Role of CaO addition in the local order around Erbium in SiO2–GeO2–P2O5 fiber preforms

Abstract

The development of materials for optical signal processing represents a major issue in present technology. In this contribution we present a study on Er-doped fiber preforms where particular attention is devoted on how the addition of CaO in the glass modifies the local environment of the rare earth. The results from photoluminescence and Extended X-ray Absorption Fine Structure (EXAFS) are compared and a clear link between the width of the emission line at 1.5 µm and the amorphous/crystalline local structure around the Er3+ ion is evidenced.Keywords: Optic fibers; EXAFS; Erbium; Nanoparticles 1. Introduction Knowledge of the environment of rare earth ions in glasses is essential for the understanding of the spectral properties of these glasses and the design of new glass compositions for photonics applications. Among the rare earths, erbium is especially interesting because the 4I13/2 − 4I15/2 transition at ≈1.54 µm coincides with the lowest attenuation window of silica glass fiber; hence, erbium doped glasses can be used to amplify attenuated signals in optical fiber telecommunication systems. While Erbium-Doped Fiber Amplifier (EDFA) was developed 20 years ago, extensive studies are still carried out to improve its properties. In particular, linear dimensions should be reduced although the severe limitations imposed by the poor rare earth solubility into silica matrix. Moreover, spectral bandwidth should be increased to improve the Wavelength Division Multiplexing (WDM) applications. To solve these problems, various approaches have been proposed namely Er-Tm co-doping [1] or Er incorporation in nanoparticles (NP). However, to be compatible with optical waveguide applications, Rayleigh scattering must be kept at a minimum. For this purpose, Tick proposed that the particles size must be less than 15 nm with a narrow distribution size and high density [2]. There are reports on thin films containing semiconductor and metal nanoparticles acting as sensitizers, absorbing the incident light with a high crosssection and exciting erbium co-doped ions in the nanoparticle vicinity through energy transfer [3][4][5][6][7][8]. Here we propose for the first time to incorporate Er into oxide nanoparticles in optical fibers to improve the spectroscopic properties of erbium ions. To our knowledge only few studies on nano-structured silica fibers were dedicated to metal ions properties [9]. In our samples, oxide nano-structures are prepared by adjusting the composition of usual modifiers in the core of the fiber. A glass with an immiscibility gap is obtained and two phases are then formed with high and low silica content. The nanoparticles are preserved when the perform is drawn into a fiber. An effective method to study the incorporation of Er in a matrix is by Extended X-ray Absorption Fine Structure (EXAFS) [10] as this technique provides quantitative information on the geometry and chemical composition of the environment of the element under study. The Er site has been successfully studied by this technique in a var...

Erbium emission properties in nanostructured fibers

“…A maximum clearly appears in a region of less than 1 µm in diameter (due to diffraction limit). This result indicates that erbium ions are located inside or very close to the DNP as it was observed in preforms through EDX analyses [20]. Note that no erbium fluorescence is detected outside the DNP.…”

“…We clearly observe some dark spots in the core which are ascribed to nanoparticles. As it was reported in preforms, DNP are observed only for Ca concentration of 0.1 mol/l or higher [20]. The absence of DNP for low concentration can be explained by the fact that, according to the SiO 2 -CaO binary phase diagram, the immiscibility gap occurs for a CaO concentration between 2 and 26 mol%.…”

EXAFS study of the Er3+ ion coordination in SiO2–TiO2–HfO2 sol–gel films

“…The PL spectra at 1.54 lm were sharper in the samples containing silica, hafnia and titania than in binary (silica-titania or silica-hafnia) compositions [4], suggesting that Er entered in one of the crystalline structures produced in this case. Note that a clear connection between crystallinity of the Er environment seen by EXAFS and sharpness of the PL lines has also been put in evidence in a recent study of SiO 2 -GeO 2 -P 2 O 5 glass fiber preform [24]. Thus, in the STHb sample Er is likely to enter the hafnia crystallites, considering the similarity of the related EXAFS spectrum with that of the HE sample.…”

Erbium-doped nanoparticles in silica-based optical fibres

“…the particles were synthesized before insertion into the silica tube-substrate. We have proposed a more straightforward technique allowing to embed RE ions within in-situ grown oxide nanoparticles in silicabased preforms [6]. The implemented principle is the spontaneous phase separation process [2] : Silicate systems can exhibit strong and stable immiscibility when they contain divalent metals oxides (MO, where M= Mg, Ca or Sr).…”