Alternative designs for high power single mode active optical fibres

In this paper, we propose, through different theoretical examples of composite fibre designs, some alternative ways to push up the limits of active optical fibres. Highly ytterbium-doped large cores are required for high power delivery and low non linear limitations. Such cores usually support several guided modes. We demonstrate in this paper that by surrounding the core with a tailored cladding, single mode transverse emission under amplification or in the lasing regime is possible. A 3D modelling of transverse mode competition in active fibres is first presented and applied to several cladding designs. This numerical model, which is wavelength sensitive, is of great interest for laser cavity design in continuous wave regime for example as both composite fibre and cavity designs are considered. Accurate modelling of modal competition requires to compute overlap between electric field and active region for each electromagnetic mode (Gamma_mn) as a function of the transverse plane coordinates Gamma_mn(x, y). The fibre design, the laser cavity, the pump power have to be considered in order to obtain an accurate description of 3D spatial population inversion and its consequences on modal competition under active regime. The numerical model is first applied to a fibre exhibiting an all solid cladding made of 2D array of high refractive index rods and then to a multi-layered fibre. The latter is used to illustrate our purpose in figure 1 which shows the comparison between lasers made of fibres with or without tailored cladding. Both fibres exhibit an Yb-doped (5625 wt-ppm) core with a diameter of 30 mum and a refractive index higher than that of silica (5times10^-3). The pump power is 60 W. When the core is surrounded by pure silica, 17 modes can propagate and LP₀₁ carries less than 20% of the total output power which is distributed among almost all modes. The laser obtained is highly multimode and the spatial quality of the beam- is expected to be very low. On the contrary, when the structured cladding surrounds the core, almost all the laser power is emitted on the LP₀₁ mode. With a 4% - 99%, 2 m long cavity, less than 8% of total power is emitted on the LP₄₁, all other modes do not exist.