Near-quantum-defect-limited all-fiber 978-nm single-frequency laser amplification through population-inversion management

Here, we propose apopulation inversion management strategy that coordinates two coupled but distinctdesign dimensions: accessibility of the required local population-inversion state andabsorption‑length matching for efficient longitudinal interaction realization.

Modelingshows that pump delivery governs accessibility of the required local excitation regime,pump and coherent seed conditions set the accumulated absorption demand, and unit-length pump absorption maps that demand onto physical length, revealing anabsorption‑length trade-off between longitudinal compression and local heatconcentration rather than a unique optimum absorption. Guided by this framework, weimplement high local excitation using 920 nm core pumping and apolarization‑maintaining Yb-doped silica fiber with a nominal 920 nm small-signalcore absorption of ~400 dB/m.