the various levels to the investigated process and that this can alter by several orders of magnitude and nonlinear susceptibilities averaged over the velocities and influence significantly the selection of optimal configurations of the transitions and of the experimental conditions for effective resonant four-wave conversion of the radiation frequencies. A numerical analysis of the derived expressions is used to account for the experimentally observed considerable differences between the spectral dependences of the output power and the conclusions drawn from perturbation theory.