Fibre lasers

One of the requirements of an optical fibre amplifier or laser is that it should have a high absorption at the pump wavelength and a very low loss at the laser wavelength. Conventionally, this has been achieved by using a high dopant concentration in a short length of neodymium-doped compound-glass or silica fibre. However, an attractive alternative approach is to use lower dopant levels and thus exploit the long interaction lengths and very low losses inherent in communications-grade optical fibres. We report here a novel extension of the MCVD fabrication process which allows the fabrication of both mono- and multimode optical fibres containing rare-earth ions at concentrations of up to 0.25 wt% in the core region. The technique is unique in that it allows the use of starting materials, e.g. rare-earth halides, which have a high melting point ( > 580°C) and hence have hitherto been unusable, since they exhibit a very low vapour pressure at the temperatures commonly encountered in reactant delivery systems for optical fibre fabrication. Initial work has concentrated on the lanthanide series as dopants, particularly neodymium and erbium, as these are of interest for both sensors and lasers. We have produced fibres with very high-loss absorption bands ( > 3000 dB/km) in the visible and near infra-red regions, while maintaining the characteristic low loss ( < 2 dB/km) of the MCVD fabrication process in the 'second window' for optical communications (1300nm). The technique may also be used to incorporate many other rare-earth and transition metals into optical fibres.