Ytterbium doped nanostructured core silica fiber with built-in Bragg grating for laser applications

摘要

We report for the first time successful inscription of high reflectivity Bragg grating in nanostructured core active fiber.Nanostructurization of the fiber core allows to separate the active and photosensitive areas and to distribute them all overthe core. As a result unfavorable clustering between germanium and ytterbium particles is avoided. The distribution ofdiscrete glass areas with feature size smaller than λ/5 results in effectively continuous refractive index profile of the fibercore.We present a single-mode fiber with built-in Bragg grating for laser application with the core composed of ytterbium andgermanium doped silica rods. The core structure is arranged as a regular lattice of 1320 doped with ytterbium and 439doped with germanium silica glass rods. The average germanium doping level within the core of only 1.1% mol allowedefficient inscription of Bragg grating. The nanostructured core was 8.6 μm and the internal cladding was 112 μm indiameter coated with low index polymer to achieve the double-clad structure. In the first proof-of-concept in the lasersetup we achieved 35 % of slope efficiency in relation to launched power for the fiber length of 18 m. The output wassingle-mode with spectrum width below 1 nm. The maximum output power limited by pumping diode was 2.3 W.The nanostructurization opens new opportunities for development of fibers with a core composed of two or more typesof glasses. It allows to control simultaneously the refractive index distribution, the active dopants distribution andphotosensitivity distribution in the fiber core.