Saturation-induced polarization preferences in two-mode oscillating gas lasers

D. Polder, W. van Haeringen

Anatolii M Tumaikin, W. van Haeringen, Daan Lenstra

Sergio Ciuchi, Maxi San Miguel, N. B. Abraham

We consider the statistical properties of the fluctuations in the orientation of the vector laser field for a nearly isotropic laser, addressing the competition between the randomizing influence of noise and the preference for linearly or circularly polarized emission induced by saturation in the laser medium and/or cavity anisotropies. We describe, through a nonperturbative analysis, a crossover regime from diffusion-induced polarization-isotropic emission to emission of nearly fixed polarization characteristics. In this crossover the linewidth of intensity fluctuations associated with one of the circularly polarized components of the vector field amplitude changes from decreasing to increasing with output power. This behavior can be expressed in terms of a single parameter that measures the pump or the total output intensity in terms of the ratio of the noise amplitude to the degree of cross saturation of the amplitudes of the circularly polarized components of the vector field amplitude. Analytical results are given for the linewidth of the intensity fluctuations. When the laser has a preference for linearly polarized emission, there is a weakly non-Lorentzian spectrum in the crossover region. When the laser has a ``preference'' for circularly polarized emission the crossover regime is characterized by a non-Lorentzian spectrum due to fast hopping between two circularly polarized eigenstates. We also describe ellipticity fluctuations induced by the diffusion of the direction of the main axis of the polarization ellipse in the presence of cavity birefringence.

Daan Lenstra

Abstract Starting from a theory in which laser operation is described as the excitation of the natural modes of an open-resonator system, the case of a Fabry-Perot resonator with plane-parallel mirrors is treated. It is shown that, in the case of weakly anisotropic mirrors, the natural modes can be grouped in pairs, such that each pair corresponds to one scalar longitudinal mode. In that case, laser operation can and will be described in terms of these longitudinal modes, which then possess a vectorial field amplitude, in agreement with the usual formalism. The polarization preference in a single-mode gas laser operating on a transition between a singlet and a triplet state is investigated, taking into account differences in the relaxation constants for the various expectation values of atomic variables. A calculation of the effect on the relaxation constants due to interatomic collisions leads to the conclusion that theory and experiment are in good agreement with each other. Equations describing the time evolution of the polarization parameters of both modes in a two-mode operating gas laser are derived. It is found as a general rule that mode-mode interaction results in modes whose polarizations are mutually orthogonal. An exception to this rule is found in the case in which the single-mode preference is for circular polarization. The influence of two specific mirror anisotropies is discussed. The polarization behaviour of a single-mode gas laser in the presence of (partly) applied, transverse magnetic fields is extensively dealt with. It appears to be essential to distinguish between the case in which the magnetic field is applied only to a fraction of the active medium and the case in which the magnetic field is uniform in the entire medium. This distinction is shown to lead to a satisfactory theoretical description of a certain class of experiments. In the case of two other experiments, good agreement with the theory is obtained, by assuming the presence of mirror anisotropies of acceptable kind and size. Finally, the influence of small anisotropies on the degree of polarization of a single-mode gas laser is theoretically investigated, when the latter is polarized by means of a strong transverse magnetic field.

N. B. Abraham, E. Arimondo, Maxi San Miguel

Christian Delsart, J. Keller

The optical Autler-Townes effect (dynamic Stark splitting) induced by a single-mode CW dye laser has been observed in neon gas with a cascade level scheme and a He-Ne probe beam. The splitting is found to be proportional to the square root of the saturating laser power and independent of the detuning. The existence of two Rabi nutation frequencies due to level degeneracy is clearly shown by changing the polarization axis of the probe beam.