Dissertation Defense

Passive and Active Fiber Laser Array Beam Combining

Wei-Zung Chang

To construct high-peak-high-average-power laser systems for high field-physics, fiber laser beam combining becomes a better option than bulk solid-state due to its potential for compactness and robustness, as well as its good heat dissipation and having diffraction-limited output even at high average powers. However, the combining approaches that have been developed so far are mainly limited to the continuous-wave regime. Even some combining scheme having potential for multi-kW power scaling such as passive coherent combining is still not fully understood, let alone the ultrashort pulse regime urgently requested by the high-peak-high-average-power applications. In this thesis, we aim at these two motivations and explore their underlying physics. In passive coherent combining, we propose a new propagation model to capture the multiple-longitudinal-mode nature of single fiber laser and the dynamical formation of coherently combined modes. We also conduct an exhaustive experiment based on a scalable 50:50 coupler configuration up to 16 channels with a 2-channel building block. Good consistency between experiments and simulation explains how the combining efficiency scales with array size. In pulse coherent combining, we demonstrate femtosecond pulse combining with up to 4 parallel fiber amplifier channels. Active phase locking is implemented by LOCSET single detector feedback technique. We achieve good combining efficiency and good match between the combined and the individual pulse durations. To explore the potential of scalability, we also derive a theoretical expression in relation to modulation depth, temporal amplitude and phase errors, which shows that multi-channel pulse combining with LOCSET feedback can be scalable to large combining number. Established on the foundation of 4-channel pulse coherent combining, we synthesize femtosecond pulses by coherently combining 3 fiber chirped pulse amplifiers with distinct spectra. Partial-overlap approach by linear detector as well as non-overlap approach by two-photon-absorption detector are compared and discussed for the sake of different phase locking conditions. Their scalable architecture with good combining efficiency opens a new way to overcome the gain narrowing effect imposed on a single fiber amplifier.

Sponsored by

Almantas Galvanauskas