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 over the core. As a result unfavorable clustering between germanium and ytterbium particles is avoided. The distribution of discrete glass areas with feature size smaller than λ/5 results in effectively continuous refractive index profile of the fiber core. We present a single-mode fiber with built-in Bragg grating for laser application with the core composed of ytterbium and germanium doped silica rods. The core structure is arranged as a regular lattice of 1320 doped with ytterbium and 439 doped with germanium silica glass rods. The average germanium doping level within the core of only 1.1% mol allowed efficient inscription of Bragg grating. The nanostructured core was 8.6 μm and the internal cladding was 112 μm in diameter coated with low index polymer to achieve the double-clad structure. In the first proof-of-concept in the laser setup we achieved 35 % of slope efficiency in relation to launched power for the fiber length of 18 m. The output was single-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 types of glasses. It allows to control simultaneously the refractive index distribution, the active dopants distribution and photosensitivity distribution in the fiber core.
We study optical properties of gradient index vortex masks based on an effective medium approach. We consider masks with single charge developed using two types of nanorods made of thermally matched low and high refractive index glasses. Optical performance of generated vortices are analyzed in terms of glass refractive index difference and spatial dimension of the components. A fabricated vortex mask has been combined with single mode optical fiber. Optical performance of the resulting fiber integrated vortex mask is characterized and discussed.
We study optical properties of the gradient index vortices obtained using effective medium approach. Vorteces with charge +1 has been was developed using two types of nanorods made of thermally matched low and high refractive index glasses. Their optical properties of vortices are analyzed in the context of glass refractive index and size of the components. Consequently vortex has been integrated with single mode optical fiber and such a system is analyzed.