Application of chalcogenide glasses for optical fibre amplifiers at 1.3 µm

The majority of optical fibre now installed in Europe and North America operates in the second telecommunications window, centred at 1.32 microns. In this region, fibre loss reaches one of its minima and chromatic dispersion is negligible. Unlike trans-oceanic cables which operate at 1.55 microns, no optical fibre amplifier exists at this lower wavelength, thus necessitating a conversion of optical signals to electrical signals in order to amplify and re-transmit over long distances. It is clear that to upgrade currently installed fibre and to remove barriers that limit the full exploitation of land-based fibre, an optical fibre amplifier operating at 1.3 microns will play a critical role. Intense interest in fluoride glass optical fibres doped with praseodymium, the PDFA, was generated three years ago with the announcement of a 1.3 micron amplifier operating with a gain of over 10 dB. This host was extremely inefficient however, with a gain of less than 0.1 dB/mW of pump power, compared to the gain coefficient of 11 dB/mw achieved with an erbium-doped amplifier operating at 1.55 microns. Sulphide glasses based on Ga2S3 and La2S3 have recently attracted attention as a promising alternate host for the rare-earth praseodymium. When doped in a suitable glass, two energy levels of this ion are separated by an energy equivalent to 1.3 microns. In fibre form, such a glass provides the potential for highly efficient amplification. In addition to the basic Ga:La:S composition, we have developed and characterized a number of related glasses with an aim to improving quantum efficiency and also the thermal properties which are critical for fibre drawing. The goal is to approach the present 1.550 micron power amplifier performance of a 20 dB gain using only a few tens of milliwatts of pump power. In this paper, we present details of the properties of these glasses, progress towards the realization of a single-mode optical fibre and the overall suitability of sulphide glasses for an optical-fibre-based amplifier operating at this important telecommunications wavelength