A spectroscopic investigation of gallium lanthanum sulphide based glasses for optical devices
The optical communications network requires an efficient and cost effective 1.3 µm optical fibre-amplifier. The Praseodymium ion (Pr3+) 1.3 µm transition is the best candidate but requires a glass host with low phonon energy to produce a high radiative efficiency. Gallium Lanthanum Sulphide (GLS) based glasses are investigated as potential low phonon energy hosts. The work reported aims to link the spectroscopic results to the observed physical and optical properties and to the composition and atomic structure of the glasses. The Pr3+ doped GLS glass shows a high radiative quantum efficiency (RQE) for the 1.3µm transition and the lifetime of the transition is also long indicating a potentially useful glass dopant system. The Pr3+ enters the glass with a large coordination number and is sulphur coordinated. The Pr3+ doped GLS glass spectroscopic, optical and physical properties are found to be extremely susceptible to oxide impurities and added La203. In the Gallium Lanthanum Oxy-Sulphide (GLSO) glass the Pr3+ spectroscopy becomes excitation wavelength dependent. The temperature range of glass formation is increased as more oxide is added whilst the RQE and lifetime of the 1.3 µm transition are strongly reduced. This is explained through new oxide coordination for the Pr3+, which experiences a higher energy phonon and a different nephelauxetic shift of the Stark levels. The increased temperature range for glass formation of the GLSO glass is beneficial since the range of GLS glass is small. By adding metal halides to the GLS glass it was hoped that the improved glass forming properties of the GLSO glass could be kept along with the high RQE and lifetime of the 1.3 µm transition. The metal halide CSCI provided the largest increase in glass forming temperature range. The RQE was slightly reduced from the GLS value but much larger than the GLSO value. The lifetime was the longest measured. The dependence of the physical and optical properties on amount of metal halide was explained by changes in the bonding of the glass formers whilst the spectroscopic properties were most strongly influenced by bulk glass effects indicating a similar coordination for the Pr3+ in the GLS and CSCI GLS glasses. The doped GLS glass was not found to obey the simple multiphonon non-radiative decay formula with each rare-earth transition needing to be assessed individually.