Nonlinear Dynamics of Self-Pulsing All-Solid-State Lasers.

摘要

The Project involved the study, both experimental and theoretical, of extreme events (EE, sometimes named Optical Rogue Waves) in two types of self-pulsing lasers: 1) all-solid-state (i.e., diode-pumped) Nd:YVO4+Cr:YAG ( slow saturable absorber); and 2) Kerr-lens-mode locked Ti:Sapphire ( fast saturable absorber). The main tool is the analysis of the time series of the laser pulses and of the images of the laser spots. Regarding (1), observations on a specially designed and constructed prototype showed the existence of EE in the pulse energy and also in the time separation between pulses. The two types of EE are uncorrelated: a high energy pulse is not preceded by a longer pumping time. Therefore, the problem is to identify the reservoir for the energy-EE. Correlations were identified between the spatial complexity of the laser spot, the Fresnel number of the cavity, the dimension of embedding of the time series and the appearance of EE. Regarding (2), two coexistent modes of operation are observed: transform-limited pulses (P1) and chirped pulses (P2). EE are observed only in P2. We found that EE arise after a threshold similar to the modulational instability is crossed. EE exist in P2 only, because P1 is unstable for the parameters values above that instability threshold. If the system is forced to start near P1, it evolves into P2 before EE can be observed in the practice. Finally, the observed privileged distances in the separation between successive EE are the residuals of the cold cavity periodicities, perturbed by the opposite tendencies of an expansive Kerr nonlinearity and contractive aperture losses.