Stokes Photons Research Articles

Quantum Theory of a Raman Laser

The quantum theory of a coherently driven optical cavity with an intracavity Raman active medium is presented. This system is shown to give rise to laser action at the Stokes frequency. A Fokker-Planck equation is derived for the Stokes mode and shown to assume the same form as that for the usual laser. The limiting noise in this Raman laser arose from the spontaneous decay of a pump photon into a Stokes photon and a phonon.

On the quantum theory of stimulated Raman scattering

A fully quantum mechanical density-matrix formalism of stimulated Raman scattering is presented. The effects of atomic motion, non-linear parameters and loss are included. The equations are solved to fourth-order in atom-field coupling constant. The steady-state Stokes-photon distribution is studied. A study of an approximate time-dependent case is also discussed. The results show that only a fraction of incident pump photons can be converted to Stokes photons.

Possibility of observing spontaneous and stimulated Raman scattering by forbidden rotational and vibrational–rotational transitions in a strong optical field

It is shown that, in strong optical fields (~109 W/cm2), Raman lines may be excited due to transitions with changes by 3 and 4 in the rotational quantum number, which are forbidden in the linear approximation. This lifting of forbiddenness for Raman scattering is due to four-photon processes in which a pump photon is absorbed and a Stokes photon is emitted in an elementary event. Under quasiresonance pumping, this type of stimulated Raman scattering may be observed due to rotational transitions. Estimate are made of the conditions for observation of the spontaneous and stimulated processes in specific media.

Possible Reduction of Laser-Fusion Target Illumination by Enhanced Stimulated Raman Plasmon Scattering

When the input irradiance $I$ exceeds a threshold value, two mechanisms cause the stimulated Raman scattering by a plasma to increase as $\mathrm{exp}({g}_{E}{I}^{2}x)$. One mechanism is a nonlinear increase in the plasmon amplitude, and the other is a second-order process of converting two laser photons into a Stokes photon and anti-Stokes photon. In laser fusion, the enhanced scattering by the residual fusion-chamber gas may greatly reduce the laser irradiance at the target. This theoretical possibility should be investigated experimentally.

Raman laser with a plane-parallel resonator pumped by a converging beam

The boundary-value problem is formulated for a Raman laser with a plane-parallel resonator pumped by a converging beam. The problem is formulated using the equations for the photon flux densities of the pump radiation and the first Stokes component. A method of numerical calculation on a computer is described. An approximate analytic technique is suggested for finding the transverse distributions of the Stokes photon flux density in the mirror planes. This technique can be used to find the principal features of steady-state emission from a Raman laser pumped by a converging beam. The approximate results are compared with those obtained by numerical calculations. The case of emission of two Stokes components is discussed.

Influence of spontaneous emission and of finite phonon lifetime on Stimulated Raman Scattering

We study the influence of the spontaneous emission and of the damping of the molecular vibrations on Stimulated Raman Scattering in liquids starting from the Heisenberg equations for the laser wave, for the 1. Stokes wave, and for the molecular vibrations. The spontaneous emission is described by fluctuating Langevin forces. In the linearized case the Heisenberg equations are solved analytically. In order to investigate saturation effects the amplitude equations are transformed into generalized rate equations, which are solved numerically. To demonstrate the results they are plotted in several figures, showing the growth and the lineshape of the 1. Stokes wave, the ratio of Stokes photons and phonons and the saturation behaviour of the phonons and the Stokes and laser photons.