6 kW near single-mode monolithic fiber laser oscillator based on a core NA design of confined ytterbium-doped fiber

In this article, we presented an investigation on the combination of the low-NA and confined doped design in Yb-doped fiber for gain filtering in the fiber laser oscillator. Theoretical simulations showed that for a ytterbium-doped fiber with low-NA (core NA < 0.055), the bending radius needs to be carefully selected. Too-tight bending allows higher-order modes to extract more relative gain while making the fundamental mode experience significant loss. With too loose bending, the higher-order mode can still survive due to lower loss. The bending strategy was analyzed and optimized in consideration of YDFs with different NA. We compared three confined Yb-doped fibers, which have respective core NAs of 0.058, 0.053, 0.048, while the core/cladding diameters and the core doping ratios maintain ∼25/500 μm and ∼80%. By using the former two kinds of doped fibers with core NAs of 0.058 and 0.053, the TMI thresholds of the constructed fiber oscillator are measured as ∼3274 W and ∼4554 W, respectively. Using the confined YDF with a core NA of 0.048, we demonstrated a 6 kW-level near single-mode monolithic fiber laser oscillator. At the maximum power of ∼5990 W, the beam quality factor (M 2 ) is ∼1.6, and the signal-to-noise ratio (SNR) of the spectrum is higher than 40 dB. Further power scaling is primarily restricted by the pumping power. To the best of our knowledge, it is the highest power reported in the confined doped monolithic fiber laser oscillator. We believe the corresponding results can be a reference for achieving a high-power monolithic fiber laser oscillator with near single-mode beam quality in the laser community.