a1 Fiber Optics Research Center, Russian Academy of Sciences, 38 Vavilov Str., 119333 Moscow, Russia
a2 International Laboratory of Glass-based Functional Materials, Mendeleev University of Chemical Technology of Russia, Miusskaya Square 9, 125190, Moscow, Russia
a3 Department of Materials Science, University of Milano-Bicocca, Via R. Cozzi 53, 20125 Milano, Italy
a4 Department of Physics “Alessandro Volta”, University of Pavia, via Bassi 6, 27100 Pavia, Italy
A spectroscopic protocol is proposed to implement confocal microfluorescence imaging to the analysis of microinhomogeneity in the nanocrystallization of the core of fibers belonging to a new kind of broadband fiber amplifier based on glass with embedded nanocrystals. Nanocrystallization, crucial for achieving an adequate light emission efficiency of transition metal ions in these materials, has to be as homogeneous as possible in the fiber to assure optical amplification. This requirement calls for a sensitive method for monitoring nanostructuring in oxide glasses. Here we show that mapping microfluorescence excited at 633 nm by a He-Ne laser may give a useful tool in this regard, thanks to quasi-resonant excitation of coordination defects typical of germanosilicate materials, such as nonbridging oxygens and charged Ge-O-Ge sites, whose fluorescence are shown to undergo spectral modifications when nanocrystals form into the glass. The method has been positively checked on prototypes of optical fibers—preventively characterized by means of scanning electron microscopy and energy dispersive spectroscopy—fabricated from preforms of Ni-doped Li2O-Na2O-Sb2O3-Ga2O3-GeO2-SiO2 glass in silica cladding and subjected to heat treatment to activate gallium oxide nanocrystal growth. The method indeed enables not only the mapping of the crystallization degree but also the identification of drawing-induced defects in the fiber cladding.
(Received July 21 2011)
(Accepted December 05 2011)