In this report, the authors from Lawrence Berkeley National Laboratory and the University of Michigan describe a phase-stabilized coherent pulse stacking scheme that has the potential to combine in excess of one hundred pulses in a relatively simple cascaded delay line configuration. The basic delay line building block employs counter-propagating beams in a ring path with the optimized reflectivity of the input beam splitter depending on the number of cascaded sections and, hence, pulse energy enhancement. By cascading a sequence of interferometers of increasing loop lengths, the enhancement factor can be increased: for example, in a cascade of two cavities at the fundamental time period plus two at five times the fundamental, 25 pulses can be overlapped. The phase stability of the cavities strongly affects the efficiency of the recombination and stacking performance, and cavity lengths have to be controlled to nanometer accuracy to achieve about 1% stability. The authors employed a technique which they call modulated impulse response phase detection, whereby the cavity round trip phase was measured and a fast feedback control loop added to lock the cavity phase.
In the experimental implementation, the 10-ps pulses from a mode-locked Nd:YAG laser were stacked in the so-called 2+2 arrangement described above (two delay cavities at the fundamental, plus two at five times the fundamental time period). Long-term stability was demonstrated with the output stable to 1.5% for over 30 hours and a pulse energy enhancement of 18.4 recorded, comparing favorably to the theoretically predicted value of 21.5. The authors are confident that optimization of the delay cavities will lead to greater stability and enhancement, with the technique potentially leading to significantly enhanced pulse energies.
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