Advanced Techniques for Spectral and Temporal Ultrashort Pulse Synthesis in Coherent Pulse Stacking Amplification

Abstract

Coherent combining methods enable scaling of average power and peak power in fiber laser systems. Coherent pulse stacking has the potential to enable high repetition rate laser-matter interactions by extending the effective pulse duration, enabling near-complete energy extraction from a fiber amplifier. This, along with coherent beam combining, provides a promising path to achieve efficient high-repetition rate laser sources for laser-matter interactions. This thesis explores the adaptable nature of coherent pulse stacking, and develops techniques to achieve repeatable, robust high-efficiency stacking. It also demonstrates, for the first time, simultaneous coherent spatial combining and coherent pulse stacking amplification. Coherent pulse stacking was analyzed, and shown to be mathematically equivalent to a deep recurrent neural network. Different pulse amplitude profiles were shown to have minimal stacking loss, which demonstrates the adaptable nature of the technique. A method for increasing pre-pulse contrast by allowing post-pulses at the cost of efficiency was further developed and explored for equal-amplitude and equal nonlinear phase burst shapes. The impact of stacking parameter errors was quantified, and required tolerances for these parameters were derived. These analyses show stacking designs with >30 dB pre-pulse contrast and >95% stacking efficiency. A theoretical framework for accounting effects of stacking alignment errors was developed, which enabled quantification of tilt and piston error impact on stacking efficiency via numerical simulations. This enabled determination of required alignment accuracies for achieving high efficiency stacking. Alignment methods to meet the required tolerances for both tilt and piston errors were developed and implemented, achieving <5 µrad tilt accuracy and <2 µm piston accuracy and speeding up alignment. Additionally, oscillator repetition rate was locked to a highly stable rubidium frequency standard, eliminating oscillator cavity drift. In aggregate, all these technical advances significantly decreased system drift and enabled robust and stable stacking operation. As a result, high efficiency stacking of >80% stacking efficiency was achieved with high repeatability, robustness, and stability of <1% RMS. Simultaneous coherent beam combining and coherent pulse stacking amplification was demonstrated for the first time. A record-high ultrashort pulse energy of 7 mJ per fiber was coherently spatially combined in a four-channel fiber array with a total energy of ~25 mJ, and stacked into a single output pulse with stacking efficiency of 70%. Stacking performance at these energies was shown to be stable and robust over long durations, with stabilization noise of 2.2% RMS. Stacking was shown to not affect compressed pulse duration, and stabilization of coherent beam combining was shown to not impact stacking efficiency. This represents a milestone in the development of coherently combined fiber lasers, enabling further scaling towards 100 mJ and beyond. Finally, a technique to synthesize flat-top bandwidth-limited pulses using coherent spectral combining is developed. This method is directly compatible with other coherent combining techniques, which enables future scaling to high energies for generation of quasi-monoenergetic gamma photons via Thomson scattering. Spectral synthesis of the required spectrum is demonstrated with five spectral channels in a chirped pulse amplification system, and techniques for phase stabilization of spectral channels are developed. In summary, this thesis work expands the understanding of coherent combining techniques, both spectral and temporal, and shows the adaptable nature of coherent pulse stacking. When the techniques developed are used, high-energy high repetition rate laser sources are shown to be practically implementable via coherent combining techniques.