We study the time evolution of the polarization state of the light emitted by a Nd:YAG laser as the direction of the linearly
polarized pump beam is changed. The accurate description of the emitted polarization state is an exciting topic encountered
in many laser systems and often depends on the peculiarities of the laser structure. However, we expect that the behavior of our Nd:YAG laser will highlight key polarization properties of many Class B lasers which include semiconductor lasers. As we shall demonstrate experimentally, numerically, and analytically, the slow evolution of the inversion of population relative to the laser fields is responsible for unusual polarization switching properties.
In this paper, we present an original detection method using three fast detectors to monitor the spatio-temporal dynamics of a bimode CO<SUB>2</SUB> laser with saturable absorber. We observe that the time dependent components of the modal amplitudes increase as the frequency degeneracy lift between the TEM<SUB>01</SUB> and TEM<SUB>10</SUB> modes is reduced. Just before locking the system undergoes a period-doubling cascade leading to chaos. We record the temporal evolution of the modal amplitudes and the optical phase difference between the two modes in the specific regimes within the period-doubling cascade. Numerical simulations are in good agreement with the experiments, in particular they confirm the existence of the period-doubling cascade.
Experimental results containing the dynamical behavior of the Nd-doped optical fiber laser are presented. It is found that the state of pump polarization affects the intensity ratio of polarization modes. The intensity fluctuation spectrum of each polarization mode reveals three relaxation peaks. The model of a class B laser with two orthogonal elliptically-polarized modes is eligible for explanation of the main experimental features of relaxation oscillation peaks.
The transverse dynamics of a highly multimode CO<SUB>2</SUB> laser is studied. The spatial distribution of the intensity is shown to remain ordered even at high Fresnel numbers. Parameters as the effective cavity symmetry or the gain bandwidth are shown to be very important and alter significantly the oscillating pattern. The interpretation of this behavior is helped by adding a saturable absorber inside the cavity of the laser. The effect of the absorber is to stabilize the modes of the empty cavity, which are so demonstrated to be relevant for the description of the transverse dynamics. Following this approach, the dynamics of the laser without absorber is shown to result from the competition of a small number of the empty cavity eigenmodes.
Further modifications of the method proposed by Ott, Grebogi and Yorke to control chaos [Phys. Rev. Lett. 64, 1196 (1990)] have been achieved allowing us to stabilize and characterize unstable states (stationary or periodic) in their whole domain of existence. We demonstrate the possibility of stabilizing unstable periodic orbits in an experiment by applying a continuous feedback method. It has been checked experimentally on a CO<SUB>2</SUB> laser with a modulated parameter. This kind of method is very attractive opening the way to the control of chaos in very fast systems.