Single-Mode P2O5-F-SiO2 Optical Fibers with Optimized Acoustic Profile: Influence of the Contrast of Optical Refractive Index and Composition of Core Doping on the SBS Gain Maximum

Optical fibers with a modified radial profile of acoustic refractive index (ARIP) have been considered. The use of two-component doping of the core in the silica optical fiber makes it possible to create simultaneously various profiles of refractive indices for an optical wave and a hyper-sound wave generated due to electrostriction, which can be applied for efficient reduction of the maximum of the amplification coefficient (gain) of the stimulated Brillouin scattering (SBS). The use of phosphor oxide (P2O5) and fluorine (F) with this purpose, when maintaining optically single-mode operation, makes it possible to realize a high-contrast ARIP by optimizing the shape of it, it is possible to effectively involve a great number of guided acoustic modes to the SBS process. This creates a broad multiband SBS gain spectrum and leads to a reduction of the absolute value of its maximum proportional to the number of acoustic modes. In this work, for the optical fibers with such a step-index profile of optical refractive index (ORIP), we have studied theoretically the dependence of the absolute maximum of the SBS gain at the ARIP parameters providing its ultimately possible reduction owing to the multimode acoustics on the value of the contrast ?n of the optical refractive index between the fiber core and its cladding. Among others, the real technological limitations on the maximum codoping of silica glass with P2O5 and F (in particular, for the method of the modified chemical vapor deposition, MCVD) are considered. In addition, an approximate analytical model of the optimal ARIP (and, respectively, the radial distributions of the P2O5 and F concentrations) is presented. The results have made it possible to come to the conclusion that the maximum SBS gain suppression (up to 15 dB) in the P2O5-F-SiO2 fibers can be achieved at ?n < 0.0045, while at 0.0045 < ?n < 0.0075, these limits are objectively restricted by 6 dB; at ?n > 0.0075, it is necessary to use other glass compositions or methods.