Performance enhancement of nonlinear pulse amplification in large-mode-area multimode gain fibers via pump configuration optimization

Harnessing multimode nonlinear pulse amplification is an effective way to generate pulses that have broader spectra, shorter durations, and higher energies. Yet, as energy levels increase, a formidable challenge arises—spatiotemporal deterioration (STD), which compromises beam and compression quality. To address these issues, this study examines the effects of co-pumping, counter-pumping, and bidirectional pumping on STD in a double-cladding, large-mode-area Yb-doped nonlinear pulse amplifier. Grounded in a rigorous theoretical framework, we introduce advanced simulation methods and present a novel spatiotemporal quality factor to evaluate modal pulse performance. Our findings highlight a significant difference in amplification dynamics: co-pumping promotes gain-managed nonlinear amplification which extends the spectrum beyond the gain bandwidth but results in considerable STD and increased pedestal energy—limitations that ultimately restrict peak power. Conversely, counter-pumping enables self-similar amplification, a method that is more resilient to STD, leading to enhanced beam quality and higher peak power after compression. By deepening our understanding of nonlinear pulse dynamics, this research contributes to the theory and design principles of high-energy ultrafast fiber amplifiers.