Polarization control mechanisms in vectorial bistable lasers for one-frequency systems

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-μm ${}_{}{}^3{}_{}{}^{}\mathrm{-}_{}^{20}{}_{}{}^{}$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.