The main goal of this study in this Action is the development and testing of measurement techniques for highly doped fibres. Also, efforts are made to study the design and characteristics of amplifiers and lasers made by highly doped fibres. The focus is on Er³⁺, Yb³⁺ and Er/Yb doped fibres but any other dopant is considered.

The characterisation of highly doped fibres is very challenging when compared with low concentration fibres. Effects like clustering and up-conversion are not yet fully understood and their measurement is still under debate in the scientific arena. Developing more accurate models for clustering and up-conversion effects is a target of this working group and will provide better tools to simulate amplifiers and lasers made by highly doped fibres.

The study of non-linear effects in highly doped fibres has a two-fold importance. First, nonlinear effects like four-wave mixing are a major problem in WDM applications. Secondly, nonlinear effects can be used for distributed measurements of other fibre parameters.

Other measurements like PMD and polarisation-dependent losses are less reliable and difficult to perform due to the high losses and fluorescence produced at the signal wavelength. The standard measurement techniques must be tested in these conditions and, if necessary, new techniques developed.

Splicing highly doped fibres to SMF or other speciality fibres is also a very important topic for development of efficient amplifiers and lasers.

Highly Er doped fibres have already proved exceptional performances in EDFA applications (very short doped fibre lead to very high bit-rate due to low PMD and low non-linear effects). This Action has activities on designing, characterisation and modelling of fibre amplifiers aiming to the optimisation of gain, gain flatness and conversion efficiency.

The activities within this Action are carried out by round-robins, personal contributions, and joint publications. Several highly doped fibres are circulated among the participants and various measurement techniques are compared. Also, modelling and simulation comparisons are conducted in order to tune the software tools of the participants. The technical sessions of the bi-annual meetings provide the floor for proposals and debates on different issues regarding highly doped fibres.