Beam quality evolution in large-mode-area specially doped laser fiber through bend-induced effective refractive index change

Abstract

The effect of bending in a specially doped large-mode-area (LMA) gain fiber on the beam quality of the laser and amplifier has been studied through simulation and experimentation. The effect on the overlap between the fundamental mode (FM) and the doping region due to bend-induced refractive index change was studied theoretically by varying the bend radius. Bend radius of the gain fiber in the range of 5–8 cm was used to study the evolution of beam quality at the amplified output. The numerical simulation of the overlap between the FM and the dopant distribution in the core of the gain fiber for different bending radii is well matched to the experimentally measured beam quality of the amplified output for the respective bending of the gain fiber. The master oscillator (having M 2 = 1.1) was successfully amplified to 35 W maintaining the near-diffraction-limited beam quality (M 2 = 1.05) using a confined doped LMA gain fiber with a bend radius of 8 cm. However, the use of a uniform doped LMA gain fiber with similar bend configuration degrades the beam quality (M 2 = 1.37) at the amplified output power of 31.3 W.