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Soviet Journal of Quantum Electronics

Investigation of self-focusing in the case of stimulated Raman scattering of light

Anna D. Kudryavtseva, Albina I. Sokolovskaya, M. M. Sushchinskiǐ

Published Jan 31, 1972• 1 citation
Publisher

Abstract

An investigation was made of the influence of different excitation conditions on the self-focusing of stimulated Raman-scattered light. It was found that the dimensions of self-focusing filaments and their number depended on the energy of the exciting radiation and the thickness of the scattering layer. In the situation in which self-focusing points were observed at both ends of a Dewar flask, the anti-Stokes scattering included (in addition to rings of class 1) scattering along the axis. Estimates were obtained of the self-focusing threshold power related to the Kerr effect and to the increase in the polarizability of molecules due to excitation. It was found that the first of these mechanisms could not give rise to self-focusing of the Raman-scattered light, whereas the threshold of the second mechanism was close to the excitation energy at which such self-focusing was observed.

DOI
10.1070/qe1972v002n01abeh003534

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An investigation was made of the influence of different excitation conditions on the self-focusing of stimulated Raman-scattered light. It was found that the dimensions of self-focusing filaments and their number depended on the energy of the exciting radiation and the thickness of the scattering layer. In the situation in which self-focusing points were observed at both ends of a Dewar flask, the anti-Stokes scattering included (in addition to rings of class 1) scattering along the axis. Estimates were obtained of the self-focusing threshold power related to the Kerr effect and to the increase in the polarizability of molecules due to excitation. It was found that the first of these mechanisms could not give rise to self-focusing of the Raman-scattered light, whereas the threshold of the second mechanism was close to the excitation energy at which such self-focusing was observed.
An investigation was made of the influence of the geometric excitation conditions on the energy dependences of the stimulated scattering of focused laser pulses of ~ 10 −8 sec duration when the sharpness and depth of focusing were varied. The experimental results obtained in this study were interpreted theoretically on the basis of a quantum analysis of stimulated Raman scattering in a two-level system. The experimental relationships could be explained only if an allowance was made for the changes in the populations of the levels participating in the Raman transition. It was possible to select the experimental conditions so that reconstruction of the wavefront of the pump radiation was observed under stimulated scattering conditions in a wide energy range.
The regular structure in frequency broadening spectrum of the self-trapped laser and Raman radiations is produced by the phase-modulation of light waves propagating in the filament with intensity-dependent index of refraction. A quantum-mechanical calculation of double quantum transition in a three-level system is shown to explain the process of self-modulation and the production of picosecond pulses in the Raman process. Coherent coupling of the laser and the Stokes waves produces mutual modulation of large amplitude, if the optical field is strong and the vibrational relaxation time is long. Numerical evaluation for CS2 gives a modulation frequency of about 1011 Hz, when an optical power of 10 kW is trapped in a filament of 2 µm radius. Besides, the effect of self-modulation on the spatial distribution of anti-Stokes emission along the filament is discussed and shown to agree with the experimental results observed by Stoicheff and his co-workers.
A calculation is made of the intensity of the Stokes radiation in the stimulated Raman scattering of ultrashort light pulses in a dispersive medium employing the asymptotic approximation for arbitrary modulation of the amplitude and phase of the pump pulse. Particular attention was paid to the effect of phase modulation on the stimulated Raman scattering. Calculations were also made of the intensity of the anti-Stokes component in the scattering of an ultrashort pulse in a dispersive medium and of the optimal angle for the observation of the anti-Stokes rings.
Polarization effects in stimulated Raman and stimulated Rayleigh scattering and self focusing are investigated from the viewpoint of the scattering of polarized photons by molecules. Both stimulated vibrational Raman scattering in liquids and stimulated rotational Raman scattering in gases are considered, and it is demonstrated that the polarization changes and relative power thresholds for different laser polarizations should be somewhat different to those predicted in other theories. For example, the ellipticity eta 0 of the stimulated rotational Raman photons in the forward direction from Delta J=+or-2 transitions in gaseous symmetric top molecules is given in terms of the initial ellipticity eta 1 by sin 2 eta 0 = - 5 / 7 sin 2 eta 1 , whereas conventional theories predict that eta 0 and eta 1 are equal in magnitude and opposite in sign. Furthermore it is shown that deviations from this relation could provide a measure of the degree of optical orientation of the molecules.
A theoretical study of the polarization dependence of stimulated Rayleigh-wing scattering and the optical-frequency Kerr effect has been undertaken. When the incident light is circularly polarized, a striking difference in gain is found for the co- and counter-rotating senses of polarization of the scattered light---the former being suppressed and the latter being exceptionally favored. The analysis begins with a model for the Kerr effect, which involves the alignment of anisotropic molecules in an electric field, but the results can be immediately generalized to any light scattering process of tensor symmetry. The nonlinear problem of the propagation of an intense elliptically polarized light wave in a Kerr-active medium is shown to have a solution in which the vibrational ellipse undergoes self-precession and self-retardation. The stimulated scatterings or instabilities of such a self-precessing and self-retarding light wave are obtained for the backward and forward directions. Birefringence, optical activity, and linear and circular dichroisms are some of the phenomena which result, but the forward direction yields substantially different results from the backward direction because of Stokes-anti-Stokes coupling.