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Oscillations, Waves, and Interactions - GWDG

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406 U. Parlitz<br />

here chaos control methods are of interest. As an example, in Sect. 6.1 multipledelay<br />

feedback control is applied to a frequency-doubled Nd:YAG laser to suppress<br />

its chaotic fluctuations. This control method was developed at the DPI <strong>and</strong> has<br />

some promising features for stabilising <strong>and</strong> manipulating chaotic systems including<br />

complex spatio-temporal dynamics (Sect. 6.2). In Sect. 2.3 a combination of dynamical<br />

system governed by ordinary differential equations <strong>and</strong> an automaton switching<br />

between a finite number of states is presented. Such hybrid systems often occur in a<br />

technical context <strong>and</strong> may also exhibit chaotic dynamics. In Sect. 3 we briefly revisit<br />

the most important concepts for characterising chaotic dynamics: fractal dimensions<br />

<strong>and</strong> Lyapunov exponents. To investigate experimental systems specific methods for<br />

time series analysis are required. Following the long tradition of signal processing at<br />

the DPI, new methods for data analysis have been devised, implemented <strong>and</strong> applied.<br />

This topic is addressed in Sect. 4 where state space reconstruction (Sect. 4.1) <strong>and</strong><br />

forecasting methods (Sect. 4.2) are discussed.<br />

2 Nonlinear systems exhibiting complex dynamics<br />

In this section three types of dynamical systems are presented which possess different<br />

dynamical features.<br />

2.1 Nonlinear oscillators<br />

A cornerstone of physics <strong>and</strong> engineering is the damped harmonic oscillator<br />

¨x + d ˙x + ω 2 0x = f cos(ωt) (1)<br />

driven by some external periodic force f cos(ωt). 2 It is well known that the solution<br />

of this ordinary differential equation (ODE)<br />

x(t) = xhom(t) + xinhom(t) (2)<br />

consists of a general solution xhom(t) of the homogeneous equation (f = 0) converging<br />

to zero due to the damping <strong>and</strong> a special solution of the inhomogeneous system (1)<br />

with amplitude<br />

xinhom(t) = a cos(ωt − ϕ) (3)<br />

a =<br />

f<br />

� (ω 2 0 − ω 2 ) 2 + d 2 ω 2<br />

<strong>and</strong> phase<br />

�<br />

dω<br />

ϕ = arctan<br />

ω2 �<br />

. (5)<br />

0 − ω2<br />

The amplitude a of the (asymptotic) oscillation of the driven linear oscillator is<br />

proportional to the driving force f <strong>and</strong> the shape <strong>and</strong> location of the resonance curve<br />

a(ω) which depends on the damping d <strong>and</strong> the eigenfrequency ω0 as shown in Fig. 1.<br />

2 By rescaling time the eigenfrequency ω0 can be eliminated (i. e., ω0 → 1) but we shall<br />

keep it here for a more transparent interpretation of the results.<br />

(4)

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