Beam Quality Optimization in Fiber Lasers via Selective Mode Attenuation Using Femtosecond-Laser-Inscribed Cladding Modulations in Few-Mode Fibers

Beam quality directly determines the output performance and application effectiveness of high-power fiber lasers. Conventional control methods, such as coiling and tapering, degrade mechanical reliability and cause localized heating, while specialty fibers suffer from fabrication complexity and poor compatibility with standard components. To address these issues, this paper proposes and validates a beam quality optimization method using mode-selective loss, based on femtosecond-laser-inscribed periodic index modulation structures in the fiber cladding. Theoretical analysis clarifies the influence of mode weight on the M2 factor in a 20/400 LMA fiber and the optimization mechanism of the structure. In a kilowatt-level laser, the average M2 improvement is 19.5% above 800 W; at 1384 W, M2 drops from 3.31 to 2.67, and the R2 between beam profile and an ideal Gaussian beam increases from 0.5412 to 0.8609. This work provides a fully fiber-integrated, easily fabricated, and tunable beam quality-control solution for high-power fiber lasers.