Doping silica fibres with rare-earth metals like erbium is far from straightforward. Typically, rare-earth elements and co-dopants are added to the fibre core using a solution-doping method that relies on diffusion.
The problem with this approach is that it is hard to control the doping levels and the homogeneity of the refractive-index profile. What's more, the doping process is usually slow, which means that the erbium ions tend to cluster into pairs.
Liekki says DND overcomes these problems by mixing the dopants in-situ during the glass deposition process. And there are other benefits, such as reduced clustering and better glass homogeneity.
Preliminary results look encouraging. For example, the team of researchers used DND to fabricate an aluminium/silica fibre containing 2200 ppm of erbium ions. A 1.2 m length exhibited a 38 dB gain at 1530 nm with 110 mW pump power. With figures of merit like this, Liekki reckons it should be possible to construct erbium-doped fibre amplifiers using much shorter lengths of doped fibre.