Theoretical study of graded-index fiber for beam quality improvement and SRS suppression.

A confined-doped graded-index fiber model was established with reference to Nufern's 42C ytterbium-doped fiber. The core NA of the fiber is ∼0.06; the doping/core/cladding diameter is 30/42/250 µm; and the gain ion doping distribution is double parabolic. Based on a forward tandem-pumped amplifier structure, the numerical simulation was carried out on the physics characters of the fiber model. The fourth-order Runge-Kutta method was used to solve the boundary value problem during the numerical simulation. The differences between the graded-index fiber (GIF) and the step-index fiber (SIF) were studied theoretically, and the results illuminated that the beam quality of the GIF was better than the SIF. The stimulated Raman scattering (SRS) power in the GIF was lower than that in the SIF. These results show that the GIF has great potential for increasing the laser output power while maintaining good beam quality and can provide theoretical guidance and reference to experimental research of the high-power fiber laser.