Single-mode P2O5–F–SiO2 optical fibres with an optimised acoustic profile: Influence of the optical refractive index contrast and dopant content of the core on maximum SBS gain suppression

We consider optical fibres with a modified radial profile of the acoustic refractive index. The use of two-component doping of the core in a silica optical fibre makes it possible to simultaneously create different refractive index profiles for electrostriction-induced optical and hypersonic acoustic waves, which can be used to effectively reduce the stimulated Brillouin scattering (SBS) gain. The use of phosphorus oxide (P₂O₅) and fluorine (F) allows one, while maintaining the optical single-mode regime, to realise a high-contrast acoustic refractive index profile (ARIP), in which, under the condition of optimising its shape, a large number of guided acoustic modes can be effectively involved in SBS. It creates a wide multiband SBS gain spectrum and leads to a decrease in the absolute value of its maximum proportional to the number of these acoustic modes. In this paper, for such fibres with a step-index optical refractive index profile, we theoretically investigate a dependence of the absolute SBS gain maximum on the core/cladding optical refractive index contrast Δn at the ARIP parameters providing a maximum possible decrease due to multimode acoustics. Real technological limitations on the maximum doping of silica glass simultaneously with P₂O₅ and F (in particular, for the modified chemical method of vapour deposition, MCVD) are taken into account. An approximate analytical model of the optimal ARIP (and, accordingly, the radial distributions of P₂O₅ and F concentrations) is also presented. The obtained results lead to a conclusion that the deepest SBS gain suppression (up to 15 dB) in P₂O₅–F–SiO₂ optical fibres can be achieved at Δn < 0.0045, while at 0.0045 < Δn < 0.0075 these limits are objectively limited to 6 dB, and at Δn > 0.0075 other glass compositions or methods must be used.