Comprehensive investigation of longitudinal spindle-shaped Yb-doped fibers for high-power laser applications.

We present a comprehensive investigation on the laser properties of longitudinal spindle-shaped Yb-doped fibers (SYDFs), focusing on transverse mode instability (TMI) and stimulated Raman scattering (SRS), by conducting theoretical and experimental comparisons with traditional uniform double-clad fibers. The underlying physical mechanisms for TMI and SRS characteristics in SYDF amplifiers are revealed through detailed theoretical analysis. The experimental comparisons indicate that the SYDF showcases a slightly weaker TMI threshold than that of the uniform fiber with the same core-to-cladding ratio, showing ∼4.6% and ∼6.1% lower in the co-pump and counter-pump schemes, respectively. But the SRS suppression ratio of the SYDF amplifier was ∼7.6 dB higher than that of the uniform fiber at the same power level of 3.5 kW. When the output power of these two fiber amplifiers scaled to 6 kW, the SYDF still exhibited a stronger laser performance as compared to traditional uniform fiber. To the best of our knowledge, this is the first comprehensive comparison between SYDFs and uniform fibers in both experimental and theoretical regimes. This work provides significant reference for the design and development of what we believe to be new silica-based fibers to alleviate TMI and SRS in high-power fiber lasers.