A structural investigation of a synthesized precursor for optical fiber applications; the heterobimetallic ErNb 2 (OPr i ) 13 M. Engholm a, * , L. Norin b , S. Edvardsson a , K. Lashgari c , G. Westin c a Department of Information, Technology and Media, Physical Electronics and Photonics, Mid Sweden University, SE-851 70 Sundsvall, Sweden b Acreo FiberLab, S-824 12, Hudiksvall, Sweden c Department of Materials Chemistry, A ˚ ngstro ¨ m Laboratory, Uppsala University, SE-751 21 Uppsala, Sweden Received 26 August 2005; accepted 5 November 2005 Available online 20 December 2005 Abstract A structural investigation of a synthesized precursor in a silica glass matrix is performed. Silica soot samples are doped with the hete- robimetallic precursor ErNb 2 (OPr i ) 13 by using a conventional solution doping technique and heat treatments to different temperatures. The precursor has also been introduced into a silica fiber preform by using the modified chemical vapor deposition technique. Analyses are made by using ultraviolet–visible–near infrared absorption spectroscopy, scanning electron microscopy, energy dispersive spectroscopy and powder X-ray diffraction. It is concluded that an immiscible system of ErNbO 4 crystallites and Nb 2 O 5 is formed in the silica soot sam- ples at high temperatures. Colloidal particles of ErNbO 4 are also formed in the silica glass fiber preform showing interesting features. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Optical fiber; Erbium; Crystallites; Energy transfer; Silica; Glass-ceramics 1. Introduction Rare-earth ions are widely used as active ions in wave- guides and optical fibers. The erbium ion in particular is important in optical fiber amplifiers at the telecom wave- length 1.55 lm. Glass materials containing niobium are often used in applications where a large nonlinear refrac- tive index (third-order susceptibility) is desired e.g. in opti- cal switches and modulators etc. [1,2]. However the nonlinear properties of niobium doped glass are not within the scope of this work. Instead we intend to explore the possibilities of using a synthesized precursor, containing niobium, in silica glass for use in active optical fibers such as fiber amplifiers and fiber lasers. We believe that doping with a synthesized precursor provide several advantages compared with the conventional method of mixing the chemicals at random. For example, several precursors with different local structures can be mixed already at the dop- ing stage and give the opportunity to make a hybrid solu- tion. This could offer a broader fluorescent bandwidth and a flatter gain spectrum, which are important properties in optical amplifiers. In this paper we present a structural investigation of the ErNb 2 (OPr i ) 13 precursor to establish if the local structure is preserved at the high temperatures involved in silica fiber fabrication. The role of the crystal field and site symmetry of rare-earth ions in glass materials need further investigations. Recently, Torsello et al. [3] have demonstrated that the symmetry of the local chemical environment plays an important role for nonradiative tran- sitions in rare-earth-doped oxides. According to ordinary Judd/Ofelt theory [4], the oscillator strength of a forced electric dipole transition between two multiplets may be simplified to an expression of the form f ed / X k¼2;4;6 X k W i U ðkÞ     W f       2 ; where jW i,f i are the initial and final states in the intermedi- ate coupling scheme. X k are intensity parameters that de- 0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.11.004 * Corresponding author. Fax: +46 60 148456. E-mail address: Magnus.Engholm@miun.se (M. Engholm). www.elsevier.com/locate/optmat Optical Materials 29 (2006) 386–391