Spectroscopic properties of Er doped and Er, Nd codoped fluoride glasses under simulated sunlight illumination Shintaro Mizuno a,⇑ , Hiroshi Ito a , Kazuo Hasegawa a , Hiroyuki Kawai b , Hiroyuki Nasu b , Mark A. Hughes b , Takenobu Suzuki b , Yasutake Ohishi b a Frontier Research Center, Toyota Central R & D Labs., Inc., 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan b Research Center for Advanced Photon Technology, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan article info Article history: Received 20 January 2011 Received in revised form 22 March 2011 Accepted 22 March 2011 Available online 19 April 2011 Keywords: Fluoride glass Er-doped Nd, Er codoped Quantum efficiency Sunlight pumping abstract We investigated the fluorescence characteristics of Er codoped Nd doped ZBLAN glasses proposed for solar pumped fiber laser (SPFL) under simulated sunlight. Er is used as a sensitizer because it absorbs a part of the ultraviolet and visible light where is no absorption of Nd. Under simulated sunlight illumi- nation, Er singly doped fluoride glass displayed four emission bands with peaks at 550, 848, 977 and 1533 nm attributed to the 4 S 3/2 - 4 I 15/2 , 4 S 3/2 - 4 I 13/2 , 4 I 11/2 - 4 I 15/2 and 4 I 13/2 - 4 I 15/2 electronic transitions of Er, respectively. The quantum efficiency measurement was carried out using an integrating sphere and under the simulated sunlight excitation showed a maximum of 73% for 0.5 mol.% of ErF 3 in ZBLAN glass. In Nd, Er codoped fluoride glass, the 1.05 lm emission of Nd was observed under 380 nm excitation what supposes the energy transfer from Er to Nd in ZBLAN glasses as Nd has no absorption at the wavelength. Er, Nd codoped fluoride glasses are promising as a sensitized laser media for solar pumped fiber lasers. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction The sun is the sustainable energy source and a solar pumped la- ser, in which the sunlight is directly changed to the laser irradia- tion, attracted the attention of researchers as one of the ways to use it. A study of a solar pumped laser was considered since laser incunabula. In 1965, C.G. Young reported the first laser oscillation by the natural sunlight excitation using a Nd:YAG crystal and a Nd doped glass rod [1]. The output power of laser oscillation and the efficiency or Nd:YAG crystal rod were improved by Arashi in 1984 [2]. An idea of solar pumped laser in an optical fiber was pro- posed by Ueda in 1997 [3]. Then, the emission oscillation from a lamp excited fiber laser was confirmed by Saiki et al. in 2003 [4]. A Nd:YAG crystal has been used as a gain medium for the first time and subsequently Nd,Cr codoped transparent YAG ceramic, where Cr performs as a sensitizer of Nd, became the mainstream. The development in transparent ceramics was further advanced by Ike- sue [5] and Yanagiya [6] in the late 1990s as they bring into use a Nd:YAG as a gain medium for large-scale solar pumped lasers. It’s also a specific feature of the transparent YAG ceramic media that can adopt Nd and Cr dopants in a high level of concentrations. Yabe et al. [7] combined a Nd,Cr:YAG ceramic with the sun tracking and light concentrating system using Fresnel lens and then reported about 10% slope efficiency and 24.4 W laser output. Saiki et al. [8] obtained the laser oscillation with the efficiency of 43% from Nd,Cr:YAG ceramic under an arc-metal-halide lamp excitation. Kanabe et al. [9] then simulated laser properties of transparent Nd,Cr:YAG ceramic theoretically. According to this background, we reconsidered a form and a gain medium of the solar pumped laser and finally focused on the devel- opment of solar pumped fiber laser. The fiber laser possesses sev- eral unique features for solar laser design, i.e., a flexibility of the shape, low oscillation threshold, good radiation performance and mode control. However, to achieve the high performance solar pumped laser a proper study of suitable glass gain media is impor- tant. Nd-doped glasses have been studied as media for a laser igni- tion of a nuclear fusion for a long time. Since, the development in fiber lasers made of Nd-doped glasses is now rapidly growing be- cause in the fiber is relatively easy to achieve downsizing, increase the power density and mode controllability, there was no apparent effort to consider using of Nd-doped glasses as solar laser media. Recently, we have reported that Nd-doped ZBLAN glass shows high quantum emission efficiency under the sunlight excitation [10]. However, we also found that rare earth ions show a discrete absorption spectrum and there is a significant leftover of the sun- light which can not be absorbed by Nd ions only. In this paper, we report Er as a sensitizing agent enhancing the energy conversion and the emission efficiency of Nd ions in ZBLAN glass under the sunlight excitation. We examined the optical characteristics of Er doped ZBLAN glasses under simulated sunlight irradiation and investigated the energy transfer in Nd, Er codoped ZBLAN glasses. 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.03.041 ⇑ Corresponding author. Tel.: +81 561 71 7781; fax: +81 561 63 6156. E-mail address: mizunos@optics.tytlabs.co.jp (S. Mizuno). Optical Materials 33 (2011) 1958–1963 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat