Fabrication of ultra-high numerical aperture GeO2-doped fiber and its use for broadband supercontinuum generation.

We report the fabrication, characterization, and application (broadband supercontinuum [SC] generation) of ultra-high numerical-aperture heavily (50 mol. %) GeO2-doped optical fiber, obtained through a modified chemical vapor deposition process and rod-in-tube method. The formation of Ge-related diamagnetic defect centers, such as germanium oxygen defect centers (GeODC) with nonbridging lone electron pairs, confirmed by x-ray photoelectron spectroscopy and optical absorption studies, inducing hypolarizable local dipoles, may be responsible in boosting the nonlinear effects and enhancing stimulated Raman scattering at pumping with high-power pulses, culminating in generation of broadband SC generation. The SC spans toward the Stokes side up to 2.4 μm, under the action of ns-range pulses launched from a smartly Q-switched erbium-doped fiber laser with operation wavelength (1.56 μm) matching the zero-dispersion wavelength of the high GeO2-doped fiber.