The domain of vectorial bistability observable in the polarization flip of a quasi-isotropic laser is theoretically and experimentally shown to decrease when light intensity increases.
The basic mechanisms of the polarization switching in vectorial bistable lasers are studied both theoretically and experimentally versus internal parameters. In particular, by scanning the frequency of a gas laser with two linearly polarized eigenstates, two quite different flipping processes with hysteresis occur according to the linear-phase-anisotropy value of the cavity. Indeed, when the anisotropy increases, rotation and inhibition mechanisms appear successively. The variations of the hysteresis loop for different anisotropy and excitation values are theoretically found to be opposed in the two processes. Additional evolutions of the polarization, such as multiple switchings in the inhibition mechanism and ``hybrid'' hysteresis loops, are predicted. The experiments, essentially performed on a monomode 3.39-\ensuremath{\mu}m $^{3}\mathrm{\ensuremath{-}}^{20}$Ne laser containing an adjustable linear phase anisotropy, confirm the existence of the two processes and their corresponding properties. These processes may also occur in other quasi-isotropic lasers versus other internal parameters, such as, for instance, the injection current in semiconductor lasers. In this case the TE and TM modes flip only in the inhibition mechanism when the bistability conditions are satisfied. The knowledge of the basic flipping mechanisms in the laser itself enables us to understand and realize the external optical-polarization control by an anisotropic feedback (one-frequency systems). Induced-rotation and induced-inhibition mechanisms are then theoretically predicted and experimentally verified. Sensitive optical gates are realized by varying the feedback phase in both processes.
The dynamics of a quasi-isotropic continuous-wave solid-state laser sustaining the oscillation of two orthogonally polarized eigenstates is experimentally investigated. A set of phase-sensitive vectorial Maxwell-Bloch equations is developed to describe theoretically the evolution of such a laser. Different temporal behaviors for the output intensities are observed, depending on the relative amounts and orientations of the phase and loss anisotropies and on the frequency of relaxation oscillations. Namely, the frequency locking of the two orthogonally linearly polarized eigenstates is shown to lead to a surprising pulsed mode-locked oscillation regime. In addition, for different values of the parameters, other peculiar operation regimes are found, such as cw circularly polarized, double-pulsed, or triple-pulsed output intensities. These experimental results, obtained using a diode-pumped Nd:YAG (YAG denotes yttrium aluminum garnet) laser, are well confirmed by the theoretical model.