A novel low-NA confined-doped long-tapered fiber for high output power fiber amplifier

Abstract

Yb-doped fiber (YDF) lasers and amplifiers have been widely employed to industrial processing, medical treatment, 3D printing, and military defense owing to the high conversion efficiency, efficient heat dissipation, compactness, and good beam quality [1-3] . However, their outstanding evolution is interrupted by stimulated Raman scattering (SRS) [4] and transverse mode instability (TMI) [5] effects. It is effective to suppress both of them by concentrating on the large mode area (LMA) YDF design. Specially designed fibers, such as constant-cladding tapered-core fiber [6] , spindle-shaped fiber [7] , low-NA fiber [8] , and gain-tailored fiber [9] , have been fabricated and demonstrated to suppress those nonlinear effects (NLEs) with effect. Hereon, we proposed a novel low-numerical-aperture (NA) confined-doped long-tapered fiber, which was fabricated by a modified chemical vapor deposition (MCVD) process combined with solution doping technique (SDT). As described in Fig. 1 (a), the core and inner cladding diameter were 25μm and 400 μm at both ends and 37.5 µm and 600 μm in the middle, respectively. The numerical aperture (NA) of fiber core was ~0.05, as shown in Fig. 1(b). Additionally, the gain dopant was restrictedly distributed in smaller area in fiber core, whose diameter ratio was 70%.