Adaptive compression of tunable pulses from a non-collinear-type OPA to below 16 fs by feedback-controlled pulse shaping
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The self-controlled compression of widely tunable pulses in the visible generated by a non-collinear-type optical parametric amplifier is accomplished by a pulse shaper based on a 4f setup with a pixeled mask in the Fourier plane which is controlled by an evolutionary algorithm in a feedback loop. Pulse durations below 16 fs are achieved by shaping the pulses such that their second-harmonic signal is maximized. The optimization process generally requires less than five minutes. It is shown that the algorithm eventually determines the shaper settings which produce the global optimum for the SH signal. Moreover, pulses having propagated through a disturbing medium which introduced additional group velocity dispersion have been recompressed to below 16 fs. An acceptable value for the phase difference between two adjacent pixels of the liquid crystal mask is experimentally found to be 1.6. The described setup provides a powerful tool for delivering ultrashort tunable pulses to any location within an experiment, as well as tailored sub-20-fs pulses for optimal control studies.