LAUSR

To mitigate the limitations of stimulated Raman scattering (SRS) associated with low dopant concentrations in low numerical-aperture (NA) fibers, this study proposes a central refractive index (RI) dip design for low-NA fibers to further increase the effective mode area (MFA). Through simulations of mode-field characteristics, bending-induced distortion, and bending loss, optimal dip parameters were determined. The simulations demonstrate improved transverse mode instability (TMI) and SRS thresholds in a forward tandem-pumped amplifier using this central dip fiber (CDF). Guided by these findings, a low-NA fiber with an ultra-low central dip was fabricated. Experimentally, under forward tandem pumping and optimized coiling, the amplifier achieved a 5.6 kW output with M2 < 1.4, demonstrating significant enhancement in both SRS and TMI thresholds by using the CDF.