High-energy ultrafast ytterbium-doped fibre laser technologies

Abstract

This thesis investigates the design and dynamics of high-energy ultrafast fibre lasers, with a particular emphasis on Mamyshev oscillators (MOs). Steady-state (SS) and dynamic (Dy) models based on rate equations were developed to analyse both the steady-state output and the transient evolution of these systems. Through systematic modelling and experimental validation, practical strategies were established for optimizing filter separation, gain distribution, and cavity architecture, enabling femtosecond pulse generation with energies up to the hundreds of nanojoules directly from oscillators. Beyond oscillator optimization, the work extends high-energy fibre lasers through the integration of advanced fibre technologies. Hollow-core nested antiresonant nodeless fibres (NANFs) were implemented both intracavity and extracavity. Within the cavity, NANF provided a flexible means of reducing the repetition rate to the megahertz regime, yielding more than 20% enhancement in pulse energy. Outside the cavity, they acted as efficient delivery channels and linear compressors, achieving stable compression of ~500 nJ MO pulses to sub-50 fs durations with ~86% overall efficiency, while avoiding nonlinear spectral distortions. These results underline the potential of NANFs as a route toward fully fibre-integrated, high-energy ultrafast pulse generation and delivery. The thesis also addresses the critical challenge of initiating mode-locking in MOs. Using dispersive Fourier transform (DFT), the spectral evolution from seed injection to stable mode-locking was directly captured under different initial inversion conditions. This approach revealed the pulse buildup dynamics, including the role of gain competition and noise evolution, and provided practical guidelines for achieving robust single-pulse operation. Finally, multicore fibres (MCFs) were explored as a prospective route for further pulse energy scaling. By enabling coherent coupling across multiple cores, MCFs offer a promising strategy to surpass the limitations of single-core designs while preserving beam quality and stability. To sum up, the combination of validated theoretical models, targeted experimental demonstrations, and the integration of advanced fibre platforms establishes a comprehensive framework for the development of next-generation high-energy ultrafast fibre lasers. The findings not only deepen the understanding of Mamyshev oscillators but also chart practical pathways toward their deployment in scientific, industrial, and biomedical applications.

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Identifiers

ORCID for Lin Xu: orcid.org/0000-0002-4074-3883

ORCID for Ben Mills: orcid.org/0000-0002-1784-1012

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