Structural dependent thermal and optical properties of rare earth doped glass with mixed glass formers P. Nandi, A. Srinivasan, G. Jose * Department of Physics, Indian Institute of Technology Guwahati, Assam 781039, India article info Article history: Received 29 October 2007 Received in revised form 15 May 2008 Accepted 2 July 2008 Available online 23 August 2008 abstract A series of glasses containing the glass formers, phosphate and tellurite, were prepared by varying the glass composition and rare earth doping concentration. The glass transition temperature T g decreased with the increase in the Na 2 O concentration and increased with phosphate addition. A structural analysis based on FTIR spectra is used to explain the observed changes in T g . The change in the bandwidth of Er fluorescence peaked around 1534 nm with glass composition and erbium concentration is reported. An analysis of the fluorescence spectra shows that a decrease in TeO 3þd and TeO 3 units present in the phos- phate modified glass as compared to the tellurite glass without it. A model is proposed for the local struc- ture of Er doped sodium phosphotellurite glasses. The spectroscopic properties of Er 2 O 3 doped and Er 2 O 3 –Yb 2 O 3 codoped sodium phosphotellurite glasses are investigated. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Glasses doped with rare earth ions has been investigated inten- sively for fabricating lasers and amplifiers since the last quarter of the twentieth century. Rare earth (RE) ions were used to probe the local structural variations in the host glasses because of their un- ique spectroscopic properties resulting from the optical transitions in the intra 4f shell. From the application point of view, the rare earth ion erbium (Er) has been successfully used in optical fibers as a dopant to amplify signals in optical communication systems [1]. Erbium doped fibre amplifier (EDFA) in which the host is usu- ally silica glass fibre, amplify signals in the C-band (1535 nm– 1565 nm) in the optical domain. The possibility of amplifying sig- nals in a broad wavelength region simultaneously, led to the rapid development of wavelength division multiplexed (WDM) optical communication networks in recent years. Erbium doped glasses has been the choice for high power and low pulse width commer- cial eyesafe lasers used in various medical and defense applications [2]. Ytterbium codoping has been attractive in most of the erbium doped gain media because of its high absorption cross-section at 980 nm, which improves the pumping efficiency for erbium lasing transition through energy transfer. It is well known that Yb 3þ : 2 F 7=2 ! 2 F 5=2 ground state absorption (GSA) wavelength (980 nm) matches with that of Er 3þ : 4 I 15=2 ! 4 I 11=2 GSA. In planar waveguide lasers and amplifiers, Yb–Er codoped glass has been in general the obvious preference [3,4]. Many different glasses have been investigated for erbium doping in the past with the aim of achieving efficient fluorescence and large bandwidth. Tellurite glasses were found to hold a lot of potential because of their high emission cross-section, higher bandwidth and wide transparency [5]. But alkali tellurite glasses have low measured lifetime ðs f Þ, low phonon energy and intense green upconversion fluorescence at even low pump powers which decreases the efficiency for 1550 nm amplification. The low phonon energy in erbium doped tellurite glasses increases the lifetime of excited state 4 I 11=2 under 980 nm pumping, resulting in excited state absorption (ESA) and hence decreasing the pumping efficiency. The low glass transition temperature (T g < 300 °C) of these glasses limited their use for fi- ber drawing and planar waveguide fabrication. On the other hand, phosphate glasses are well known for high phonon energy (1200 cm 1 ), good thermal stability (T g  450 °C) and higher rare earth solubility. Therefore, the addition of P 2 O 5 in a TeO 2 glass would increase the glass transition temperature and maximum bulk phonon energy of the glass. As a result of the increased pho- non energy in a P 2 O 5 –TeO 2 (phosphotellurite) glass the 4 I 11=2 ! 4 I 13=2 nonradiative decay would be fast and the upconver- sion probability must decrease. Hence these glasses could be pumped using a 980 nm laser diode more efficiently. Another important aspect regarding phosphotellurite (PT) glasses is the interesting properties that would result from the use of mixed network formers, phosphate and tellurite. Studies on the structure, color and electrical properties of PT glasses have been previously reported [6]. Weber et al. studied the optical prop- erties of neodymium doped PT glasses [7]. Recently PT glasses con- taining heavy metal oxides are reported for ultra broadband fiber 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.07.003 * Corresponding author. Present address: Institute for Materials Research, University of Leeds, Houldsworth Building, Clarendon Road, Leeds LS2 9JT, UK. Tel.: +44 11334325369; fax: +44 1133432384. E-mail address: g.jose@leeds.ac.uk (G. Jose). Optical Materials 31 (2009) 653–659 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat