Bending Induced Temperature Increases in Double-Clad Fibers for High-Power Fiber Lasers

Abstract

Rare-earth-doped optical fibers are one of the most promising solid-state lasers. In these fiber lasers, a cladding-pumping scheme using large-mode-area double-clad fibers (DCFs) is utilized to increase the overall conversion efficiency of pumping light and to overcome the restriction owing to the onset of stimulated Raman scattering. On the other hand, it is extremely challenging to increase the fiber core size while retaining the excellent beam quality because fibers with large core size allow propagation of several higher-order modes (HOMs), except for the fundamental mode (FM). In order to suppress HOMs, DCFs are bent with a relatively small bend radius. Failures at bends in an optical fiber are caused by light leaking from the core when the fiber is accidentally bent tightly with a high power input. For the DCFs with core radii of 10 and 20 µm, the relationship between the bending induced temperature increases and the bend losses in the bent DCFs was investigated theoretically by the explicit finite-difference method using the thermochemical SiO x production model. The temperature at the boundary between the inner and outer cladding layers increased with increasing optical power P at 1.080 µm and was a large value higher than the softening temperature T s of the silica glass when P = 5 kW was entered to the bent DCF with a bend radius R of 150 mm and a core radius of 10 µm. On the other hand, the temperature at the boundary was a small value lower than the T s when P = 10 kW was entered to the bent DCF with a large R of 245-275 mm. Furthermore, it was found that the initiation of the fiber fuse was fairly easy under a certain conditions where the temperature at the boundary was higher than the T s .