Parametric Generators of Hyperbolic Chaos

Abstract

Excitation transfer between alternately active oscillators with phases transformed in accordance with chaotic maps, like that in the previous two chapters, may be regarded as some general principle of design of systems with attractors of the Smale-Williams type. Appropriate and convenient for implementation of this principle, are parametric oscillatory systems (Mandelshtam, 1972; Louisell, 1960; Akhmanov and Khokhlov, 1966; Rabinovich and Trubetskov, 1989; Damgov, 2004), The term relates to a class of systems with excitation of oscillations caused by periodic variation of some parameter. Parametric oscillations occur in systems of mechanics, electronics, acoustics, nonlinear optics, etc. In mechanics, a commonly known example relates to a person on a swing, who gradually increases the amplitude of the oscillations due to variation of the body position that corresponds to periodic change of the effective length of the equivalent pendulum (Fig. 6.1 (a) and (b)). In electric LC-circuit one can increase amplitude of the oscillations step by step if vary periodically the capacity: increasing the distance between the plates of the capacitor at the instants of its maximal charge, and decreasing it back at the instants of maximal current in the inductance, when the charge of the capacitor is close to zero (Fig. 6.1 (c)). Clearly, the mechanical work against the force of attraction of the charged plates in the course of the process converts into the energy of electromagnetic oscillations in the circuit; so, the amplitude will grow gradually. The main part of this chapter starts with consideration of the three-frequency parametric oscillator (Louisell, 1960), in which the excitation takes place in two coupled oscillators of frequencies ω1 and ω2 due to periodic variation of the coupling strength with the pump frequency ω3 = ω1 + ω2. Saturation of the excitation may be ensured by nonlinear damping in the scheme, which manifests then regular sustained regime of parametric generation. After that, we turn to designs, in which phases of tire parametrically excited oscillators undergo transformations corresponding to the expanding circle map in some period of time. Accounting compression of the phase volume in the state space along other directions, these will be systems with attractors of Smale-Williams type in the stroboscope Poincaré maps. In the systems examined here, the phase manipulation is performed as energy is transferred between oscillators, whereas the key role in systems discussed in the previous two chapters is played by a non-oscillatory external source of energy that compensates for oscillator losses, rather than by the energy transfer.