Spectral shaping and bifurcation suppression for pulsed laser amplifiers

Abstract

Lasers are a flexible and powerful tool in many modern technological and scientific applications. A special type of lasers used in these applications are high-energy pulsed lasers, which need special optical amplifiers to achieve the required energy levels. Among these amplifiers are regenerative amplifiers (RA), which use multiple passes through a gain medium to amplify incoming seed pulses. RAs can show bifurcation due to the coupling of subsequent pulses introduced by the dynamics of the stored energy of the amplifier, which is undesirable for most applications. A control oriented reduced-order mathematical model of such an amplifier is investigated in this thesis in order to suppress these bifurcations using model-based control methods. This model uses small gain estimations of the individual passes through the gain medium combined with slowly varying envelope estimates and spatial averaging to achieve a simplified state-space model of the observed dynamics. Based on this description, a linear quadratic regulator (LQR) combined with an extended Kalmanfilter (EKF) is designed. The suppression of bifurcations due to feedback stabilization achieved by the LQR then enables the use of output pulse shaping techniques using iterative learning control (ILC) at arbitrary operating points. The designed system is shown to achieve pulse shaping for different operating points while staying within the input constraints introduced by the optical filter.