SBS Gain Suppression in a Passive Single-Mode Optical Fiber by the Multi-Mode Acoustic Waveguide Design

A novel method of stimulated Brillouin scattering (SBS) gain suppression in passive single-mode optical fibers is proposed and experimentally verified. The effect is achieved due to special propagation conditions created for a number of the core-bounded acoustic modes effectively involved in the SBS, such that its gain becomes approximately evenly distributed over a wide spectral range, thereby proportionally reducing its maximum. To verify and investigate this method in depth, single-mode large-mode-area (LMA) fibers with correspondingly optimized acoustic index profiles are fabricated, experimentally tested, and analyzed. Uneven complex doping of the core with two additives, P
₂
O
₅
and F, forms a high-contrast gradient acoustic index profile while maintaining a quasi-uniform optical refractive index profile. With a contrast of P
₂
O
₅
of more than 6 mol.%, we achieved 8 dB of SBS suppression over conventional uniformly doped LMA fibers. Moreover, it is shown that more than 10 dB of SBS gain suppression can be achieved if the optimal dopant distribution is realized.