Quasi-stationary polarization of a single mode gas laser in a magnetic field

Anatolii M Tumaikin, W. van Haeringen, Daan Lenstra

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.

A. G. Petrashen, V. N. Rebane, T. K. Rebane

W. van Haeringen

The polarization properties of a single-mode operating gas laser in a small axial magnetic field (field splitting\ensuremath{\ll}natural linewidth) with an initial cavity anisotropy are considered in detail. Expressions are given for the effects on the polarization parameters due to the active medium, the external field, and the initial anisotropy of the cavity. The cooperation of these three effects is discussed. Existing theories are in this respect shown to be either incomplete or inadequate. Specializations are made to several types of atomic transitions and cavity anisotropies. A group of low-field polarization phenomena observed on the He-Ne 1.153-\ensuremath{\mu} mode is theoretically discussed. In particular, it is shown that the observed polarization flip by tuning through the line center and the hysteresis effect, observed by Kannelaud and Culshaw, can be completely understood assuming a cavity whose main anisotropy is a linear phase anisotropy. At zero or very small magnetic field the theory predicts a preference for linear or circular polarization, depending on the type of atomic transition.

Guy Michel Stéphan, A. D. May

This tutorial paper is based on lectures delivered by G M Stéphan at PELS'97. The idea was to tell the students about the methodology used by the authors to understand and to quantitatively compute the flips observed between two polarization states in a single-frequency He-Ne laser. The study is limited to this phenomenon. Contents are: introduction, quasi-isotropic lasers and polarization flips; the passive Fabry-Perot interferometer, an illustrative example; a vectorial description of a two energy-level system with magnetic degeneracy; two useful tools: adiabatic elimination of fast variables and use of Lyapunov exponents to study the stability of a solution; medium polarization in the adiabatic approximation; the laser equations for the field. These contents are organized for people coming into the field: rather than being too general, we have preferred to focus on limited and well defined examples from which other cases can be studied.

L. Lynds, Albert W. Angelbeck, B. Burdick